AR10 Light Primer strikes

[chindogg]

My buddy just put together an AR10 DPMS build. I went North to the land of liberty and hit his range with him to shoot it for the first time. First round went off great. After that no bang-light primer strikes. Hand cycling the BCG it would work, follow up rounds not. He is a little older and not so internet savvy so I often help him in situations like this. From what I gather it is most likely a buffer or buffer spring issue. Any experience out there?

 

[GM-GUY]

Does he have an adjustable gas block?

Where are the rounds ejecting to? 1 O’clock, 3 O’clock or 5 o’clock?

How are the rounds ejecting? Dribbling out, next County over, damaged?

Strip down the BCG completely, lube it, give the chamber a good brushing too.

 

[In God We Trust]

Is he using reloads? Cheap ammo? If the first round goes off fine but the follow up shots don’t, could it be a cycling problem?

 

[92G]

The description is unclear. “Light strike“ implies no ignition despite the firing mechanism being cocked and deployed. however it sounds like the rifle required hand cycling to function? Honestly that could be a very large # of issues. Without more info it’s impossible to provide any type of diagnostic.

 

[citoriguy]

What weight buffer were you and your friend using? Length of buffer tube (rifle v. carbine)?

 

[chindogg]

Does he have an adjustable gas block?

Where are the rounds ejecting to? 1 O’clock, 3 O’clock or 5 o’clock?

How are the rounds ejecting? Dribbling out, next County over, damaged?

Strip down the BCG completely, lube it, give the chamber a good brushing too.

Ejecting to 3 O'clock. No ajustable gas block, Casings were scratched with a line but nothing crazy. It was lubed well prior and re lubed during to no avail.

What weight buffer were you and your friend using? Length of buffer tube (rifle v. carbine)?

This is the buffer tube kit he has :

AR-10 Commercial Spec Buffer Tube Set (.308 Cal) » Guntec USA

Check out our AR-10 Commercial Spec Buffer Tube Set (.308 Cal). We carry a complete selection of AR-15, AR-308 and AR 9mm accessories!

guntecusa.com

From what I gather on the website it is most likely a 3.8 oz buffer

 

[Mbous]

Check to see if the gas block is moving and not drawing the gases in. I had that exact problem with a complete 6.5mm creedmoor upper I bought. The gas block moved far enough forward that the hole in the barrel was completely exposed. I've got to replace the set screws because I can move the block back but I could already see one set screw starting to strip with basically no torque applied. Mine is not an adjustable gas block, just one that liked to move down the barrel. The rifle turned from semi-automatic to pull the charging handle every single time to cycle the BCG. My and a friend both thought it was the BCG or the buffer but it was the gas block.

 

[teamRR]

Step 1 - headspace check as it could cause something weird like this.

 

[TrashcanDan]

Rounds loaded to long?
Primer type?
Primers seated all the way?
Not fully chambering?
Case shoulders not sized down?
Case walls not allowing it to chamber all the way?
Broken firing pin?
Garbage in the bolt carrier group not allowing the firing pin to travel?
Bolt face rotating all the way and locking in?
Hammer spring weak or not in right? Broken?
Does it have a forward assist?

Odd that it worked on the first and not the rest

 

[bfm]

Hammer spring weak or not in right? Broken?

A lot of people install the spring for the hammer backwards. It will drop the hammer but with not much force.

 

[wmass]

It sounds like a short stroking issue. I would start at the gas block.

 

[jpm]

hammer spring in wrong or you just need a stronger hammer spring. Happens with 7.62x39 ARs too because that ammo has harder primers.

 

[MisterHappy]

Pull the trigger harder?

 

[oldguy68]

A weak hammer spring, hard primers or firing pin out of spec and not protruding enough.

 

[teamRR]

The part that doesn't add up for me is when he says it light strikes when running but if manually cycled it fires ok.

If it cycles ok but just doesn't fire I think that eliminates springs, buffers, gas, etc as it is able to eject and reload, ie not short stroking..

Given it fires ok the first time or if manually cycled I don't see how thats a trigger or firing pin problem.

To me sounds like the firing pin is ok but isn't making it far enough, possibly bolt/lugs/carrier/chamber off somehow. Loose barrel could be in the ballpark too. Sorry for the repeat but wanted to add some color to it - remove the extractor and ejector, reassemble the BCG minus those parts, and perform a headspace check. Note how the end of the BC aligns with the upper when fully closed too (this is a loose tolerance but if something is F'd it will hang out or way in when shut)..

If thats all ok, put you ejector and extractor back in there, verify that at a 45 degree angle you can drop the bcg down the upper and witness it engage typically. A smooth running AR works this way, if something is off or sharp, needs a little wearing in, such will generally not work - grease lugs, try 100 cycles by hand via the charging handle, and repeat the test.

 

[citoriguy]

Are you shooting .308, 6.5CM, or some other AR-10 caliber? Are you using the right BCG for that caliber? I don’t believe the .308 and 6.5CM BCGs are interchangeable.

 

[teamRR]

Are you shooting .308, 6.5CM, or some other AR-10 caliber? Are you using the right BCG for that caliber? I don’t believe the .308 and 6.5CM BCGs are interchangeable.

They often are. The difference is mostly a smaller diameter firing pin, maybe some minor changes. With Aero for instance they only sell one BCG rated for both and they use an in between sort of firing pin diameter.