The science of recoil

[1911Tuner]

Until the bullet is moving, there is obviously no recoil going on.

Obviously.. No argument there.

What you haven't thought about is that the bullet has to travel a small distance before it leaves the case. Essentially, the case itself is a "gun barrel" of sorts...smoothbore, of course...and the case head makes up a "breechblock" for that smooth barrel. So, a metallic cartridge essentially makes it a "gun within a gun."


And...If there's a little headspace, the case starts to move rearward in "recoil" before the recoil force acts on the true breechblock...but a few thousandths of an inch can pretty much be disregarded. I doubt if the time could be measured without some very expensive and sensitive equipment. This is why rifle cases manage to stretch when the pressure nails the forward section to the chamber walls.

 

[Matt304]

If you look at my graph, that shows you directly how long the energy takes to be applied. There is no delay anywhere in reaction; it is instantaneous. The force profile looks theoretically exactly like that. The recoil accumulation represents a direct relation to surface area of that graph at any given time.

Imagine that you drew a line straight down from the peak of that graph. On the left of the line, compare the graph area to the area on the right side. Already about 40% of total energy has been released by the time the highest pressure peak is reached.

Assume that the total recoil velocity of your firearm reaches 10fps from each shot you fire. Based on my prediction, this would tell you that if we stopped time on that line I mentioned earlier, the gun would already be moving 4fps in your hand as peak pressure is occurring.

Right along the lines of what 1911Tuner was saying, soon we have a gun which is nearly at full recoil as the bullet is just beginning its journey down the barrel. The whole remainder of the time the bullet spends traveling down the barrel, it is riding inside of a gun which is already traveling at near its full recoil speed.

The moral here is that a proper and consistent hold is everything.

But there's more. The good side to this is that as the gun is at its full recoil velocity, our hands have much less of an influence on it. And at around 1 millisecond elapsed time, the barrel doesn't get to pivot very far off course. The hotter the load energy, the more of an influence it will have on the shot, as there will be more off-center velocity and gun movement in the same given timeframe. Hold and technique is still very much important at all times, just remember that. The final deceleration over a much longer time period is what allows for the gun to continue its headed course and finally kick up.

 

[1911Tuner]

There is no delay anywhere in reaction; it is instantaneous.

.

Absolutely...and you'd be surprised at the number of people that I've fought with over the fact that action and reaction begin at the same instant...that the gun is in recoil before the bullet leaves the barrel. I even had one guy insist that the force vector "splits" with half of it pushing the bullet...then after the bullet exits...the other half starts pushing on the breechblock. In the same breath, he claimed to have had two years of college physics. I was laughin' so hard that I couldn't see to type.

Ah well. Onward...

 

[Matt304]

I even had one guy insist that the force vector "splits" with half of it pushing the bullet...then after the bullet exits...the other half starts pushing on the breechblock.

Oh boy, it looks like you've run into a real prodigy.

Yes, I myself have noticed many arguments about recoil physics in which people claim that there is no way the gun moves by the time the bullet leaves the barrel. I guess if you aren't inclined with physics whatsoever, it can be an easy incorrect assumption to make. So long as they accept the right answers and move on, no harm done. But this is definitely the way it works.

On a more interesting notion, this does make you think about the effect of barrels which are too long, and if they may have some negative effect on POI due to an unbalanced gun design. Revolvers come to mind. I suspect the amount of time may be too short to have much effect for a few extra inches, since they would be adding mere 1/10000s of a second, but it does make you wonder.

 

[DaveBeal]

Absolutely...and you'd be surprised at the number of people that I've fought with over the fact that action and reaction begin at the same instant...

You bet. Momentum is conserved at all times. Before you pull the trigger, nothing is moving. But as soon as the bullet starts to move forward, something must move backward. At first it may just be the case. Then it's the slide, then it's the frame and your arm. Eventually, the whole earth recoils. But not by much.

 

[1911Tuner]

Eventually, the whole earth recoils. But not by much.

That's a stretch...

Actually, "it" doesn't have to move. In order for both objects in an action/reaction pair to accelerate into motion...the force applied must be sufficient to overcome the resistance offered.

If you push on a house, you can't move it...but that doesn't mean you didn't apply a compelling force. It just means that the force wasn't large enough. The house "felt" acceleration, but its mass was too great. If you push hard enough to make yourself move...technically, there is an action/reaction event.

It's a little like the tree falling with no one around to hear it question. If you place the butt of a rifle against a tree and fire it...does the fact that the rifle doesn't move backward mean that it didn't kick?

 

[DaveBeal]

That's a stretch...

Actually, "it" doesn't have to move.

I admit that it's entirely theoretical, but if you could attach a gun with absolute rigidity to the earth and fire it, the earth would move a bit. It has to, because there was no net momentum in the earth/gun system before you pulled the trigger, and there has to be none after. Since the gun is fixed and can't recoil by itself, the earth has to.

In your house example, if I lean against a wall of my house and suddenly push with enough force to throw myself backward, something has to move forward. Probably just a portion of the wall will move by flexing, but theoretically it could be the whole house if it is rigid enough.

 

[1911Tuner]

Well...I guess hitting a gnat would alter the speed of a F118 at full throttle if ya get down to brass tacks.

 

[Walkalong]

The earth would absorb the energy, just like the brick wall of the house, but the Theory is OK.

 

[Matt304]

Getting a little astray, but what the heck. It's fun to think about.

In theory, if the world was outside of an orbit and just floating out in space at a complete rest, and there was complete equilibrium of all other forces on Earth, a gunshot could put the planet into motion. It would start moving at a rate so slow it could nearly never be measured, but it would have to begin a motion at some extremely low speed.

Since we are being pulled upon by the sun's enormous gravity, nothing on earth could stand a chance at changing our path.

 

[1911Tuner]

Getting a little astray, but what the heck.

Okaaaayyyy. Since we've literally gone into orbit, it might be worth mentioning at this point that while weight and mass are related...they're not the same.

In the vast frontier of space, outside the gravitational pull of another body, a baseball and a mile-wide asteroid weigh exactly the same. Nothing.

BUT! You can easily accelerate a baseball with one hand in space...but not the asteroid. Mass is still mass.