[shooter503]

Does anyone have a reference to a good text on the theory of recoil operated (non-locking) actions?

I am interested in the timing relationship between when the case starts to grip the chamber walls as powder combustion pressure rises, the reduction of this pressure as the bullet moves forward and the time when the back-thrust of the cartridge case initiates the inertia bolt rearward movement. This information is needed to better understand a cartridge reloading problem.

This question is a bit techy I suppose but I would appreciate any input on this event. Thanks.

[1911Tuner]

Both blowback and locked breeh designs work via Newton's Third law that states: For every action, there is an equal and opposite reaction. The main difference is in the method used in delaying the breech opening.

Action-reaction requires that a pair of objects come under the influence of a pair of forces. Those forces can come from the inside pushing the objects in opposite directions, or pulling from the outside. Force forward equals force backward. If that force is sufficient to overcome the object's inertial and frictional resistance to acceleration, the object moves. Once it starts to move, it also starts to conserve momentum.

With the blowback, the force that pushes the bullet through the barrel also pushes on the breechblock through the cartridge case. The breech opening is delayed by way of the breechblock's mass and the action spring's resistance. The barrel is fixed to the frame, and the slide moves independently of the barrel.

With the locked breech/recoil-operated design, the barrel moves and is attached to the slide for a short time and distance, which serves to add mass to the slide for that duration, thus negating the need for a heavy slide and/or heavy spring. The slide is driven rearward under the same level of force and at the same instant that the bullet is driven forward. As the slide's mass accelerates and gains momentum, it draws the barrel backward with it, until the barrel reaches the timed point that it may disengage from the slide and allow the breech to open safely. Because the bullet accelerates at about 40 times the rate of the heavier slide, by the time the barrel starts to disengage and the breech starts to open, the bullet is gone, and pressure falls. The slide continues rearward by way of conserved momentum.

The bullet must be there for the action/reaction to occur. If the bullet is gone before the slide moves, the slide won't move...at least not so you'd notice. Air and powder gasses also have mass, and therefore will cause an
action/reaction to take place...but it won't move much. Maybe .010 inch or so, depending on the power charge weight, bore diameter, slide mass, spring resistance, etc. Don't forget to figure in the hammer's mass and its resistance...mainspring too.

The bullet must move forward in order for the slide to move. When the bullet hits the rifling and is swaged into the bore under high pressure, it naturally holds the barrel forward. When the bullet moves forward, that allows the barrel to move backward. So, while the bullet is being pushed through the barrel, the slide is literally pulling the barrel off of the bullet. If the barrel is unable to move backward, the slide won't move backward either. They're conected by the locking mechanism.

With the straight blowback, the slide will move independently of the barrel, so bullet movement isn't necessary for the slide to cycle.

Consider a bolt-action rifle. Force exerted on the bullet is also exerted on the boltface, and drives it rearward. Because the bolt is tied to the receiver by the lugs and recesses, whichever direction the bolt goes, so goes the receiver. Because the barrel is threaded into the receiver...so goes the barrel. Because the stock is attached to the receiver...so goes the stock.

The last thing to understand is that a straight-walled pistol case doesn't grip the chamber walls tightly enough to prevent the case from being shoved backward against the breechblock. it will seal gasses, but it'll also slip.
The bottlenecked rifle case...say a .308 or .30-06 for instance...is operating under 2.5 times as much pressure as say...the .45 ACP, and also has a restriction at the shoulder. The forces that are working equally in all directions also work to hold the case forward in the chamber...so that any excessive headspace will cause the case to stretch at the thinnest section, usually just forward of the web...producing what is known as "Incipient Head Separation" or outright separation if the headspace is excessive enough. So the two cases behave differently under pressure. One slips and the other stretches.

Straight blowback designs rely on breechblock mass and spring tension to delay the breech opening. This is why high pressure or heavy recoiling cartridges aren't generally seen in blowback designs unless there is another part of the design that works to further retard the timing of the opening of the breech. These are known as "Delayed Blowback" actions, and the mechanism for that delay varies form one design to another. Research Heckler and Koch's roller delayed blowback design to better understand it. They sucessfully incorporated it into the H&K 91/G3 .308 caliber rifle.

Bottom line: In either of the four basic designs...locked/recoil...blockback...delayed blowback...and locked/gas-operated...the breech must remain closed until the pressures have dropped to a safe level.

[Jim K]

I am not sure I agree with all of that, but the bottom line is that in a blowback action the case wall gripping the chamber does little or nothing to retard the blowback action. The sheer inertia of the weight (mass) of the breechblock/slide keeps the breech closed until pressure drops.

Even attempts to use the case walls to retard blowback (as in the Kimball pistol in .30 Carbine, which had rings cut into the chamber), have been failures.

Jim

[1911Tuner]

Jim wrote:

>but the bottom line is that in a blowback action the case wall gripping the chamber does little or nothing to retard the blowback action. The sheer inertia of the weight (mass) of the breechblock/slide keeps the breech closed until pressure drops.<
*****************

+1 on that. More likely to produce serious failures to extract.

Jim...If you'd come down here to North Cackalackey to see me, I'd demonstrate all of it for ya...and you'd have the pleasure of not only my company for the duration...but that of my charming S.O. (Not to mention her culinary skills, honed by generations of her mountain heritage)...and a few of the prettiest and finest Old Fashioned Farm Collies in the state. Then, there's always that "Turboffee" that's gainin' me a bit of notoriety around the boards. C'mon...It ain't that far from Maryland, and easy to find straight off of I-85 south. No need to bring shootin' irons. I've got'em aplenty for the choosin'.

Standin' offer...You call it.;)

[shooter503]

Thanks Guys,

The part I was really looking at was the possibility of the case adhering to the chamber walls under pressure - I think someone once called this "Stiction". P.O. Ackley devised a practical demonstration of stiction in his book "Handbook for Shooters and Reloaders - Volume 1". However, he was using a 30.30 cartridge for the tests and so the stiction effect would be very different from that of a short pistol case.

My practical question condideration was:

Under the combined effects of bolt inertia and recoil spring pressure would the pressure in the case have time to rise to a level where stiction would take place? This, of course, would allow the pressure in the case to be maintained at a higher level before bolt movement took place allowing more powder consumption and higher bullet velocity. Eventually the case pressure would fall and the stiction effect would disappear and the usual inertia extraction would take place.

It was apparent to me that a very fine balancing of bolt inertia, spring pressure and powder burn rate would be required to make this idea work. It is also apparent from Jim's comment that this idea has been tried before.

On second thoughts:
The sheer inertia of the weight (mass) of the breechblock/slide keeps the breech closed until pressure drops.
Jim, read at face value that idea is counter-intuitive. Should I read that as "The sheer inertia of the weight (mass) of the breechblock/slide keeps the breech closed for a sufficient period of time for the pressure to drop.". Clearly, if the movement of the bolt was dependant on pressure it would move while the chamber pressure is high i.e. early on in combustion. I think I understand that the bolt does not move while pressure is high due to inertia causing a time delay. After the time delay caused by the pressure overcoming the inertia of the bolt has expired the bolt begins to move by which time the chamber pressure has fallen to a safe value.

Obviously, these considerations have strong influences on reloading procedures.

[ribbonstone]

Worked quite a bit with a .401WSl...not a pistol, but might be the most powerful commercal blow-back caliber marketed. Used one devil of a big weight, an extention of the breech block, hidden under the fore end.

None of the blow backs I know of run at real high pressure (not in the 50-55K range). The grip of the case to chamber wall though pressure does happen, but doesn't seem to be a major factor in how they operate.

Some ill-advised loads (I switched bullets without working up the lad from below) in that .401 did manage to open before pressure had dropped...large spark display and a whole lot of noise.
------
Will mention this. If you get the load balanced just right, the recoil senstaion is very soft. Balanced so that the breech has lost nearly all of it's speed at the end of it's travel, ther is no sudden jar of a speeding breech meeting a metal stop. Of couse those loads are well below max., but it is an interresting sensation.

[grendelbane]

None of the blow backs I know of run at real high pressure (not in the 50-55K range).

While it is more of a retarded blowback than a straight blowback, the 5.7x28 pistol, smg, and carbine manufactured by FN operate in the 50k range. In both examples, the barrel is just slightly free to move. Some reloaders report discovering loads that cease to operate the action when they are increased past a certain point.

Some individuals have assembled blowback 9x23mm AR15s, which is another cartridge that operates in the 50k range. Not too many of them out there, so I don't really know how well they operate.

The last little blowback oddity I can think of is some one was experimenting with 23mm length pistol brass that had a very thick head, leaving a powder space similar to that of the 9x19. The theory here was that 9x19 ballistics could be achieved in a lightweight pistol with a blowback action. So far as I know, the idea never went anywhere.

[Jim Watson]

The last little blowback oddity I can think of is some one was experimenting with 23mm length pistol brass that had a very thick head, leaving a powder space similar to that of the 9x19. The theory here was that 9x19 ballistics could be achieved in a lightweight pistol with a blowback action. So far as I know, the idea never went anywhere.

That is the Tanfoglio - Lapua F.A.R. (Fast, Accurate, Reliable) system. They still advertise it but I don't know if they are selling any, even in Europe.

The biggest (commercial) blowbacks I know of are the Winchester 1905,1907, 1910 repeaters. Rifles with big bolt extension weights in the foreend to control the blowback by sheer inertia.

There was a guy who did some experiments to see how much energy was sapped by functioning the action of an automatic. He used a Ruger .22 so he could improvise a rig to clamp the bolt shut without too much trouble. He was much surprised when he got higher velocity from the gun when it was allowed to cycle normally than when the bolt was blocked closed. He had some theories but no proven explanation.

[1911Tuner]

Jim Wrote:

>He was much surprised when he got higher velocity from the gun when it was allowed to cycle normally than when the bolt was blocked closed. He had some theories but no proven explanation.<
**************

Interesting. There's gotta be an explanation, but I couldn't guess what it is.

I did a little experiment a few months back to show that bullet velocity has little to do with recoil...in which I reamed the rifling out of the barrel of a 1911 pistol (what else?) for a finished ID of .498 inch, for almost .025 inch
clearance per side between barrel and bullet. The gun was otherwise bone stock, right down to the small-radius firing pin stop that I'm so fond of.

I fired it with a few handloads made up of a 225-grain lead bullet and 6 grains of Unique. Not a powderpuff, but not quite up to full hardball level. The gun not only cycled smartly, and kicked the brass to about the same distance as it would have with a stock barrel...but the bullets buried up in dry, packed NC clay and even a couple of hardwood trees. I didn't shoot it across a chronograph, but apparently the velocity levels weren't too far removed from what they would have been with a rifled barrel.

Due to the extreme illegality of this creation, I removed the barrel and beat it flat with a large hammer shortly after the testing was done.:p

To touch on the recoil question again for a minute...

Why doesn't a Bazooka or similar shoulder-fired rocket launcher kick?
Answer: No breechblock. If a fixed breechblock were added to the
rear of the tube, you may rest assured that the rocket launcher would recoil so vigorously that it could no longer be shoulder-fired.

Projectile and breechblock, with force exerted equally on both...Two opposing objects and two opposing forces...Action/reaction force pairs. Simple, what? :cool:

[grendelbane]

I have noticed that everyone has holes in their knowledge, and the first example for me in 2007 was the F.A.R. system. At least I had seen some thing about it, and was not totally ignorant of its development, or lack thereof.

Seems like they went after the wrong market. Not sure which market would be the right one, but the system does look like it has potential for small, lightweight sub-guns with a high ROF. With the current metals price situation ammunition would be expensive, and also heavy. Perhaps a brass/polymer composite cartridge could be developed.

To get this thread back on course, if you want to see a situation where the case grabs the chamber, simply fire some aluminum cased Blazer in a blowback action like a 9mm AR15. Functioning becomes sluggish, and you will be lucky to see 67% actually feed. One of these days, I may substitute a 5.56 buffer for the heavier 9mm buffer, and see if this improves reliability.

[shooter503]

The firearm in question is a recently acquired Marlin Camp 9. The exercise is purely a theoretical one because for regular use I am happy with the standard 9mm load.

The 9mm from a P1 pistol produces about 1250 fps. The same cartridge from the Marlin produces 1400 fps. I guessed that in the carbine a larger amount of a slower burning powder would produce the same overall energy as the standard 9mm allowing either the same velocity with lower pressure or potentially higher velocity with the same pressure. With a bolt gun this approach would work. With a simple blowback action many other variables enter the calculations - hence my original post.

One concern was that the slower chamber pressure rise might allow extraction to commence before sufficient powder was burned producing fireworks. "ribbonstone" shows this was a valid consideration.

I deliberately chose a slowish powder to begin experimenting, Alliant 2400. Even at low loads the action cycles correctly however resultant bullet velocity is very low, around 900 fps. I think that, with this powder, case capacity will be reached way before normal 9mm bullet velocities are achieved. Apparently I need to try a faster powder. Perhaps Blue Dot.

It looks as though it would be a very easy matter with the Marlin action to jam the breach closed during firing. This could be done by placing a suitable length of steel between the rear of the bolt and the back of the steel action upper. This would allow a very accurate comparison between blowback and locked breech performance. Any comments on this idea?

[1911Tuner]

2400 is a bit too slow for the small case capacity. Blue Dot has produced some of the fastest velocities in 9mm that I've seen, but it's a funny powder to work with. Blue Dot has the distinction of being the only powder that I've ruined a .357 Magnum revolver with using a manual recommended charge.
Probably wouldn't be the same risk factor in the 9mm case...but one never knows. Proceed with all due caution.

[grendelbane]

If I were looking for maximum velocity in a 9mm I would start with Power Pistol first. I don't like the bright flash that it produces, but in a carbine length barrel that should be no problem.

I have never fired a Marlin 9mm, or loaded for one. Most of what I read from people who have lead me to believe it is not the best action for +p loads.

I have loaded for a 9mm AR15, which has a much heavier bolt & buffer. In it I got the best results from Power Pistol.

Power Pistol is an amazing powder. It is very easy to work with. I just wish it did not have that nasty flash. Currently, I am working with Longshot, which is much better behaved in that department.

It is possible that the bolt of the Marlin is so light that you will not be able to improve upon standard loads. This is my theory, anyway.

[shooter503]

Thanks.

grendelbane
Thanks. The caution about +P loads is exactly why I want to use a slower powder for the same velocity.

1911Tuner
Thanks also. I had heard reports of unexpected pressure rises with BlueDot which Is why I went to 2400 first. As you say, it is too slow. It has been a few years since I have been an active reloader so new powders have appeared since then. In view of your comments, Grendelbane's PowerPistol should probably be the next candidate. I had the same pressure problems that you had with BlueDot using Accurate Arms 223. I was reloading for an XP100 in 223. Fortunately this is a very strong action. With a slight increase in powder the "bang" turned into "cough" - and a lot of smoke blowing out like a steam engine around the action. It took complete disassembly of the action and a Dremel tool to get the case out.

I put a Wolff 21# recoil spring in the carbine. I doubt if this will make much difference to the initial bolt movement since the added spring pressure, over the standard spring, is probably quite low in the "bolt forward" position.

[Clark]

I did a calculation of a .223 blowback

assume: Peak chamber pressure = 50kp/i/i
assume: average chamber pressure = 25kp/i/i
assume: Peak bullet velocity = 2500 f/s
assume: Barrel length = 16i = 1.33f
assume: brass case inside diameter = .35 i
calculate force from chamber = PA = [25kp/i][.35i/4][.35i/4][3.14]= 600
p
calculate time of chamber force = 2 1.33f/[2500f/s]=.001 s
assume: action 2.5 i long = .208 f
assume: spring force = 20 p
calculate spring energy =fd=20 .208 =4.17 fp
calculate distance chamber pushes bolt = E/F =4.17fp/600 p = .0069 f
This means the bolt will be accelerating back for .001 seconds until it
has gone .0069 feet back and then it will be slowed down by the recoil
spring for 2.42 inches where it just runs out of speed as it reaches the
back of the action.
calculate bolt peak velocity = 2D/t = 2 .0069 f/.001s = 6.9 f/s
calculate mass of bolt = 2 E/VV = 2 4.17fp/ 6.9 f/s / 6.9 f/s = .175
pss/f
calculate weight of bolt = GM= [32.2 f/s/s] [.175pss/f] = 5.6 pounds

If you increase the recoil spring force, the bolt wieght requirement
will go down.


Usually, blowbacks are for wimpy cartridge, but I offer the two following counter examples from my experiemts:



CAUTION: The following post includes loading data beyond currently published maximums for this cartridge. USE AT YOUR OWN RISK. Neither the writer, The High Road, nor the staff of THR assume any liability for any damage or injury resulting from use of this information.

I have pushed a .380 blowback to see how much power I can get.
It is more than a 357 mag max load in a smaller case.
Browning M1903 8.5 gr. Power Pistol, 158 gr. XTP, seated to 1.090",1187 fps

I have pushed a .380 locked breech to see how much power I can get.
It is less than 9mm Luger power.
.380 Kel-Tec P3AT, 90 gr Gold Dot 6.1 gr Power Pistol, .968" 1110 fps



What does it all mean?
The difference between blow back and locked breech in a pistol is not as important as:
1) Case support
2) Chamber wall thickness
3) Slide mass

[shooter503]

So, if we consider the bolt only, it is subject to Newton's Law, Force=mass x acceleration. Or for our purposes Acceleration=Force/mass since the bolt mass and the cartridge are constants in any particular test firearm.

If we consider the bolt only, no recoil spring, the implication is that ANY force, even the small force produced when powder combustion just begins, will start the bolt moving although the rate of acceleration of the bolt is very low. As chamber pressure increases the force applied to the bolt increases and it moves more rapidily rearward. This effect stops when the case is extracted to the point where the case mouth is pulled clear of the chamber. At this point all rearward applied force on the bolt ceases and it continues to travel rearward based on it's inertia only.

Now let's add a recoil spring to the bolt. This spring can vary in length and it can vary in poundage. If the free length of the spring is exactly the same length as the "bolt forward" distance between the rear of the bolt and the spring contact point on the receiver there will be no spring pressure on the bolt. Therefore, the poundage of the spring has no effect. In fact, as far as initial movement of the bolt is concerned, it is as though there was no spring fitted and initial rearward movement of the bolt will take place at the beginning of chamber pressure rise.

If we now substitute a longer recoil spring a forward force will be applied to the stationary bolt dependant on the length and the poundage of the spring. In this case, the bolt will not start to move on initial chamber pressure rise. On ignition, the chamber pressure will rise to a certain value where it overcomes the pressure of the recoil spring. At this time the bolt will start to move but its acceleration will not only be controlled by chamber pressure rearward but also by the recoil spring pressure forward that increases as the bolt moves further rearward. When the mouth of the case clears the chamber the rearward inertia of the bolt is opposed by the increasing pressure of the recoil spring and the rest I understand.

A crude review would suggest that for maximum cartridge power we would simply use a very heavy bolt backed by the strongest recoil spring we could find of appropriate coil dimensions. In that case why do we not see this idea applied to even relatively low power rounds such as the 9mm? It is conventional wisdom that a 9mm pistol must use a locking breech. Is the problem increased pistol size and weight or are there other factors?

[Jim Watson]

In that case why do we not see this idea applied to even relatively low power rounds such as the 9mm? It is conventional wisdom that a 9mm pistol must use a locking breech

And just why is it you think we do "not see this idea applied to even relatively low power rounds such as the 9mm?" I've seen a bunch, it has been done and done again, but without much commercial success. The only volume production example I know of is the big Astras, the 400 in 9mm Largo, and the 600 in 9mm P for their Nazi chums. They tried again with the post-war Falcon but somehow lost the balance trying to make it look more conventional with an exposed hammer; recoil was hard and the gun still looked odd. Oh, wait, how about the Hi Point?

But Walther couldn't make a blowback 9mm fly in the Model 6 or the first version MP, High Standard couldn't compete with S&W and Colt even by putting a Mann ring in the chamber of the T3 to slow it down, the somewhat similar Detonics Pocket 9 didn't get much farther than Don Johnson's hideout in early Miami Vice, I haven't even thought about a Sirkis in years, and maybe we shouldn't even try to think about a Dreyse Military, which had so strong a spring that it had to have a disengaging latch to let you rack the slide... and hope it didn't let go when shot.

[shooter503]

Thanks for straightening me out Jim. As you see, my knowledge of pistol history is not that good. I was "rifles only" until I bought my first collector handgun a couple of years ago.

I suppose I should have asked why such pistols were not presently on the market.

I really appreciate the input from you experts. I will be lying awake in bed now trying to invent a better blowback pistol !!!!!!!