What makes a soft shooting AR?

[ummagawd]

I've read countless threads on multiple forums about this, but am having a hard time putting all the factors together.

The general consensus with the 3-gun folks (who arguably have a fair experience on how to build such soft shooting rifles) is that you get an 18" barrel with rifle length gas, and then lighten up your operating system and buffer.

The logic behind a lighter operating system makes sense to me so we can save that discussion.

Lets start with the barrel:
If we were to negate the gas system completely, does barrel length by itself contribute to felt-recoil? For example, if we had a 14.5" inch barrel and an 18" barrel where both did not have a gas port.... which would shoot "softer"? Would the difference be marginal?

Gas system
So yea, the rifle length is the "softest" of the group. But is it the dwell time (length of bullet travel from port to muzzle tip)? Or is it the length of the tube that the gas has to travel? Or is it the volume of gas flowing through (i know it gets tricky here considering tube diameter and gas port size come into play).

I ask this because lets say that the barrel length from gas port to muzzle tip on a 14.5" middy barrel = 5.5" (not sure if this is the exact measurement... but stay with me here). And for an 18" rifle length it is 6".

Would this mean that the 14.5" middy be softer due to less dwell time? Or if anything... shoot very very similar to the 18" rifle?

I'm sure the answer is that all of these items working in concert with each other to make up the shooting characteristics but I guess i just want to understand better.

I'd also appreciate if folks would refrain from the "these things would not be noticed by the average consumer" remarks.

Appreciate the input guys...

[Eric D.]

There are two stages of recoil: 1. the impulse created by the exploding powder acting on the bullet. 2. The BCG expending any left over kinetic energy into your body when it reaches the rear of the buffer tube. Having said that, barrel length does not affect recoil in your hypothetical example. Barrel length does, however have an indirect effect on recoil based on how much energy is given to the BCG if that makes sense

Others, correct me if I'm wrong but I remember reading that a midlength gas system on a 16" bbl has the same dwell time as a rifle length gas system on a 20" bbl. Everything you mentioned comes into play: gas tube volume, gas port size etc. Think of the gas port as a regulator: small hole means high pressure, low volume. Big hole means low pressure, high volume. Because gasses are compressible, they will not flow through a nozzle faster than mach 1 - because of this, a small gas port will have higher pressure right at the gas port and it will take longer for the entire system to reach peak pressure. A larger gas port will have less pressure, again directly at the gas port, and the entire system will reach peak pressure sooner- the reason sbr's have larger gas ports than mid or rifle length gas systems.

To wrap up, there are two things that reduce recoil.
1. Lighter bullets. The impulse stage of recoil is pretty much inevitable.
2.Finely tuned gas system/buffer/spring combination. The idea is for the BCG to have a velocity of 0, thus 0 kinetic energy when it reaches the rear of the buffer tube.


I've read countless threads on multiple forums about this, but am having a hard time putting all the factors together.

The general consensus with the 3-gun folks (who arguably have a fair experience on how to build such soft shooting rifles) is that you get an 18" barrel with rifle length gas, and then lighten up your operating system and buffer.

The logic behind a lighter operating system makes sense to me so we can save that discussion.

Lets start with the barrel:
If we were to negate the gas system completely, does barrel length by itself contribute to felt-recoil? For example, if we had a 14.5" inch barrel and an 18" barrel where both did not have a gas port.... which would shoot "softer"? Would the difference be marginal?

Gas system
So yea, the rifle length is the "softest" of the group. But is it the dwell time (length of bullet travel from port to muzzle tip)? Or is it the length of the tube that the gas has to travel? Or is it the volume of gas flowing through (i know it gets tricky here considering tube diameter and gas port size come into play).

I ask this because lets say that the barrel length from gas port to muzzle tip on a 14.5" middy barrel = 5.5" (not sure if this is the exact measurement... but stay with me here). And for an 18" rifle length it is 6".

Would this mean that the 14.5" middy be softer due to less dwell time? Or if anything... shoot very very similar to the 18" rifle?

I'm sure the answer is that all of these items working in concert with each other to make up the shooting characteristics but I guess i just want to understand better.

I'd also appreciate if folks would refrain from the "these things would not be noticed by the average consumer" remarks.

Appreciate the input guys...