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Feb
20

Flatlining Your Gun

This is the fourth article in a series of posts talking about “flat”. If you haven’t read the others, you might want to do that first. Where we left off last time, we learned what properties about the sighting systems on the gun can affect your ability to call shots, and what we can’t do about it – speed up how fast our eyes actually see. If we can’t speed up our eyes, perhaps can we slow down the gun? Or, more properly – slow down the movement of the sights? Ah… now we’re on to something.

The question is, what are the factors that affect how quickly the gun (and thus the sights) move away from the spot they’re pointed. For the purposes of this post, I’m going to assume we’re talking about a 1911 pattern handgun, but the discussion applies to others, as well, to a greater or lesser degree.

Remember why the gun actually flips? The barrel and the other moving bits sit above the hand, and so the forces pushing toward the shooting also tend to have leverage to rotate the hand upward, thus causing what we see as flip. But, the thing is, that motion doesn’t really occur until after the bullet has left the barrel. While the bullet is in the barrel, it’s actually pulling the barrel forward – this is what keeps the gun locked up until the bullet has left and the pressure has dropped enough in the barrel to make it safe to unlock the barrel. The whole system is effectively static while the bullet travels down the barrel, though the slide actually does start to move just a very small amount rearward.

What happens right after the bullet leaves the barrel and the slide starts to move rearward is that the firing pin stop contacts the hammer, and here we find our first resistance to the rearward travel of the slide, and therefore the first point that the gun really starts to move upward. There’s an interaction here between the geometry of the lower rear corner of the firing pin stop and the hammer, which is being pressed against the firing pin stop by the tension of the mainspring. The sharper the corner on the firing pin stop, the less leverage the slide has against the force of the mainspring, and therefore, the more the gun tends to move upward in this phase. If we can both increase the leverage the slide has against the hammer and mainspring, and decrease the force the mainspring is providing to press the hammer against the slide, we can keep the gun a bitter flatter in this earliest phase of recoil. The trade off is twofold. Decreasing the weight of the mainspring can risk inducing light strikes as the hammer doesn’t strike the firing pin hard enough to consistently ignite primers. Changing either property also decreases the amount of energy required to cock the gun, and therefore leaves the slide with more velocity. This could potentially lead to accelerated wear, and can lead to more flip later in the recoil cycle. There’s a balance – and it depends somewhat on the weight of the slide, the weight of the recoil spring, whether or not it’s a compensated gun, etc. You probably don’t want to monkey around too much with these things without understanding the potential side effects. But, the end result is, the gun will appear to stay flatter very early in the recoil cycle if the slide can unlock relatively easily.

The next thing that starts providing resistance to the slide’s rearward movement is the recoil spring. Essentially, what the spring is doing is robbing energy from the slide as it moves rearward and storing it up so that it can be returned to the slide to move it back forward. In the process, though, the spring couples the slide to the frame to some degree, and also causes the gun to flip. The heavier the recoil spring weight, the larger the degree of push into the frame and the larger the degree of flip in this part of the cycle. Reduce the weight of the spring, and the gun will appear to flip less during the early part of the recoil cycle. There are tradeoffs here, too, though. The spring needs to store enough energy to close the gun fully every time, even if the gun gets dirty. Go too light, and you’ll have failures to feed or failures to go into battery. Also, the lighter spring leaves more energy with the slide, too, much like the changed firing pin geometry and lighter mainspring, and it has the same net effect – that energy has to go somewhere, and it will… later in the recoil cycle.

The final place flip shows up can’t be helped much. This happens when the slide smacks into the frame (or, more correctly, the guide rod head). Any energy remaining in the slide is firmly applied to the frame, and results in a sudden, sharp flip and a smack in the palm. So, you can see the effects of the tradeoff we made earlier – with regards to flip, what we’re really doing is just moving where it occurs in the recoil cycle. It can be a relatively smooth lift with a bump up at the end, or it can be a relatively flat travel with a sharp bump up at the end, or various degrees in between. It’s easier to see with an Open gun, but you can actually see two different phases to recoil in the sight movement – all the stuff leading up to the bump at the end, and then the bump itself.

As you can see, changing the firing pin stop geometry and lowering the weight of the recoil spring both reduce the amount the gun rises from the point of aim. The slower the gun moves away from point of aim, the more clearly your eyes can pick up where the sights are as they start to move. Guns that lift more slowly in the that first phase of recoil are what we perceive as shooting “flat”. So, here – finally – we arrive at an important conclusion. Flatter shooting guns make it easier to call shots more accurately.

Next up – some Open gun specifics!

About the author

DaveRe

USPSA Grand Master, NRA Instructor, http://re-gun.com/about/

Permanent link to this article: http://re-gun.com/2012/02/flatlining-your-gun/

2 comments

  1. Bruce Braxton says:

    I call those phases of recoil “primary, secondary, and tertiary recoil”.

    Primary = recoil from the acceleration of a bullet down the barrel. Newton’s Third, plain and simple. Variables include powder burn rates, bullet weights, and adjustment of Power Factor.

    Secondary = the rocket effect from gases following the bullet out of the muzzle. Newton’s Third again, but can be adjusted via powder selection (burn rates and volume) and compensators/muzzle brakes.

    Tertiary = the bump when the recoil spring contacts itself, thereby becoming solid. Controlled by tuning spring weights to adjust slide velocity when it hits that point or adjusting that impact point (i.e., the Dawson “Stroker” guns).

    1. DaveRe says:

      Bruce,

      Those aren’t bad terms. In the end, though, what you call primary and secondary start taking effect at the same time. Actually, if you think about it a bit, you’ll realize that it’s actually the gas that causes recoil (it’s the source of the “push”). But, neither one is apparent in the system until it’s all uncorked. This is because the force required to push the bullet down the barrel is working on the bullet (which pulls the barrel forward) and the slide (via gas pressing into the case against the breechface) equally until the bullet clears the rifling and stops pulling on the barrel. This is also what lets the gun actually unlock – it’s an ingenious little insurance policy against the gun unlocking while pressure is too high.

      The other factor you’re leaving out is how the weight of the recoil spring affects muzzle rise before the tertiary part you describe. If you shot the gun without a recoil spring, it would basically not move upward until the slide bangs into the frame (I say “basically”, cause we don’t have a frictionless system). This is because there’s nothing to provide force against the frame until the slide hits it. The recoil spring couples the slide to the frame during the recoil cycle and is what allows the slide to transmit force against the hand and cause flip before the slide hits the frame.

      Hopefully, you’ve got a bit of a typo there – if the recoil spring is going solid, you’re going to break stuff in the gun! :) The recoil spring should still have just a tiny amount of space left between the coils when the recoil spring plug touches the guiderod head, and should never go solid.

      Dave

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