Gunsmithing 6.5CM AR10 Cycling Issues

[elmuzzlebreak]

Ok need some input.
Set up is Aero M5 receivers, 22” proof 6.5 CM, rifle+2 gas tube, superlative arms adjustable gas block, rifle buffer tube with rifle Ar10 buffer and spring, and JP BCG.
So the main issue is with the weapon not cycling. The bolt will not lock after firing the last round on a mag even when the SA gas block is open all the way. Also when opening the gas block I experience double feed malfunctions and he weapon not re cocking. A round will be ejected however when pulling the trigger back after reset the hammer will not fall. Shooting Hornady ELD 140gr and Prime 130gr.

Any input is appreciated.

 

[honuswagner]

Interesting... I just spent the last several weeks tuning a 6.5 CM in AR10 platform. I'm shooting suppressed, so problem was too much pressure.

I'm not the expert in AR builds, but my first thought is that you might need a lighter buffer spring. Hopefully some more experts will weigh in here.

 

[elmuzzlebreak]

I am leaning towards a lighter spring and keeping the rifle buffer.

 

[Ripdog28]

To clearify, Double feed (2live rounds) or a failure to eject with a live round pick up?

The failure to lock back, first check magazines, cheap fix.

Try a standard gas block. $45. In 3 gun, the vast majority of problems I see with cycling are guns with adjustable gas blocks.

The trigger not falling after reset, clean trigger assembly or take it out and reinstall. Likely if the trigger properly locks back but fails to fall it is a trigger issue. If it follows the bolt then it is the bolt not cycling all the way.

 

[LRI]

Direct gas operating systems are a function of pulse and dwell.

The pulse is how big of a squirt the bolt carrier sees from the gas tube and the dwell is how long it lasts. The gas key doesn't propel the bolt rearward like a bottle rocket. The work is actually done down inside the bolt carrier group. Gas rings are piston rings and the bolt and bolt carrier literally try to blow apart from each other. That 1/2" or so of travel is all it takes to accelerate the bolt carrier to the rear, extracting the case, cocking the hammer, and peeling a new one off the magazine as it travels forward again.

Kinda' impressive in how they work.

So, if you stuff is failing to lock back on the last round, start looking at these things:

Hole size in barrel (pulse size)
Gas block (pulse mostly, adjustable? If so, open it up!)
Distance from gas block to muzzle (this influences the dwell as a "shorter" barrel from gas block to muzzle effectively reduces the time that the gas system stays pressurized)
Bullet weight (This also effects dwell cause heavy stuff moves slower)
Charge weight (pulse size)

One way to easily check: If you have a can or access to one, thread it onto the muzzle and go shoot it. Back pressure increases significantly and often times this will make a lethargic gun start functioning properly. If it does, you either need a hotter load, heavier bullets, or work on the gas port.

Worn out gas rings can also create this symptom, but unless this rifle has a TON of rounds through it, I'd bet your fine. The way to check:

Pull the bolt carrier out of the upper. Extent the bolt as far as it'll go. (out of battery position) Now, set it on a table. The resistance in the gas rings should overcome the weight of the bolt carrier so that it does not collapse. If it does, they are shot. Replace them. If it supports it, make a fist and bang the table hard. (hammer fist) The shock should collapse the bolt carrier and close it up.

Hope this helps.

C.

 

[lima]

What is the actual diameter of the gas port in the barrel?