Physics related Q regarding bolt travel distance

[wtturn]

There is a lot of debate among AR-9 users about shortening versus lengthening the bolt travel and its effects on recoil impulse and dot bounce.

A lot of people shorten travel by putting spacers in the buffer tube or using extended-length buffers (i.e. a VLTOR A5 buffer in a carbine tube). I understand some do this so as to not break mag catches. For purposes of this discussion, let's ignore LRBHO and the mag catch issue and focus solely on recoil effects.

Some swear shortening the cycle returns the bolt to battery sooner and thus dampens the recoil impulse. This seems paradoxical to me, since the mass of the bolt is imparted with more energy over a shorter distance at the point the buffer bottoms out in the tube. Wouldn't that cause the felt recoil at the rear and the force of the bolt returning to battery be much greater?

Would it not be desirable to make the travel distance of the bolt as long as possible to spread out the impulse and let the spring help decelerate the bolt? Interested to hear about others' experiences and maybe even some math/theory because I am no physicist.

[StealthyBlagga]

After running some shooting tests, I found myself unexpectedly in the "shorter=better" camp. I posted the test data in the rifle forum a few months back - search using my screen name to find it. Maybe I should ask a mod to move it here. 

I am not a physicist, but am a scientist with training in statistical methods. 

Edited December 6, 2016 by StealthyBlagga

[wtturn]

I have read that post of yours several times and found it very helpful.

Is the reason one of my considerations is to use the A5 buffer.

Sent from my MotoG3 using Tapatalk

[PHX]

This is all assumption since I dont have my rifle yet to test with.  I think you have the right idea about a longer length of travel while the bolt is moving back into the buffer tube.  That should help decelerate the bolt and soften the felt recoil.  however you will then have a longer length of travel for the bolt to go back to its forward position.  This could increase muzzle dip/dot movement since the bolt will have more time to speed up.  My plan is to find a happy medium between the two.    I have read multiple times that shortening the bolt stroke with spacers will make the rifle feel like it is recoiling harder, but will not impart as much dot movement.  That leads me to believe the bolt is slamming to the rear with more force (hard felt recoil), and then going back forward with not as much momentum so it doesn't impart dot movement.  I will definitely be trying this out next weekend when I have my rifle in hand.  Im thinking of using cork to help soften the blow of the bolt.  I think it would be better than metal on metal.  If it falls apart which I think it probably will, Ill try rubber or delrin.  

[Bamboo]

I installed an NFA buffer spacer which limits the stroke (to about 1/4" past the mag catch) and doesn't increase the buffer spring tension like using stacked quarters or other spacers.   It is made of plastic and costs about seven bucks.  So far it looks to be pretty durable.  I felt it made a positive improvement in the dot bounce.   

[2011BLDR]

Keep in mind that the A5 doesn't function the same as a standard buffer their is a lot more going on than just cycle length with the A5:

1. Deceleration of the weights in recoil

2. Unloading of weights in closing/

3. Elimination of carrier bounce

[MikeRush]

On 12/5/2016 at 2:33 PM, wtturn said:

Some swear shortening the cycle returns the bolt to battery sooner and thus dampens the recoil impulse. This seems paradoxical to me, since the mass of the bolt is imparted with more energy over a shorter distance at the point the buffer bottoms out in the tube. Wouldn't that cause the felt recoil at the rear and the force of the bolt returning to battery be much greater?

Would it not be desirable to make the travel distance of the bolt as long as possible to spread out the impulse and let the spring help decelerate the bolt? Interested to hear about others' experiences and maybe even some math/theory because I am no physicist.

Shortening the stroke does return the bolt to battery sooner but does not "dampen" the recoil impulse. If anything the gun feels more harsh as it does reach the end of its travel with more remaining velocity. The felt recoil is higher, but the bolt returns to battery with less velocity the shorter the stroke gets. It works both ways.

Lots of people try to set things up as "soft" as possible. This isn't always the fastest setup, although it can be. Whatever produces the best splits and hits for you and your style is what you want to run. Fortunately it is pretty easy to throw some quarters in the back of a buffer tube at the range and run a timer.

For physics check out Dmetrieff's Submachine Gun Designer's Handbook.