Gunsmithing Tuning gas on an AR10

[360moa]

I'm wondering if there is any scientific (or failing that, ad hoc) guidance on turning the gas system of an AR10 (or AR15). The one I'm working on is a 6.5CM, but in general there is an interaction between the mass of the bolt and carrier, the buffer mass, and the amount of gas allowed back to the bold carrier. For example, on this rifle I have the JP silent spring system which allows me to change the buffer spring weight as well as the mass of the buffer assembly, and I have an adjustable gas block which allows me to turn the gas down to zero or up to "max" (whatever psi that is).

Some questions I have are: Is it better to add weight to the carrier, add weight to the buffer, or adjust gas flow? Should I go with more gas and heavier moving components, or less gas and lighter components?

Some people say heavier moving components increases lock time, which I think would be a good thing. Others say lighter components means you can run less gas pressure, which is somehow better in their view. I'm simply wondering if anyone has studied this scientifically rather than relying on shade-tree wisdom.

Also note I know I can (and already have) tuned the gas to just enough to cycle the bolt with a given load. But that is not my question.

 

[LRI]

How I've done it and its worked well for me:

Start with really small hole about 8 inches or so back from muzzle. Use a load (charge and bullet weight) that you plan to run. If using a can, have it installed as the back pressure increases a bit and makes the cycling a bit more "hyperactive".
Shoot it one time, single bullet in magazine. If the bolt fails to lock back, increase the hole size. SMALL amounts. Direct gas operating systems are twitchy. They work in a relatively narrow range. Once the bolt locks back reliably, your set.

Go too far and case rims/heads start to take a beating. It can be a tedious process getting this nailed down so be patient.

 

[kingzero]

I pay attention to the condition of the brass. Adjust the gas till the rifle is working reliably and check the brass. If the brass is getting beat up, add weight/ increase spring rate to help keep the bolt closed a little longer to let the brass shrink back down before extraction. Rinse and repeat till the brass looks good and the rifle runs reliably. Ejection should be around 3 o'clock. There's probably a better way but thats how I do it.

 

[Red_SC]

I do it basically the same way as Chad, although I do it without suppressor installed. I just did one yesterday, I started about .006 under where I expected to end up and increased by .002 until it locked back on an empty mag.

As far as bolt carrier/buffer weight balance, I use judgement with the application. This one was a 22" 6.5 Creedmoor with +2 gas, so my preference was a JP LMOS and SCS. The 18" .308 I did a week ago, I used full weight components.

 

[Eagle1899]

I've done these a few times. Changing the gas hole size is great if you only run one load with or without a can all time.

I prefer a gas bleed block as opposed to a gas restriction block. My current favorite is from Superlative Arms. I also run a heavy stainless steel buffer.

Tuning can be interesting, as Chad said the window is narrow. But the process remains the same.


On a related subject I run an AR15 for competition. 18" Rifle gas barrel, PVA Mad Scientist Brake, Superlative Arms Gas Bleed block, AIM Surplus lightweight carrier and a solid delrin carbine pattern buffer. The recoil is incredibly light and it has been 100% reliable. I would love to try a similar set-up on an AR10/308 style rifle.

 

[360moa]

This process worked well for me on 2 rifles (one AR10 , one AR15) but I'm having trouble on a third (another AR15). The bolt will not lock back even with gas adjusted to maximum (and even with the gas block adjustment screw completely removed). I've rechecked the gas hole alignment in the barrel to the block, it is lined up. A borescope confirms alignment there. The system has enough gas to eject the fired round but does not lock back on the empty mag.

I'm now looking at the buffer spring. I think it is possible the spring is out of spec (rate to high or possible a bit too long --10-3/4" in a carbine tube). Unfortunately I didn't have any spare springs to swap in quickly but have some coming from Brownells.

 

[dakewlndn]

Slash's AR10 Heavy Buffer took all my cycling issues away (which I only had one every 50 rnds or so)
Now I haven't had any issues on both of my 308 and 6.5 builds

Google them, they have several different options. Great company