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

[briansmithwins]

Recoil works in pistols because for them, it's the cheap/simple choice.

The Browning short recoil, telescoped bolt* design pistol works great on pistol caliber cartridges. It's easy to make, cheap, and strong enough to hold together for the typical life of a pistol. Scaling short recoil up to handle a rifle cartridge, not so much.

On rifles like the AR, SCAR, AK, or ARX the highly stressed components are the barrel, barrel extension, and bolt. The gas piston, cylinder, and bolt carrier are stressed too, but aren't exposed to the full peak pressure of the cartridge. Because it's cheap, tough, and strong pretty much everyone makes these out of steel.

On those designs the receiver mostly just hold the stressed components in the right place and guides them as they move around. Aluminum, sheet steel, or even plastic will work.

A short recoil rifle adds to the stressed parts the barrel buffer/recoil mechanism, and the receiver also need to handle the stresses of stopping the barrel, and maintaining the bolt lock until gas pressure falls. You either are going to wind up with a much larger barrel extension (made out of steel) or a steel receiver.

BSW

*Yes, Browning invented the telescoped bolt decades before it was applied by the CZ Model 25 or Uzi: http://en.wikipedia.org/wiki/Telescoping_bolt

 

[barnbwt]

"...the receiver also need to handle the stresses of stopping the barrel, and maintaining the bolt lock until gas pressure falls."
I'd be using an AR barrel/extension. All the reciever does is guide their relative translation/rotation. Even an AR upper/lower has to handle the recoil forces. The tube would be under no greater strain than the takedown or hammer pin seats. That is, so long as recoil doesn't need to be amplified for function. I agree the Browning tilt barrel lockup would be untenable, here. This is more like the Barrett, Lynx, or possibly PX4 to a limited extent.

TCB

 

[briansmithwins]

I'd be using an AR barrel/extension. All the reciever does is guide their relative translation/rotation.

What stops the barrel extensions rearward motion? What stops counter recoil?

7075 aluminum is very strong for its weight, but it isn't hard. using it as a impacting surface against steel is going to lead to a very short life.

BSW

 

[barnbwt]

Barrel bushing woukd likely either hit a crosspin or come to the end of a slot to arrest its movement. With any sort of buffer spring or material, this would be no more violent than the AR's resistance of its barrel's movement; same recoil force driving the numbers. What stops counter recoil in an AR when the bolt hits home? What I'm saying is, I imagine these loads coukd be quite a bit larger than the AR's without being an issue, and even if they were, some quick reinforcement would likely overcome it.

As much as people bag on the R51, the steel hitting the little aluminum shelf under bolt thrust is a lot more durable than they'd expected. Some erosion due to the sliding motion at the edge, but no peening or shearing is ocurring on mine/others.

TCB

 

[Nom de Forum]

Possible enlightenment for Slamfire1 and possible opportunity for barnbwt

I broke out my Brassey's and looked at the section on short recoil. The author called the MG42 mechanism, a roller bolt, short recoil, accelerator mechanism, an extremely efficient mechanism. I also browsed my book on the roller bolts and something I think germane to this discussion is the amount of analysis that goes into designing a successfully "half locked" delayed blow back mechanism. The book "Full Circle" shows some of the force diagrams. At least for the roller bolt, calculations for mass, angles, forces, the kinematics of the system, is quite involved. This has to be carefully done or the mechanism will either have too much energy, or too little. Gas operated systems, you can move the gas port up and down the barrel, change the port size, there is more leniency after preliminary design to twiddle with things.

So, maybe that is one reason recoil mechanism are not that common, it takes more work up front!

Just looked at the FAMAS mechanism. It is not a gas mechanism, nor is it a short recoil mechanism, Wiki calls it a lever delayed blowback. This is one of the few service rifles that is does not use a gas operated mechanism. Nor does it have that "militarily essential" bolt lock!

Slamfire1,

You are right about the FAMAS, but you should look at more than the mechanism of the FAMAS because it could be enlightening. The French always do things a little different with varying degrees of success. Some of the cause of this a national characteristic of needing to be very distinctly French to differentiate themselves from, and occasionally claim cultural superiority over, the rest of the world. Prior to 1981 the French were using the MAS 49/56 7.5x54, a DI gas system operated combat rifle that is one of the most reliable and durable combat rifles ever made by any country. Here is where the distinctly French thing occurs; weirdly the 49/56 does not latch on to magazines, magazines latch on to 49/56s. What is not distinctly French or weird is the 49/56 has a last shot bolt hold open as a feature. In 1981 the French replaced the 49/56 with the MAS FAMAS 5.56x45, a lever delayed blow-back operated combat rifle that is not one of the most reliable and durable combat rifles ever made by any country. In addition to foolishly not installing a last shot bolt hold open device they spent the money that could have paid for it on designing and buying a proprietary 25 round box magazine instead of using M16 magazines. The French also built the FAMAS with poor quality plastic furniture and operating parts that quickly broke in the field. That was only the beginning of the consequences of lack of forethought and poorly thought-out decisions. A decade later the French having identified several deficiencies and disadvantages fielded the improved G2 version of the FAMAS. One design feature of the new G2 was use of STANAG, essentially M16, magazines. This decision was made after having to cooperate with NATO during Operation Desert Storm. Of course there is just one small fly in the ointment for cooperating with NATO in using those STANAG magazines. The FAMAS does not like NATO ammunition. The root cause of this goes back to the distinctly and weird French decision to use a delayed blow-back operating system in the FAMAs that, despite using a fluted chamber like the H&K G3, frequently tears the heads right of brass cased NATO ammo. The FAMAS uses French steel cased ammunition to prevent this malfunction. This is the consequence of choosing an operating system that time has shown to be unpopular with most nations and will probably never be used again in a new combat rifle design.

So now the French as of 2014 have realized that the FAMAS must be replaced with something better and more NATO ammunition compatible. Many of the current rifle designs they will be considering have a last shot bolt hold open feature just like the old reliable MAS 49/56 the FAMAS replaced. I would not want to have to bet that the next combat rifle the French adopt will not have a last shot bolt hold open feature. I would bet that most French soldiers will be pleased to have that feature back after a three decade absence from the standard issue combat rifle. Oh, by the way, some elite French police units and military special operations units already use combat rifles that have a last shot bolt hold open feature.

barnbwt,

Here is your great opportunity. The French are looking for a new combat rifle design. Being French they are not afraid to gamble on something distinctly different, since they love the advant garde. This is the nation after all that fielded the first smokeless powder combat rifle (Lebel), and fielded the first accurate recoil compensated quick firing field gun (French 75mm). Now is the time for you to convince the French, that with your paid help of course, what they need and could have is a state of the art long recoil operated combat rifle. It will be even easier to convince them if you tell them it can be a bull-pup so they will not worry the World will think they are admitting a mistake for going bull-pup with the mediocre FAMAS. You do need to realize though that if you and the French create a successful rifle the Germans, British, and Americans will steal the idea and make a better rifle. At least that has been the pattern for over 100 years. Personally I think the French should have skipped the FAMAS and redesigned the MAS 49/56 using more modern materials, reduce its size and weight for 5.56, use a non-steel receiver with steel inserts for high pressure areas, and dump the weird self-latching magazines.

As a side note of long recoil operation, Jane’s mentions it only has one advantage. That advantage is "reducing the forces" on gun mounts for larger weapons. I suppose they are implying long recoil operation spreads out the recoil force over a longer period of time to avoid a wrenching jolt to the gun mount. Perhaps what they mean by stating long recoil has only one advantage is that is the only advantage not canceled out by other disadvantages, and characteristics equaled by other operating systems.

P.S. barnbwt if the French take you up on a long recoil rifle I would like a piece of the "action" for suggesting it to you.

 

[briansmithwins]

Another disadvantage of long recoil is that in fully automatic weapons it leads to slow ROF.

Your maximum ROF is limited to how fast the action cycles.

BSW

 

[Nom de Forum]

Another disadvantage or long recoil is that in fully automatic weapons it leads to slow ROF.

Your maximum ROF is limited to how fast the action cycles.

BSW

That is probably not much of a disadvantage for a combat rifle unless you are striving for an extremely high, before full recoil, burst mode. So far designs striving for that have not been very successful in being adopted as the primary combat rifle of any nation. Any possible slow ROF on long recoil actions in full automatic mode for close range and suppressive rifle fire could be somewhat compensated for by adoption of short/fat cartridges, telescoped cartridges, and the holy grail a.k.a. caseless ammunition with all the advantages of cased ammunition but none of the disadvantages of all previous caseless small arms ammunition. Elimination of ejection could possibly facilitate faster return of the breech block into battery.

 

[sawdeanz]

Yes but short recoil action has a faster ROF, as evidenced by the MG42.

 

[Nom de Forum]

Yes but short recoil action has a faster ROF, as evidenced by the MG42.

A Browning AN/M3 .50 cal using a long recoil action has just as fast a ROF as a MG42.

 

[briansmithwins]

MG42s do have a high ROF, achieved in part by the muzzle booster.

None of the M2 variants that I'm aware of ever got anything like 1200-1500 rounds per minute ROF with any decent reliability. And the base M2 design is short recoil, not long.

BSW

 

[Nom de Forum]

MG42s do have a high ROF, achieved in part by the muzzle booster.

None of the M2 variants that I'm aware of ever got anything like 1200-1500 rounds per minute ROF with any decent reliability. And the base M2 design is short recoil, not long.

BSW

The Browning AN/M3 is a modified M2. It was intended for aircraft use and was the product of research started before WWII to increase rate of fire. At the very end of WWII some U.S. fighter aircraft were armed with the AN/M3. By the time the Korean War began U.S.A.F. fighters such as the F-86 Sabre had it as standard armament. Unfortunately by that time it was obvious 20mm and larger cannon were really needed to be effective because .50 caliber bullets had become insufficiently effective in air to air combat. This is something the U.S.N. realized in WWII and had already armed their post-WWII fighters with 20mms prior to combat in Korea.

 

[barnbwt]

"The French are looking for a new combat rifle design. Being French they are not afraid to gamble on something distinctly different, since they love the advant garde. This is the nation after all that fielded the first smokeless powder combat rifle (Lebel), and fielded the first accurate recoil compensated quick firing field gun (French 75mm). Now is the time for you to convince the French, that with your paid help of course, what they need and could have is a state of the art long recoil operated combat rifle."
Yo no hablo francais. I could pitch at the Spaniards, but they're even broker than the French, these days. I expect the frogs will go with the ARX, precisely because it is the cheapest option out there from a NATO country (and, it's still a fairly decent weapon, after all --they could do worse).

I still think long-recoil's ship has sailed. Aside from very specialized high recoiling applications where you want as long a time to hydraulically damp out that heavy recoil as possible (see: cannon) so to avoid damaging fixed mounts, there don't seem to be many advantages. It's like long-stroke vs. short stroke pistons; having the extra reciprocating mass moving around in tandem is nice to guarantee authoritative cycling, but that extra authority is really just at the expense of far greater recoil. Authority you could get by simply accelerating the smaller bolt faster in the first place (provided your magazine can keep up) or designing the gun to be less prone to binding/blockage. Past some point I think it's foolish to force an action (as with the forward assist, if the inertia of a bolt body running full-tilt isn't enough to strip or chamber a round, there's probably something terribly wrong ), but I also recognize there wasn't much alternative in the early days of autoloaders, when guns typically had obnoxiously close fitment and tons of openings for debris ingress. Toss in the terrible lubricant/park finish/mag designs of the day, and you had a whole lot steeper hill for the action to climb than nowadays with our Boron and Teflon coatings.

As far as slow short stroke actions, recall that recoil op pistols tend to have extremely high rates of fire. Bubba'ed 1911's frequently clock north of 1000 rounds per minute, which I think only the MAC11/9 comes close to in terms of stupid-high bullet spray. FAMAS is also ridiculously high, if memory serves. I imagine the highest rate of fire attainable is far more a function of how strong and well buffered the back of your receiver can be, than your chosen operating system

Nom,
I don't know jack about the AN/M3 (sounds cool, though), but what I suspect is that the reciprocating mass was reduced a bunch in order to increase ROF --that's just basic mass kinetics, after all-- but that this had the unfortunate effect of opening the action way too early to be safe. So they increased the length of dwell time to where the action could unlock safely once more. Iterate this process more than once, and you realize that if you hold the dwell time as long as your parts' length allow (i.e. long recoil), you can crank of the ROF as fast as your parts' geometry will let you. However, the side effect is absolutely hellacious recoil; not only are the barrel/bolt slamming to the rear in tandem, they are also going as fast as the action can possibly accommodate without unlocking early. That's what the fancy damping systems were for; I bet the aircraft superstructure only "felt" every other shot from the guns since the whole system was carefully tuned like a car suspension. Very compelling for a mounted gun, but that recoil might well be untenable for an infantryman.

A double-shot burst setup using the same premise and a hydraulically damped stock might be kind of cool, though (I bet it'd be wildly impractically heavy, though)

Since I don't plan on going to the trouble of eking out an existence in the cut-throat world of gun manufacturing, here's a rough animation of my accelerator-collar concept;

As you can see, the accelerator adds exactly one additional part to the mix. The stiff spring is the same one that returns the barrel forward; there is not really any reason the barrel & collar need to return forward on their own, since the bolt will drive them back anyway (not shown is a relief in the collar so it clears the magwell no matter its position/orientation)

I like it since it looks easy to make . Aluminum tube receiver, steel tube accelerator collar, and a steel pin in the receiver to cam it (can't really see the inclined cam surface cut on the back side of the collar, but it's there). I'll order one of those nifty cooling-finned AR barrels Green Mountain is selling --to go with the retro sub-gun look-- and some 1.25ID & OD tubing for the receiver/collar. I've already sourced a bolt assembly and have a PSA AR15 LPK that can be worked into a modified lower. I think some savvy shopping, and lucky guesses during design (to limit the redesign iterations) could keep costs below 300$ for the project

If it blows a case (I'll be remotely firing it for testing ), just make another tube with the unlocking cam incline moved further back, and try, try again

TCB

PS: yeah, the spring compression and contact mechanics aren't perfectly accurate yet; bite me (learning this new CAD program has me on edge. I'll eventually figure out how to use the animation/simulation module)
PPS: The attached is the overall receiver layout; to cock, you can either pull straight back and pull the barrel, or pull up first and then back to manually disengage the lugs. Shown is a full length M16 barrel model, a carbine would be shorter and have a shorter shroud section. Stick an AR15 lower under it, and that's pretty much what it'll look like

TCB

 

[Nom de Forum]

Very cool barnbwt! I am off to bed as I have an early start for a drive to California tomorrow. I'll get back to you though when I can as I think you tentative design is very interesting. Don't let not speaking French detour you from pitching to the Frogs as far more of them speak english than they are willing admit. Appeal to their vanity for embracing the advant garde and they will speak english.

 

[briansmithwins]

Couple of questions:

1) Looks like you’re using the receiver for a opening cam. If the receiver is aluminum and the cam pin/CH is steel, it seems like the receiver will have a very short life.

2) The bolt looks like it’s locked to the barrel extension/insert. But it looks like the barrel extension/insert is only attached to the barrel by a spring. Is that correct?

It looks like the barrel recoils, and then the barrel extension/insert recoils and cams separately. What drives the insert movement and camming?

BSW

 

[Elkins45]

Couple of questions:

1) Looks like you’re using the receiver for a opening cam. If the receiver is aluminum and the cam pin/CH is steel, it seems like the receiver will have a very short life.


BSW

Cast or pin a hardened bearing surface into the receiver, then put a roller around the cam pin.

 

[barnbwt]

1) There's a roller bearing on the charging handle to prevent galling, and the accelerator (dark grey collar about the barrel extension) keeps the barrel/bolt from having to recoil as fast while locked together --the spring only discharges once the cam is fully up the ramp. The ramp will also be pretty shallow to further reduce contact loads. I think hard anodizing or powder coating would be tough enough for a good, long time.

2) The bolt and barrel/extension are standard AR15. The dark grey piece stays stationary while the other parts recoil, compressing a spring against the barrel extension until the bolt unlocks. The accelerator collar is held stationary since it can on move along a helical path, and the bolt handle blocks its rotation until it is unlocked. That's why the collar around the barrel extension stays in place initially.

The goal was to harvest accelerator energy from the initial recoil phase as opposed to from barrel overtravel after unlocking like the Barrett, so the heavy barrel has to move as little as possible.

TCB

 

[barnbwt]

Ordered the 'fancy' finned barrel and tube-stock, so hopefully I'll be on my way soon. Made a trade for a bolt head assy, and hopefully will make another for a firing pin. I've got the AR trigger parts, already.

I'll start by making an aluminum receiver for the proof of concept phase (hmm...not sure how to simulate recoil for a short-recoil action without subjecting it to actual recoil --gotta figure something out for that), and a DOM steel double for at least the initial run. The steel is mostly to try to contain an OOB rupture to some degree during remote test fire while I verify the action timing is safe. Once that is shown good, I can swap the parts into the aluminum tube and see if it starts galling/peening off the bat.

I suppose I'll still need to source a buffer spring, and of course I need to settle on how to do the lower. If anyone's got an aluminum ~80% they screwed up the buffer tower on, I'm all ears

Oh, one thing I'm going to try on this build; polymer bolt body. The bolt head is stuck into a cylindrical body that fits the larger receive tube diameter, which would normally make the bolt assembly very heavy if made from steel (or even aluminum, really). I bought some Delrin and UHMWPE rod to drill/turn to size. Neither would be as good for sustained fire as metal, obviously, but this is a semi-auto, after all. I suspect the high lubricity of both will somewhat negate the tendency to gall against the much harder metal parts, and Delrin at least, has very good thermal stability. The UHMWPE was mostly as a backup in case the Delrin is too brittle (both are very cheap compared to metal, too)

TCB

 

[Nom de Forum]

Nom de Forum -
A Browning AN/M3 .50 cal using a long recoil action has just as fast a ROF as a MG42.

MG42s do have a high ROF, achieved in part by the muzzle booster.

None of the M2 variants that I'm aware of ever got anything like 1200-1500 rounds per minute ROF with any decent reliability. And the base M2 design is short recoil, not long.

BSW

You are indeed correct about M2 variants being short recoil operated and I suffered a serious headspace and timing problem with the post on the AN/M3 .50 because it is indeed a short recoil operated MG and does not support the contention that a long recoil operated MG could achieve just as hight of ROF as an MG42. That post was made too late at night and somewhat influenced by my default reaction that the MG42 is somehow special in having the capability of a 1200 round ROF since the AN/M3 does it without a muzzle booster. I still suspect a long recoil action could achieve a ROF more than sufficiently high for a full automatic rifle.

 

[Nom de Forum]

barnbwt - The goal was to harvest accelerator energy from the initial recoil phase as opposed to from barrel overtravel after unlocking like the Barrett, so the heavy barrel has to move as little as possible.

Wow you are a busy man considering you are still working on this project last I checked in!

http://www.thehighroad.org/showthread.php?t=750964

How is the little carbine going since your last post in mid-October?

Oh, one thing I'm going to try on this build; polymer bolt body. The bolt head is stuck into a cylindrical body that fits the larger receive tube diameter, which would normally make the bolt assembly very heavy if made from steel (or even aluminum, really). I bought some Delrin and UHMWPE rod to drill/turn to size. Neither would be as good for sustained fire as metal, obviously, but this is a semi-auto, after all. I suspect the high lubricity of both will somewhat negate the tendency to gall against the much harder metal parts, and Delrin at least, has very good thermal stability. The UHMWPE was mostly as a backup in case the Delrin is too brittle (both are very cheap compared to metal, too) TCB

Why bother with trying Delrin first instead of the UHMWPE? Perhaps I am mistaken due to my limited knowledge but doesn’t the polyethylene have far more going for it than the polyoxymethylene? It is after all so tough that it is “bullet proof” when used in armor and I think even more resistant to chemical degradation. For a semi-automatic it may very well be in your application just “just as good for sustained fire as metal”.

 

[barnbwt]

"Wow you are a busy man considering you are still working on this project last I checked in!"
Nah, I just excel at wasting time, so much that I can waste time on multiple pursuits simultaneously

"How is the little carbine going since your last post in mid-October?"
The Skorparev is still rocking out where I left it, but things plateaued about the time I needed to drill the first hole . I actually have a small bench-top drill press now, so I will be revisiting it probably after the New Year. Also still trying to come up with something I like for attaching the removable barrel. First, I'm trying to finally knock out the M90 shotgun mod/build, that's been taking up space for two years (it's getting there; see attached). I came to the conclusion that I need to actually finish a couple of my projects to free up Tupperware bins for new projects

"Why bother with trying Delrin first instead of the UHMWPE?"
Depending on exactly how "ultra high" the molecular weight is, polyethylene can be a soft/waxy material (think milk jugs) that wouldn't be the best option for this application. Just how the polymer is processed also has significant impact on properties (the mer chains are so long, the stuff is almost like a fiber composite, and the orientation/organization of the strands determines bulk properties). The site said low impact resistance, so I was also a bit uneasy about it smacking the rear of the receiver tube, as well as the cam pin set into it bearing against the cam track abruptly (not an impact, but close). Delrin seems to be pretty consistently hard stuff, but also brittle. I did buy some aluminum bar stock as a fall-back-fall-back, though. I think Delrin is also a thermoset, rather than a thermoplastic, so not as many worries about operation at elevated temperature.

I did get my metallic stock for this project today, so we're on our way. It does appear I'll need to turn down the 1.25" diameter stuff that rides inside the receiver; it's not quite telescoping in the larger tubes (by like a thousandth)

TCB

 

[barnbwt]

Got most of my remaining materials in today, namely the barrel --Snoopy dance!

I have to say, these Green Mountain ringed barrels are pretty cool; I have no idea why they aren't showing up in more ultralight builds (cuts for the rings aside, they are actually a pretty narrow profile across their length).

Looking at the barrel in relation to the receiver tubes (which are 1/4" larger than the barrel extension flange), the proportions are quite a bit different from what we're used to in a 223 AR platform. I think the overall size is about the same --after all, free float tubes seem to be in the 1.5" OD range-- but it looks way fatter due to the circular vs. 'tall' profile.

However...

I think I can make this work to my advantage in an incredibly cool way. Behold;

The VG1 'last ditch' gas-delay rifle also has a fat-pipe receiver with a little pencil barrel nosing out the front (the protruding barrel on mine is nearly a foot, though), but still looks cool owing to the beefed-up "submachinegun" profile. I think the ringed barrel would look awesome with that, especially if I forgo the AR components in the lower and use 223 AK magazines & a sheet metal FCG box/grip on the bottom.

On the other hand, if I instead use a modified AR lower, there are plenty of advantages as well. The mags automatically sit much higher so they can feed into a tubular receiver situated above, like mine. Same goes for the hammer design. I think there wouldn't be a whole lot more work besides chopping off the buffer tower. I don't think the looks would be quite so striking, but with a Beta drum, I can't help but think this would look like a smaller MG15 w/ doppeltrommel.



Any thoughts as to one or the other?

TCB