Why are Locked Breach 'Softer Shooting' than a Blow Back Action?

Discussion in 'Handguns: Autoloaders' started by Johnm1, Apr 11, 2021.

  1. Johnm1

    Johnm1 Member

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    Forgotten Weapons video on gas delayed blowback actions. Just fodder for discussion. There is a mention of felt recoil being less because the slide doesn't have to have added mass to delay the lock up/opening.

     
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  2. bannockburn

    bannockburn Member

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    As a former owner of an Astra Model 600 I can readily confirm what you're saying about straight blowback Astras!

    During my range sessions (always saving the Astra for last), I was usually good for about 50 rounds and then I was done!
     
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  3. Slamfire

    Slamfire Member

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    I have not shot enough high power blowbacks to really know if the feel is different just due to the mechanism. I will say, I watched a fellow shooter blaze away with one of these:

    Qcq24KL.jpg

    The P7 is an amazing pistol.



    The slide cycles so fast, that it feeds and extracts faster than the perception of a human observer. I looked, watched, and the slide looked in battery the entire feed and extraction cycle. And if it was not for the noise, and the ejection of the case, you would not know the slide cycled.

    The P7 also uses chamber flutes. These chamber flutes are specially there, to break the upper 2/3 rds of the case from the chamber, reducing case to chamber friction, and to increase the case thrust on the breech face.

    fluted rifle chamber

    PB9SaEH.jpg

    Remember that when someone tells you that case lubrication unpredictably and dangerously raises pressures, and "increases bolt thrust". The first is false, and the second is desired in a blow back action. A high friction case imparts less energy to the bolt, and bolt thrust is what cycles the action. The less bolt thrust, the more likely a jam.
     
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  4. Jim Watson

    Jim Watson Member

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    When Bruce Gray set up a P7M13 for USPSA, he used a smooth chamber for 9mm Major. And the compensator was in the slide, not on the fixed barrel, so gas expansion would assist the piston in retarding blowback of the heavy loads. No reports on durability, the 20 shot scope guns soon obsoleted a 13 shot iron sighted pistol.

    I had a standard P7, I think of the second type, still with a heel magazine catch.
    It was smooth and accurate to shoot but I did not feel comfortable with the squeeze cocker which, if you were taught to take a firing grip in the holster, led to a cocked gun in the holster.
    I never dumped enough magazines in a hurry to be bothered by heating over the gas cylinder in spite of hearing of it.
     
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  5. Johnm1

    Johnm1 Member

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    That is a much more intuitive video of the gas delayed system. Thanks for posting that.
     
  6. Slamfire

    Slamfire Member

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    I believe there were two factors in why the P7 went on the ash heap of history. It is an intricate pistol to make, nothing on it is cheap. And the second, when someone is under stress, they can confuse the sequence of operation on this pistol.

    I have found that the P7 can be fired by pulling the trigger first, and then squeezing the front strap. Obviously that is not the sequence the designers thought would happen, everything I read assumed the strap would be squeezed, but the trigger only pulled to fire the weapon. But I bet, the pull trigger and then front strap happened. Some LEO pulled out their P7, pulled the trigger, and then remembered to squeeze the front strap, Opps! someone got killed who should not have been killed.

    And if anyone thinks this is BS, just remember recently the Cop lady who forgot she did not have the yellow taser gun in her hand, and shot the guy in the car with her service weapon. She said "taser, taser, taser", and then "Oh ****, I just shot him"

     
  7. gc70

    gc70 Member

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    I have always looked at the difference in recoil in this way:
    All of the recoil force in a blowback design goes into moving the slide.
    Part of the recoil in a locked-breach design moves the entire pistol and the remaining recoil force accelerates the slide rearward even more.
     
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  8. shoobe01

    shoobe01 Member

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    This is well discussed in firearms engineering circles, military acceptance committees etc. They get it. A lot of things go into perceived recoil; louder guns recoil "more" to people, who will then shoot them worse. Yes, even if strapping sensors to them, same thing.

    For the specific discussion, it is about the recoil impulse over time as has been said various ways by various people here. Some of that is perceived, but plenty is actually reducing the momentary impulse as well; buffers, from elastomers in the back of a rifle to hydraulics in artillery, are performing the same function of spreading out the total force over time.

    This has been played with by people who draw blueprints so others cut steel. You can, if you buy several guns, try it. Closed cam paths are cut to be a specific shape, not a part of a circle. So if you drop a cammed barrel into a 1911 (they used to make them) the recoil impulse is totally different due to the cam shape compared to a swinging link, and a lot softer. The S&W 3rd/4th gen autoloaders had a few guns in the "TSW" line, that changed the angles of the ramp on the bottom of the barrel, which increased the dwell time (the time the chamber remains locked) and had a very noticeable impact on the felt recoil. Now, this was milliseconds, so the guns didn't shoot slower, but you can still feel the difference. I suspect the same lessons we're kept in mind when S&W did the Shield EZ line, which is how they get away with the minimal recoil spring; they run a hugely long dwell time to take out all the energy they can in initial extraction.

    Much fun to be had balancing systems like this.
     
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  9. Johnm1

    Johnm1 Member

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    I had been thinking about this earlier today. If the breached remaind locked for just a little longer than required for the bullet to exit the barrel, would that allow a little more pressure to be released before opening and have less 'force' remaining to open the slide? Thus one could use a lighter recoil spring or even hammer spring in hammer fired guns.
     
  10. JohnKSa

    JohnKSa Moderator Staff Member

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    Assuming that both guns fire the same bullet at the same velocity, the recoil will be the same since recoil is exclusively a function of the mass/velocity product of what comes out of the muzzle.

    Felt recoil is another story and is affected by a lot of things.

    Focusing just on one factor makes things simple. So assume two essentially identical guns, both firing the same cartridge from the same barrel length, one blowback, one a locked breech recoil operated design more or less based on Browning's design.

    Assume that the slide weight is about the same in both guns.

    Let's assume that the recoil spring force is about the same in both guns for simplicity.

    In the blowback gun, the recoiling mass consists of the slide. In the locked breech gun, the recoiling mass consists of the slide and barrel.

    You can find the velocity of the recoiling masses by taking the velocity of the bullet at the muzzle and the mass of the bullet and multiplying them together and then dividing that value by the mass of the slide.

    Since the recoiling mass in the blowback gun is less (only the slide instead of the slide and barrel) the slide will be moving FASTER in the blowback gun. Which means it will whack the frame with a good deal more force at the end of its travel.

    That's the first reason we would expect a blowback gun to have more felt recoil.

    The second factor is that the only recoiling part in the blowback gun is the slide and it is stopped all at once when it hits the frame at the end of travel. In the locked breech design, the recoiling mass is split up and stopped at two different points. The barrel unlocks from the slide and is stopped relatively early in the recoil cycle while the slide keeps moving--it goes all the way to the rear and is stopped at that point.
    The breech is opened by recoil, not by pressure.

    The recoil moves the slide/barrel combination to the rear together initially. They have to move together because they are locked together. As the combination moves to the rear, it reaches the point where the travel unlocks the barrel from the slide. That is done by a number of methods, a link, an inclined plane, a rotating barrel, etc. But the point is that pressure doesn't force the two apart, it's the movement of the combination to the rear that does it. You can see this is true by running the slide by hand--there's no pressure in the chamber, but still at some point in the travel, the barrel unlocks from the slide.
     
  11. Slamfire

    Slamfire Member

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    No doubt in some mechanisms, such as long recoil mechanisms, pressure is zero when the breech is unlocked. Long recoil mechanisms have a lot of mass, and are slow, so I suspect that the residual pressure curve is zero at unlock. The rotating cylinder and gatling gun mechanisms the pressure is surely zero at unlock.

    I do recommend all read Chin Vol IV The Machine Gun at Hyperwar https://www.ibiblio.org/hyperwar/USN/ref/MG/index.html Vol IV is the theory of operation for automatic mechanisms. The other books are more pretty pictures of historical mechanisms.

    https://www.ibiblio.org/hyperwar/USN/ref/MG/index.html

    In the blowback section LTC Chin shows that blowback, retarded blowback, gas operated mechanisms are designed to unlock when pressures are less than 650 psia or close to that. The pressure at unlock has to be less than the rupture strength of the cartridge case . The action is moving before then, but unlock occurs at this point for several reasons. One of which is to length the amount of time energy is available to function the mechanism, and also, to pop the case out of the chamber. Chin calls this the residual blowback affect.

    This is the Garand mechanism, and you can see if you follow the pressure curve and time, the mechanism is unlocking while there is still pressure in the barrel, and unlocks around 650 psia.

    oMRSvid.jpg

    This pressure drop is why powder choice is critical for gas operated, and even blow back actions, because powders with a high residual breech pressure at unlock will over accelerate the mechanism, have high case friction issues at unlock, which results in jams and torn case rims.
     
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  12. Jim Watson

    Jim Watson Member

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    When Mr Pedersen designed the Remington PA51 around Mr Browning's Colt patents, he established the unlocked travel of the internal breechblock at 0.18". Which is the thickness of the .380 case head. Not by coincidence, there is no unsupported case wall under significant pressure.

    The Tanfoglio 9mm FAR takes the idea a step farther. The gun is straight blowback. The cartridge case is the length of a 9x23 but the volume of a 9x19. The extremely thick case head is supposed to be all that is exposed in blowback before the bullet exits.
     
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  13. JohnKSa

    JohnKSa Moderator Staff Member

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    Just to be clear, I'm not saying that there's no pressure at the unlocking point in a Browing inspired recoil operated locked breech design, I'm just saying that the pressure isn't what causes the unlocking. The unlocking is caused by the movement of the slide/barrel combination in recoil.
     
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  14. Slamfire

    Slamfire Member

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    We need Newton to explain things. Surely F=MA is doing things in weird and wondrous ways. How the equal and opposite is transmitted through the mechanism would be interesting to understand. You know, the slide and barrel are actually moving before the bullet is out of the barrel. Saw the slow mo pictures.





    If someone has a pressure curve for the 1911 then we could definitely state that one way or another. I doubt John Browning had the equipment, or even Colt, measure the pressure curve and pistol unlock against time. I would be interested if anyone has an idea of how they might have done that in the pre transistor era. It would have been very clever.

    But, based on visual indications, such as the drag marks on these cases

    YOlW7nc.jpg

    And longer and more severe ejection patterns with slower burning powders such as Unique and Blue Dot, at the same velocity as Bullseye powder, I am going to claim, that the 1911 does open up when there is residual pressure in the barrel, and the pressure at unlock does affect slide velocity.

    But, without data, it is speculation.
     
    Last edited: Apr 19, 2021
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  15. Johnm1

    Johnm1 Member

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    This is great stuff and I'm learning a lot that applies to not only shooting and recoil but getting a better understanding of how the design of a firearm implies certain considerations in reloading. We all knew that a Garand required a certain pressure curve. I suspect that many loads for blowback or locked breach pistols need to at least consider the action type. Maybe not as important as bending a gas rod in a Garand, but at least for function.

     
  16. Johnm1

    Johnm1 Member

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    I make a lot of assumptions and that can lead to confusion. I understood that the recoil motion opened the breach. And I assumed that the timing of opening was just as the bullet existed the barrel when the pressure had not returned to zero. My question was if the opening was delayed for a little longer after the bullet exited the barrel if the pressure would drop enough to not require as strong of a return spring or increased mass of the slide. I wonder if this is the principle of the S&W EZ systems?
     
  17. Slamfire

    Slamfire Member

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    The guys who screw on silencers on their AR's are reporting changes in cycling. The silencer holds a volume of gas and releases it slowly. I forget exactly what their silencers are doing to their rifles, but it changes things.
     
  18. JohnKSa

    JohnKSa Moderator Staff Member

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    Correct. As soon as the bullet starts moving, so does the slide/barrel combination. Newton's laws demand it.
    I believe that you are correct that there is some residual pressure in the barrel at unlock. The design insures that it's not enough to cause any problems by the time the case is actually beginning to be extracted from the chamber to any significant extent.

    However, slide velocity at the point that the bullet exits the muzzle is determined completely by the momentum of the ejecta (the mass velocity product of everything that comes out of the muzzle). It is not affected by pressure at unlock.
    The pressure in the barrel/chamber drops extremely fast once the bullet exits because there's nothing to hold it in place any longer once the bullet isn't plugging the barrel. Delaying the opening any more than is required to allow the bullet to exit doesn't buy you anything. The slide velocity is determined by the momentum of the ejecta--the pressure doesn't have anything to do with it.
    ARs are operated by gas tapped from the barrel, not by recoil or blowback. Silencers keep the pressure higher longer and therefore change the amount/pressure of the gas that is tapped off to drive the bolt rearward and unlock the action.
     
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  19. Johnm1

    Johnm1 Member

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    That makes sense.
     
  20. Slamfire

    Slamfire Member

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    What we lack is a displacement versus time versus pressure curve, which is what I posted for the Garand gas system. I think that test was using an early 7.62 round.

    But this is a 30 Carbine pressure curve The inset shows the important pressure drop versus time used for the residual blow back affect on the 30 Cal M1 carbine.

    ajaLyaj.jpg
     
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  21. Slamfire

    Slamfire Member

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    I was under the belief that the 1911 was a retarded or delayed blowback, and I was searching for information to substantiate that opinion. Well, it seems the majority opinion is, the 1911 action is a locked breech, short recoil mechanism.
    These discussions can get pretty hot towards the end of the thread. :cuss:

    Short Recoil" Vs. 1911 Locked Breech

    Action type and physics

    I looked at Hatcher's Textbook of Revolvers and Pistols, and Melvin Johnson's Automatic Arms, both authors explained how the pistols worked, but were not concerned about definitions and categories.They avoided much ado about nothing.

    The 1911 does have a lot of mass in its slide, and that has the momentum to do the job of unlocking and ejection of the cartridge. But, based on my experiences with my 1911's during load development with Bullseye, Unique, and Blue Dot, its is evident that the slower burning powders cause a hard ejection with a long ejection distance. So, I interpret this as residual pressure in the barrel accelerating cartridge ejection, if not, adding acceleration to the slide. I don't have pressure gauges, but I do have a chronograph, and if the velocity is equivalent, and I tailor my 230 grain loads to 800 fps in a 5 inch 1911, then what else is going on, other than a slow pressure drop with slow powders?

    I am currently trying to slow down the slide of my RIA 1911 in 45ACP, and I am not going to claim that his pistol is soft in recoil, despite it being a locked breech, short recoil mechanism. I have a 22 lb recoil spring, a 25 pound mainspring, a flat bottomed firing pin stop, and this pistol is now tossing cases mostly at 4 O'clock, but some of the buggers are going 20 feet. I think the majority are within 10 feet of me, some hit the range roof and some cases still hit me!

    this has been the primary load, because I am trying to shoot up this 90's keg of AA#5

    FdliUXS.jpg

    w2d4YUL.jpg

    At 25 yards, yesterday, out of 28 rounds, less than half of the rounds were in the black. Most were on the repair center. This pistol is hard to shoot at distance. Might be the recoil, as I flinch.
     
  22. zaitcev

    zaitcev Member

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    I'm pretty sure the slides remained unchanged, but frame was was beefed up. Even so, it would not be enough where the slide was whacking them, so at least on the Beretta 84 the recoil spring was also made narrower. That permitted to make the opening smaller and add some aluminum there. This way the slide did not land right at the edge anymore, and mushrooming was reduced or eliminated. I am not intimately familiar with Tomcat, however.
     
  23. JohnKSa

    JohnKSa Moderator Staff Member

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    I can think of a couple of possibilities.

    1. The velocity of the slide/barrel is based on the momentum of the ejecta. That's not just the bullet, but also the gases resulting from the powder combustion. The gases are very light, but they are also traveling very fast. So, if one load uses a relatively small amount of powder while another uses more, that will increase the ejecta momentum and therefore the slide/barrel velocity, resulting in more violent ejection.

    2. The slide/barrel combination velocity at the moment of bullet exit, is determined by the momentum of the ejecta. After that, things can start to slow the slide down. If one load causes the case to expand more against the chamber and make extraction more difficult, then that will bleed off more slide velocity before ejection which could make a difference in the ejection distance of two loads with identical bullet/muzzle velocity profiles.

    The problem with the residual pressure accelerating cartridge ejection is that the bullet is LONG gone by the time the ejection takes place. Once the bullet exits the muzzle, the pressure drops to zero extremely fast. By design the unlocking happens only after the bullet is out of the bore which means by the time extraction really gets underway, the pressure should essentially be zero.

    The pressure curves for gas operated guns are kind of a red herring. The gas is tapped off while the bullet is still in the bore and the pressure is still significant--it has to be since the gas pressure is what works the action. In Browning's pistol design (and variants of it) the action is operated by momentum/recoil, not gas pressure and the design is specifically set up to insure that pressure is not a factor by the time the action unlocks and extraction begins.
     
  24. mjsdwash

    mjsdwash Member

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    I'm sure there are many good answers here, but I can sum it up.
    The longer the recoil force takes to fully transmit, the less peak force you will experience.
    An easy analogy is a fall arrest harness. A straight strap will almost kill you, a shock absorbing attachment will make the fall comfortable, and for the same reason.
    Another analogy is give zones in cars meant to slow a car in a collision.
     
  25. chicharrones

    chicharrones needs more ammo

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    Here are some pics of the 81 and 81BB slides.
    https://www.ar15.com/forums/Handguns/Beretta-Model-81-and-81BB-differences-photos/4-190649/

    And some pics of the Tomcat's slides.
    https://www.mtrcustomleather.com/whats-the-difference-between-the-wide-and-thin-version-of-the-beretta-tomcat-3032/
     
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