lack of recoil-operated semi rifles?... why?

[briansmithwins]

The bolt does not move until after the bullet has left the barrel with any gun design.

Nope.

On blowback and delayed blowback firearms the bolt starts moving back as soon as the cartridge is ignited. You can get away with this on low pressure, straight walled pistol cartridges, rifle rounds tend to get the case heads torn off because the bolt is opening while the front of the case is still stuck to the chamber walls.

This phenomenon is why HK's delayed blowback weapons have fluted chamber to keep the cases from sticking. The other work around is to oil or grease the cartridges.

BSW

 

[briansmithwins]

On springs:

The spring hardly adds any force to keeping the bolt closed on a blowback operated weapon. Say 9mmP runs at 30k psi. The pressure acting on the head of the case (~0.1^2") is 3000 psi. What spring are you going to use to counteract the 3000 lbs of force that trying to blow the bolt open?

The spring can slow down the bolt (less force acting over a longer travel) and stores the energy needed for cocking, stripping the next cartridge, feeding, and chambering, but it doesn't hold the bolt closed against the force of the cartridge in any design I'm aware of.

BSW

 

[cat9x]

As other's have said there are advantages and disadvantages to recoil operated firearms. The Remington Model 8 & 81 are among the better known long recoil rifles. There were plans once upon a time to scale up the 8/81 action for the 30-06 cartridge but this of course never came to fruition. Felt recoil of 8/81's in 35Rem and 81's in 300Sav are not that bad, but they have more than you would think the round would have. For a video of seeing how the 8/81 platform can be shot, check out the youtube video below,

http://www.youtube.com/watch?v=zILCO-SzVdY

 

[.45Guy]

Great vids!

 

[PercyShelley]

That's odd I don't find mine unpleasant at all.

Yes indeed, I noted the discrepancy. I wonder if the barrel spring was worn on the example I fired, or if there's a friction buffer like in the auto 5 that gets worn out and was in need of replacement.

Obviously, it was some mechanical reason, and not at all possible that I just have daintier shoulders than you do.

~27 pounds if I recall correctly. (M2 ball)

I have lent out my copy of Hatcher's Notebook, but that's about the figure I recall from it for a theoretical straight-blowback .30-06.

Interestingly, straight blowback does scale up very well on cartridges with rebated rims. The Becker family of aircraft cannons (including the type 99 zero-sen wing cannons and the Mk 103 ME-262 nose cannons of WWII fame) are essentially straight blowback. Yes, the firing occurs while the bolt is still moving forwards, but that contributes far less to the reduction of bolt mass than the case head design. I think these used lubricated cartridges.

FWIF, the Barrett M82 is not known for being especially accurate, compared to other styles of .50BMG rifle. In fact, Anzio builds a stabilization system to try and help improve things by controlling/limiting barrel movement.

That is also my understanding. Are M82s even used in competitive shooting alongside bolt action .50s?

I hadn't heard of the Anzio aftermarket tweaks before. That is interesting, thank you.

 

[Sam1911]

I hadn't heard of the Anzio aftermarket tweaks before. That is interesting, thank you.

Looks interesting...

http://www.anzioironworks.com/BARRETT-M82-UPGRADES.htm

 

[Matthew Courtney]

In a gas operated weapon, the bolt does not begin to move until after the bullet has passed the gas port and the pressure begins to manipulate the cycling mechanism, relatively long after the powder is ignited. You can also see in many recoil operated firearms how the bolt and barrel remain locked together until a certain point. All of these variables are manipulated through weapon design (timing, tension, mass, etc).

Watch the first few seconds of this pistol here: http://www.youtube.com/watch?v=Rqu9jCuR5P0

The force of the brass pushing rearward moves the bolt rearward against the locking lug surfaces. If the bolt didn't move back until the lugs mating surfaces were firmly against one another, the lugs would not be needed.

 

[Matthew Courtney]

On springs:

The spring hardly adds any force to keeping the bolt closed on a blowback operated weapon. Say 9mmP runs at 30k psi. The pressure acting on the head of the case (~0.1^2") is 3000 psi. What spring are you going to use to counteract the 3000 lbs of force that trying to blow the bolt open?

The spring can slow down the bolt (less force acting over a longer travel) and stores the energy needed for cocking, stripping the next cartridge, feeding, and chambering, but it doesn't hold the bolt closed against the force of the cartridge in any design I'm aware of.

BSW

3000 psi only equals 3000 pounds of rearward force when the area to which said pressure is applied equals one inch.

 

[68wj]

The force of the brass pushing rearward moves the bolt rearward against the locking lug surfaces. If the bolt didn't move back until the lugs mating surfaces were firmly against one another, the lugs would not be needed.

Yes, force is exerted on the bolt, but that is not moving the bolt outside of any play between the parts. The bolt is not considered a moving part until it begins to unlock.

 

[briansmithwins]

3000 psi only equals 3000 pounds of rearward force when the area to which said pressure is applied equals one inch.

I was using a rough number of 30,000psi for the pressure generated by the burning gas, and I used 1/10th inch for the area. Using those numbers would indeed give a force of 3000psi on the bolt face.

BSW

 

[barnbwt]

Yes, 'tis a necro thread, but this one deserves a resurrection

Lots of good info here (mostly towards the end) and a lot of misunderstanding/misinformation. It's mostly semantics, in my opinion, so no need to debate the minutia (yet).

I've found myself thinking about this very question, of late, since recoil operation actually is very common in certain classes of firearms; namely, service pistols, and heavy machine guns. Odd, how wide the gulf is there, isn't it? Granted, the same can be said of blowbacks (weak pocket pistols, SMGs, and the Oerlikon cannon ) but their appeal is quite plainly their simplicity of function.

I think recoil operation offers a similar simplicity to designers and that's why it was as popular as it was, when it was. We get our cake (powerful chamberings) and the ability to eat it too (both simple and light enough to manufacture and field)

There's two different types of recoil operation; short and long. Short is the only one that actually relies on timing/pressure curves to any extent, and if done wrong, you get case ruptures from early unlocking. Long recoil unlocks veritable eons after pressure has left the barrel, so it's pretty much foolproof in that regard, though typically more mechanically complex. In early machinegun designs, there were both recoil and gas operated schemes, but the latter were almost universally extremely massive (piston/op rods as heavy as the barrel, and nearly as large/long), relegating them to fixed/mounted guns. The Hotchkiss was "portative" not portable. This is because lack of gas-operation knowledge and period technology meant they had to be very aggressively driven, or "over gassed," to function reliably. Everything had to be big to take the needless abuse.

However, recoil operated guns were svelt lightweights operating with miniscule forces by comparison; MG15's, Maxims, Madsens, and later MG34's and 42's for the first true Light Machinegun roles deployable to mobile infantry. The Madsen tagged along in lesser numbers . Long stroke, being very safe so long as the locking surfaces are sufficiently strong, came earliest, with later improvements working to decouple the barrel/bolt sooner and sooner --creating the short recoil principle that only has the barrel reciprocate enough to unlock the bolt, and thereby limiting the total moving mass hitting the buffer at the same time.

At least in heavy machine guns, recoil operation is very scalable, from pistols all the way to rifles, to LMGs, HMGs, to large-bore cannons like the KPV in 14.5mm.

Another recurring theme in all these platforms seems to be ease of production for a quality, reliable arm. Recoil operation kicks more, but barrels are neither gas-tapped nor chambers fluted. Receivers are frequently simple tubes with a camming surface cut into them. They may be a bit heavier (since the barrel typically needs to be bushed at the muzzle end, necessitating a long/heavy shroud) but they also require far less cleaning and are far simpler in construction.

After learning that the Johnson/DROR action was basically an AR15 bolt used in short-recoil, I wondered if that might not be the basis for the most simply made of high-powered homebrew firearms;
-Off the shelf AR15 bolt assembly
--The bolt is pinned (by way of the cam pin) into a tubular bolt body that glides in the receiver tube, and is slotted for the hammer (simplified AR BCG)
--The cam pin extends out to form a bolt handle, and carries a roller bearing to act as the camming surface against the receiver (like the Johnson/DROR)
-Off the shelf AR15 barrel of some standard profile
--Simple cylindrical steel/bronze barrel bushings are pressed/threaded onto the gas block (sealing the hole) and barrel-nut seats, and keyed against rotation/over-travel with cross-pins/slots
--Muzzle brakes can either amplify or mitigate recoil as necessary for the configuration used (and a lot more easily than swapping gas systems). Could possibly even be adjustable
-Tubular steel (or possibly aluminum, if galling can be prevented) receiver
--Receiver is slotted on one or both sides for the cam roller track; recoil unlocks bolt after a short distance, and the returning bolt pushes the barrel slightly past its resting place momentarily to cam itself closed (the forward buffer greatly diminishes the stresses on the receiver, possibly allowing the use of aluminum)
--Cam roller slot is oriented/sized so it forms the ejection port (and possibly the magwell on the opposite side if the gun is made left/right swappable)
-Either a simplified sheet metal lower, or setup for buffer-less AR piece

Oddly enough, what I've described is basically a hybrid of the Johnson and the Soviet KPV, but way simpler than either. The layout would basically look like a stretched-out tubular sub-machinegun Now, to go order an AR bolt head and barrel assembly...

TCB

 

[tark]

I have a Remington model 81 in 300 Savage. It shoots two inches at 100 yards with 180 GR ammo. The recoil is not unpleasant, it's just...well....weird.

The gun is a long recoil design so the bolt and barrel recoil together for a full three inches; then the barrel runs itself forward while the bolt stays put. When the barrel stops moving the bolt and bolt carrier are released, to strip another round from the magazine and lock . It is more or less a centerfire version of the A-5 shotgun.

The recoil feels like someone has hit the muzzle of the gun with a baseball bat. It is very sharp and quick, but not all that punishing. Then there is the counter recoil as the whole mass of parts crashes forward again. It feels unlike any other rifle or shotgun I have ever fired.

The problem with the gun, and I think this is to some degree why you don't see more recoil operated guns is this: it is a complicated design and more expensive to manufacture than gas operated designs. My model 81 is a marvelous collection of finely machined and carefully hand fitted parts. But it is an absolute whore's nightmare to try and disassemble. I am quite sure it would not stand up to dirty conditions and / or abuse.

I think you don't see more recoil operated sporting guns out there because gas operation is easier, cheaper, more reliable and overall all around better.

 

[MJ]

Anyone doubt the accuracy of the modern BAR hunting rifle in 7mm MAG?

As what the USMC raiders or early WW2 thought about the Johnson rifle and LMG or the Green Devils of the pre 10th Mountain Div. thought.

Pointless he said she said argument post.

 

[Slamfire]

It has been a couple of years since I replied to this and I have read up a bit more on mechanisms and the history of machine guns. I have read Chinn’s Vol 1 and several other of his volumes and I think I see a pattern on the evolution of the machine gun. Chinn was interested in big machine guns, not service rifles. What I see in big machine gun evolution is a need for a higher rates of fire. Even during WW1, machine gun cyclic rates had become marginal for air combat, and the most advanced fighter planes of that war were only going about 100 to 120 mph!. As the speed and armor of aircraft increased so did the need for more lead on target in shorter intervals of time.

During the mid 1930’s the 20mm Oerlikon machine cannon, an advanced primer ignition mechanism that required greased cases, became the primary “mid weight” machine gun cannon of the allies. This machine gun was used on basically anything and everything, from Spitfires, landing craft, anti aircraft batteries, you name it. The basic greased round model was used by the Allies, Germans, and Japanese, see chapter 5 http://www.ibiblio.org/hyperwar/USN/ref/MG/I/MG-5.html and it would be interesting to know if any other firearm mechanism was as widely used, by all participants, in WW2.




This is the Polsten mechanism, no locking lugs, advanced primer ignition, and it required greased rounds or the case would have been ripped apart on extraction.



The Hispano was the US Army’s WW2 primary 20 mm machine gun. The mechanism was an advanced primer ignition delayed blowback and required greased rounds to operate. The Oerlikon was a similiar mechanism requiring greased rounds used by the Navy. This table is from Chinn’s Vol III and his book describes the multiple modifications that were made, almost all during WW2, when General Hatcher was in charge of the Ordnance Department.




A primary requirement for both the Army and Navy was 1000 rounds per minute. This is about 17 greased rounds per second. No analysis is provided in Chinn’s book, but it must have dropped out of a lethality analysis based on how many rounds it took to destroy an aircraft going 350 to 400 mph. At those speeds a plane would be in your sights for less than a second and given the ruggedness of the planes, you had to get several hits on target for a reasonable kill probability.

The advantage of an advanced primer ignition mechanism delayed blowback is a high cyclic rate. Take a look at this table that came from AMCP 706-260




A theoretical cyclic rate is being calculated and if you notice, it takes time to unlock, rotate, the bolt. Eliminate those steps and the cyclic rate increases.

By the early 50’s, jets have replaced propeller planes, and even the fastest single barrel reciprocating machine gun can’t shoot fast enough. Multiple barrel, externally powered Gatling type cannons with a cyclic rate around 6000 rounds per minute replace the Oerlikon and Hispano. As air speeds increase even more, explosive warhead missiles replace the Gatling cannons for air combat and most ground air defense systems.

Cyclic rates of 6000 or even 1000 rounds per minute are not needed (though they may be wanted by a ground pounder) in a hand held, man portable weapon. A long recoil mechanism has to travel the full length of the cartridge, but even at rates of 300-500 rpm, a long recoil mechanism would get unwieldy due the weight of the reciprocating mass. My Auto 5 shotgun, a long recoil mechanism, has its own unusual perceived recoil. I can feel the shotgun weight shifting once the barrel is in battery and the bolt moves forward. I believe that something firing a highpower centerfire rifle cartridge with a heavier barrel would really rock and roll under fully automatic fire.

This is what Melvin Johnson says in the Oct 1936 Issue of Army Ordnance about long recoil:


“The worst criticisms of this system are the length of barrel recoil, and the number of tricky parts. The barrel movement requires a “brake” system. Two return springs are necessary. The locking system is complicated and not particularly rugged. Provision for the barrel movement necessitates a tube around the rifle barrel, and this tube retains heat excessively. Of course the movement of the barrel takes care of the unlock and retraction of the breechblock. Using Mark I cartridges, the recoil distance must be increased an undesirable amount, usually over four inches.

Semiautomatic rifles of the long-recoil type have been consistently rejected for military use on the above and other grounds. If the barrel is movable to the extent of more than one-half inch, the weapon will require a complicated receiver or mount, and is quite likely to prove undesirable……”

Melvin Johnson was criticizing all actions types, other than short recoil, so as interesting as this article was about the disadvantages of gas, blowback, delayed blowback, and long recoil mechanisms, it does provide any information disadvantageous to the short recoil mechanism. This should not be a surprise since Melvin Johnson was lobbying to sell the US Army his short recoil rifle! Whatever merits the Johnson rifle might have had, it was doomed never to be adopted, because by the time Melvin Johnson created a suitable working model, the M1 Garand was already in low rate production.

Johnson light machine guns were issued during WW2, but did not last long in service. Don’t know why. I can’t remember an adopted full power short recoil service rifle. It seems to me that the short recoil mechanism would not vibrate excessively, I don’t think it would be as accurate as a fixed barrel gas gun, but service rifles don’t need target accuracy. I am baffled as to what made the short recoil mechanism extinct for high power centerfire rifle cartridges, unless it was a lack of familiarity, or cost.

 

[briansmithwins]

Recoiling barrels in personal weapons suffer from a weight penalty and the problem that touching the barrel on anything will likely result in a stoppage. If you surround the barrel with a shroud (like the recoil operated MG42) you get even more weight.

Rates of fire are influenced the doctrine of the using army. The U.S. Army wanted slow rates of fire (can't be wasting ammo, you know) and they got them, with the M1919, BAR, and M60. The Germans wanted high rates of fire (once you start shooting the enemy goes to ground) and got them with the MG34 and MG42.

The data about the American auto cannons is nice, but neither the Navy or USAAF got a working auto cannon out in time for WWII or Korea.

BSW

 

[Slamfire]

The data about the American auto cannons is nice, but neither the Navy or USAAF got a working auto cannon out in time for WWII or Korea.

What do you mean by "auto cannon"? Thinking of something else? Chinn says over 150,000 Oerlikon's were made and issued during WW2, and that is just what the Navy bought. They were in service up to the Vietnam war.

Great pictures here:

http://www.ussslater.org/tour/weapons/20mm/20mm.html

 

[barnbwt]

Yay! Folks are still interested in this

I agree that recoil operation tends to bring with it a weight penalty, but I think the penalty is more from the large reciprocating mass than anything. I would actually agree entirely with Mr. Johnson about long-recoil's inferiority to short recoil, since there really is precious little that is particularly advantageous about the operation for most shoulder-fired weapons.* Long recoil is more complex, mechanically, involves a lot more mass impacting/reciprocating in unison, and I suspect that a muzzle booster is needed to cycle any rifle-length barrel's mass if the cartridge is less powerful than 30-06 (this sort of applies to short recoil, too). That's a lot of downsides, and in areas specifically important for infantry purposes (simplicity, operation that doesn't interrupt sighting, heavier recoil from boosted action). The only upside is you are guaran-darn-teed not to blow the action by unlocking prematurely. Ejection is also more gentle, if the barrel is pulled off the case by a spring instead of it being ripped out at full tilt (which actually was probably a concern for the old-school paper shotgun shells, or brass ones that have always been pricey)

I think a big portion of the reason heavy machine guns tend to be recoil operated is simply because those guns, being mounted, tend to be much more barrel-heavy to preserve barrel life under sustained fire, making that piece sufficiently massive to actuate belt feed mechanisms and large cartridges without additional gas systems being added to the overall weight. If the barrel is too heavy, the addition of a booster cone is an insignificant weight gain. Being mounted, driving all that weight as hard as needed to function has few ramifications for the operator (even a vehicle as light as a technical truck isn't pushed around that much by a KPV)

The important thing about recoil operation, short or long, is recoil velocity; it must be fast enough for the masses involved to cycle the gun. The barrel as a rule is one of/the heaviest parts of the gun. I suspect that fact ends up making the reciprocating/total mass ratio very high. That's bad for infantrymen trying to control the muzzle, but great for mounted guns worried about total mass; I would argue that recoil operation is almost always lighter than a contemporary gas-driven design, with the possible exception of direct impingement, which is likely limited to small-scale weapons.

But, I also think short recoil could circumvent that basic rule about barrel recoil velocity. If the decoupled bolt body was thrown back under mechanical advantage (like a delayed blowback bolt is) the initial recoil velocity of the barrel could be reduced. Combine that with the fact a short recoil design arrests the barrel before the bolt body, and your felt recoil would be greatly diminished, along with the motion jarring the rifle as it cycles. The advantage over a simple delayed blowback would be that the leverage device wouldn't need to be nearly so beefy (it's seeing recoil forces, not bolt thrust), and high pressure cartridges could be safely used without the need for chamber fluting or lubricant coatings. I've actually thought about the possibility of such a 'delayed recoil' operation being used to make a 5.7x28 sub-compact mouse gun, since a normal recoil action that small would be horribly overdriven.

Another way would be to simply cock a spring between the bolt/barrel as they unlock, launching the bolt back as it is freed, and cushioning its return to battery. If the spring-loading could be kept low enough to avoid lug-surface damage, that would seem to make for a very pleasant recoil impulse. It's worth remembering that the decoupled bolt would only need the same momentum as a gas-op bolt+carrier assembly to extract/feed, but could potentially be much lighter/smaller without the carrier attached to it, or the gas system and its weight/recoil.

Lastly, it is worth bringing up the fact that modern infantry rifles use cartridges that allow for narrower, and more importantly shorter barrels. Remember that reaching minimum recoil velocity is the key to a short-recoil gun's operating threshold; our barrels are lighter than ever and very short. That means that peak recoil velocity can be achieved with less recoil impulse (forget boosters and their attendant recoil), and whatever shroud you might think is necessary to shield the barrel (or to mount a launcher/bayonet/accessories on) will shrink as well. Heck, what is a free-float quad rail but an SBR-length barrel shroud, and it looks very likely those will eventually find their way onto our issue rifles in some form.

TCB

*The Gepard GM6 Lynx anti-materiel rifle is long recoil (or at least, very, very long short recoil; I think the bolt actually does unlock before the barrel fully recoils, but not by much). But in that rifle's case, both the massive recoil of heavy cartridges and huge case volume (read: pressure duration) necessitate an operation that is as slow as possible, while still siphoning enough energy/force to extract and strip those big ugly 50BMG rounds assertively. That means lots of slow moving momentum; mass. The heavy barrel damps out the recoil impulse enough to avoid damaging a shooter, and there is no need for additional parts/mass (added to the total) from heavy pistons/gas tubes powerful enough to cycle the gun.


Just becuz

"the problem that touching the barrel on anything will likely result in a stoppage."
Dare you to try inducing a stoppage in the Lynx by grabbing the barrel . Or an S-18/1000 anti-tank rifle (similar barrel layout, but short recoil operated, and functionally similar to the KPV). I totally agree about smaller stuff, but I personally think the real limiting factor is the loss of the ability to put stuff on the barrel (bayonets, launchers, lights, sights). A free float tube and SBR length barrel would likely overcome those, at least in an infantry gun. I always thought it was funny how the Johnson was derided for poor reliability with a bayonet; I would think the far more serious concern would be how it would function with about 5X the normal "boost" when firing rifle grenades "KABOOM! ...boom (in the distance)"

 

[Nom de Forum]

Anyone doubt the accuracy of the modern BAR hunting rifle in 7mm MAG?

Reasonably accurate for hunting for a gas operated action.

 

[rcmodel]

The only 7mm Mag BAR I have played with much was an easy 1MOA rifle with a bunch of mixed brand factory loads.

In fact, it was incredibly accurate with certain loads it liked.
And it was very easy on the shoulder.

I liked it, a lot!!!

rc

 

[barnbwt]

...and my 308 FNAR is under an inch at a football field. More accurate than my paltry practice has been able to attain, same as my bolt action 300 SPS.

The BAR/FNAR is piston operated, too, for those folks who'll claim that also makes a gun inaccurate

TCB

 

[Nom de Forum]

The only 7mm Mag BAR I have played with much was an easy 1MOA rifle with a bunch of mixed brand factory loads.

In fact, it was incredibly accurate with certain loads it liked.
And it was very easy on the shoulder.

I liked it, a lot!!!

rc

The point I was attempting to make is that the BAR Hunting Rifle is not a recoil action.

 

[Nom de Forum]

The data about the American auto cannons is nice, but neither the Navy or USAAF got a working auto cannon out in time for WWII or Korea.

BSW

The United States made thousands of copies of the Hispano HS 404 during WWII that had reliability problems. This was primarily due to oversized chambers that eventually were shortened but still not the right size compared to British HS 404s. None the less, except in aircraft were clearing was not possible, they were reliable enough that the U.S.N. abandoned the .50 BMG as fast as possible for the 20mm as the preferred smallest AAA and armed some F6Fs and F4Us with them. Both 20mm and 37mm autocannon were used as armament of American fighter aircraft. The 37mm was also know for reliability problems, a rare thing for a Browning design. By the Korea War the problems with 20mms were corrected. Only the foolishness of some in USAF insisted on .50BMG in fighters until the realities of war changed their minds. The U.S.N. did not suffer from the same foolishness, their Korean era fighters used 20mm cannons.

 

[briansmithwins]

I was referring to auto cannons in U.S. airplanes in WWII, sorry for not making that clear.

As Chinn points out in his book, aircraft weapons need high ROF, extreme reliability, and light weight. 20mm Oerlikon cannons are reliable but not light (400lbs w/200 rounds) and are slow firing (~450 RPM) for aircraft.

BSW

 

[Nom de Forum]

I was referring to auto cannons in U.S. airplanes in WWII, sorry for not making that clear.

As Chinn points out in his book, aircraft weapons need high ROF, extreme reliability, and light weight. 20mm Oerlikon cannons are reliable but not light (400lbs w/200 rounds) and are slow firing (~450 RPM) for aircraft.

BSW

Chinn is right, but you are missing some critical information on actual cannon use in aircraft during WWII that your comment from Chinn is not considering. In WWII six of the eleven American fighter aircraft types used 20mm cannons in at least one their variants. The 37mm cannon was used in the P-39. All of these cannons were manufactured in the United States. The successful engagement range in air to air combat during WWII was nearly always less than 200 yards and often closer. Rates of fire for autocannons as low as the 90 rpm of the M4 37mm were successfully used to engage aircraft because one hit was usually so devastating it resulted in the destruction of a fighter and often a bomber. The Russians appreciated the 37mm so much they armed their Korean War era fighters with at least one 37mm and ended American B-29 missions during daylight. The most numerous autocannon used on American fighters was the American AN/M2 version of the French Hispano HS404 20mm, a combination gas and blowback operated weapon. As a side note, Americans often denigrate the French military, but most who do are probably unaware of the larger number of French military weapons and weapon designs the American military relied upon in both World Wars. The rate of fire of the M2 .50BMG at 600-850 rpm versus the AN/M2 20mm at 600-700 rpm is not significant enough to match the useable terminal ballistic performance of the 20mm. The U.S.N. considered one 20mm to be three times as effective as one .50cal. The 20mm cannon at 112 pounds is only 43% heavier than the .50BMG at 64 pounds. In most fighters the three wing mounted .50s could be replaced by two 20s. Even with a lower quantity of ammunition a 20mm armed fighter has much greater usable destructive power. This really became apparent when computing gunsights available latter in the war. Attempting to hose the sky with many projectiles was shown to be far less effective than short well aimed bursts of fewer projectiles. The major reasons why the 20mm cannon was not the predominant weapon of American aircraft are: shortage of cannons, abundance of M2s and established manufacturing, sloppy American manufacturing of the 20mm resulting in lower reliability, and American military shortsightedness and obstinate resistance to change. If American fighter aircraft during WWII had been required to attack large numbers of heavy bombers similar B-17s and B-24s the famous reputation for destructive power the M2 .50BMG enjoys today would be greatly diminished. Only the ability for the relatively large and strongly built American fighters to carry six to eight .50s with large ammunition supplies against smaller and lighter build adversaries helped to compensate for the lack of 20mm cannons. The most numerous fighter aircraft of WWII, the very effective Bf-109 a.k.a. ME-109, usually was armed with just one 20mm and two MGs. The fact that the armament was fuselage mounted greatly contributed to effectiveness.

 

[sawdeanz]

A related question I am hoping to have answered is why the HK series of arms that are delayed-blowback still have some type of op rod on top?

I'm curious in the main points of the thread too since I love firearm design and learning more about it, and I may look into some of those publications for fun when I have some time.