When does recoil start?

Cool posting on “when does recoil start?”

When Does the Pistol Slide Start to Move?

It has some pretty good gif/animation and explanations.

Short version: once the bullet starts moving, Newton’s laws kick in. I think one of the most fascinating part of the animations is the smoke spurts; the 1911 has some at the breech, too, but they all have smoke exiting the barrel before the bullet. Cool stuff.


13 thoughts on “When does recoil start?

  1. Cool stuff!

    Yes, well of course the laws of nature are always in effect, and so they don’t, technically speaking, have to “klick in” being as they never took a break.

    An example of the same issue is that if you look at most any handgun closely, but mostly any revolver (because there’s no slide to temporarily decouple recoil forces from the grip frame), you’ll see that the sights have “negative elevation” built in– The front sight sits higher over the bore than the rear sight, so the bore is pointing below the point of aim. The barrel climbs enough under recoil, as the whole gun pivots during launch, that the actual launch angle is higher by the time the bullet exits and so it hits POA. This is most apparent on revolvers or single shots regulated for heavy bullets at moderate velocities, like a 45 Colt or etc. It’s probably less apparent on an auto because the slide initially recoils more straight back, with only the recoil spring coupling the force to your hands through the frame prior to unlocking. By the time the barrel hits the unlocking bits and stops hard in the frame, the bullet is gone.

    Notice too how much faster the shock wave travels out from the muzzle upon “uncorking”, compared to the bullet. Ouch, my ears.

  2. I’ve never seen a breech spurt like that before, not even on other slo-mo videos of 1911s. Is that a bad example, or have I not paid close enough attention?

    • The 45 is a low-pressure round, comparatively speaking. The expanding brass normally seals the breach, but if you see soot around the neck of the brass, that’s because the pressure is low enough to not expand the brass against the steel of the chamber yet. If the soot goes all the way back down the brass, then some of it will escape behind the brass. A light target load is a lot more likely to do that than a full power “serious” load. It’s present on the Glock in one shot on the video down the page, too, but much less.

      • Heh. That’s kind of counter-intuitive, that low-pressure rounds don’t seal as well as high-pressure rounds. Thanks for the explanation!

      • I’ve had 9 mm spit back at me like that. I blamed it on the (cheap) ammo, which had crummy tolerances (some undersize, some oversize, when checked against SAAMI specs and my gauge). But the low pressure point is interesting. Thanks.

    • It happens even in much higher pressure rifle rounds, to some extent, at least initially. Also there is some gas leakage past the primer.

      As I’ve had to explain to many a customer; there’s gas blowing out of places you might not imagine. If you look closely after a shooting session, you’ll probably find powder fouling on the barrel where gas is leaking out between the gas block and the barrel on your AR or AK too. It blows out around the AR gas tube at the gas block too, and all out from the lower handguard and up through the rear sight block in an AK, and even back through the takedown button hole in the dust cover into your face, and through past the AR charging handle into your face. And your bolt face will likely eventually exhibit gas cutting around the area of the primer, depending on the loads you use.

      You can just barely see a gas plume exiting straight up out of the Glock too. They all do it, whether you can see it in the video or not.

      To say that the primer cup expands under pressure and “seals” gas in the case, or that the cartridge case expands and “seals” gas in the chamber is a relative term. It’s never 100 percent, and even then it takes some time for the pressure to build up enough to do that expanding.

      That primer is forced back too, partially “de capping” the case, until the case head is forced back breech face under higher pressure to reseat the primer. We know that because under-pressure loads will often leave the primer sticking out beyond the case head (called “primer set-back”) and standard pressure loads will leave the primer flush with the case head. I once tried wax bullets in my revolver as a “gallery load”, propelled only by the primer, but the pressure was so low that the primers set back so far they tied up the rotation of the cylinder. Maybe it’s counter-intuitive at first, but it happens just thata way. Point being that, depending on headspace and upon whether the gun has a plunger ejector or not, the primer may be getting un-seated up near to half way or so, for some portion of a millisecond or so, until the pressure rises enough to tighten things up. I bet that if you set off a primer in an empty case in free space, say by using a flame to cook it off, it would probably fly free of the case altogether, depending on the fit (some primers seat a lot harder in some cases than other primers in other cases).

      Crazy stuff goin’ on in there.

      Crap; now it’s late. Thanks a lot, assholes, for getting me interested in gun esoterica.

      • You are welcome 🙂

        Thank you for your added comments, too; much better than the comments left on the other website. Yes, I’ve seen a variety of effects on primer depth after firing that seem odd, or counter-intuitive.

        I wonder if there is a similar video out there on a flintlock or traditional side-lock percussion-cap gun going off for comparison; likely seem glacially slow, and even more messy.

      • That bit about primer setback in low power loads is interesting. I’ve seen recommendations to enlarge the primer flash hole in cases to be used for wax bullets or rubber practice bullets propelled only by the primer. That makes sense as a way to limit the amount of pressure buildup at the primer and allow the primer gasses to flow more quickly into the main case to propel the bullet.

  3. Oleg Volk had a photo from several years back of, I think it was an MP-5 or similar, being fired. It showed jets of gas coming out through the witness holes in the magazine.

  4. I didn’t see much if anything in the way of explanation in that article. And the commenters were rather confused, wondering if +P would act fast enough to prevent this phenomenon. Well, no.
    Something occurred to me: depending on the action, the barrel will tilt while the bullet is still traveling through it. The 1911 does this, for example. Glock only once the barrel engages the unlock block, I think. And some actions, like the rotary one (Boberg and some Berettas) don’t do any tilting. That tilting would have an effect on point of aim similar to the barrel flip of revolvers, I would think.

  5. The videos illustrate the principle of Conservation of Momentum.

    Momentum is mass times velocity.

    Just before firing, the momentum of the whole firearm, including all its parts, and the ammunition, is zero, as nothing is moving, so the velocity of every part of it is zero. Then the trigger is pulled, the hammer falls, and the primer is dented. Still it is easy to understand that the total momentum is zero, as nothing is moving at that instant. Then it gets more complicated, but the total momentum remains zero at all times.

    The powder begins to burn, turning into a gas at an increasing pressure. The gas tries to expand, and in doing so it pushes the bullet and the breech face away from each other. The bullet begins to move down the barrel, and the barrel begins to move the opposite direction.

    Remember, velocity is a vector; it has a magnitude and a direction. If we call velocity in the direction the bullet is going positive and velocity in the opposite direction negative, then the momentum of the bullet and most of the powder gas is positive, while the momentum of the gun and a small bit of the powder gas is negative. The total momentum, which is the sum of the momentum of the powder, gas, and gun, is still zero. The total momentum is conserved.

    Think of the computation: (momentum of bullet and gas) + (momentum of gun) = zero. The momentum of the bullet and gas is a low mass times a high velocity, while the momentum of the gun is a high mass times a low velocity, and it is in the opposite direction. Numerically, they are equal in magnitude but opposite in sign, such that the sum of the two is zero.

    Think of the physics: The gun begins moving rearward the instant the bullet and gas begin moving forward. It has to be this way to conserve the total momentum at all times.

    This helps understand why a heavier gun, other things being equal, has less “felt recoil” than a lighter gun. Its mass is greater, therefore its recoil velocity is less.

    • Well said.
      I’m thinking about the question on the other website, whether using +P ammo would change the result. I could write the equations and figure it out, but off hand I think the distance over which the slide moves back (for a given gun) depends only on the bullet mass, not the firing pressure (projectile velocity). So for a given bullet mass, regular or +P affects how fast things move (the bullet as well as the slide) but not how far the slide moves before bullet exit. On the other hand, a lighter round will leave faster, so perturbations caused by recoil (muzzle flip, barrel tilt) would be reduced.

      • Ok, this is trivial. Assuming other momentum terms such as escaped gases are not significant, slide velocity (magnitude) is bullet velocity * bullet mass / slide mass. Distance traveled is integral of velocity dt, and constants go outside the integral, so the distance traveled by the slide up to bullet exit is barrel length * bullet mass / slide mass.
        Bullet velocity doesn’t figure in that answer, so the answer to the original question is: if your +P is loaded with the same weight bullet as your target load, the slide will travel the same distance before the bullet leaves. (Afterwards, probably a bit more with +P since the mass of propellant gases is larger.)

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