6.5 CM AR 10. FTF, dimpled primer, bolt won't open...help

[LunaCustomGuns]

Good Morning! I could use some help.
Sighting in my backup rifle for the hunt last night. 5 round mag, 4th round, pull the trigger and nothing. Go to clear the round and the bolt is locked closed so tight that I can't clear it. I had to angle the rifle against my stomach while the barrel was still down range and use a mag to pull the bolt back. This is the second time this has happened in as many times shooting.

Before trying to pull the bolt back I did separate the lower from the upper, manually cocked the hammer and tried firing the round again to no avail.

Once I got the bolt back and the round ejected, i noticed the FTF round had a dimpled primer.

I am at a loss as to what to look for here so any advice would be helpful.

Stats: Rifle is a DPMS low profile upper on an Aero lower. Geissele trigger, NiBoron BCG, Ballistic advantage barrel, can't remember the buffer. The ammo is Desert Tech match 140 gr, I will have to caliper it when I get home tonight.

 

[The King]

Did you check headspace?

Also, remove the bolt and hold the BCG in your hand. If you try to push the firing pin all the way forward does it protrude well through the hole in the bolt face with the bolt pushed rearward?

You may have crap or machining debris in your firing pin channel.

Next opportunity is around the bolt not closing 100% in the barrel extension. You can take the barrel out and push the bcg against the extension. It should close until the carrier is against the extension itself. I bet on this or headspace.


Finally the trigger may be inop.

 

[LunaCustomGuns]

That's what I figured last time I went shooting and ran the bolt through my ultrasonic. I however did not tear down the bolt to inspect.

After some brief googling, I had no idea that 6.5 was so finicky in the AR platform. Not necessarily a negative, I just figured I could build it like a .308 and run it. Sounds like I need to reevaluate a lot of my parts. And I NEED to add an adjustable gas block.

 

[The King]

it isn’t usually finicky in this way. it’s other things that end up giving you issues.

and yes adjustable gas is key. Or the JP buffering bolt carrier. or both.

 

[Darqusoull13]

Yeah, adjustable gas block an extended rifle length gas system and a heavy buffer or heavy silent captured spring go a long way in increasing 6.5 creed operating window in a gas gun. AR10's aren't exactly plug and play and I prefer to buy a complete rifle from a single builder rather than piece them together.

Definitely have a gunsmith check your headspace or do it yourself. Also inspect the round in question. Even factory ammo can sometimes be out of spec. Clean your chamber and inspect that too in case there's a piece of crud preventing rounds from fully chambering.

 

[LunaCustomGuns]

Appreciate it.

 

[LunaCustomGuns]

So I do have a heavy buffer. Trying to remember the brand of the BCG. I have contacted the barrel manufacturer to see what they recommend. I am leaning more and more towards headspace. I wonder if I should try some non match ammo first just to see if it is randomly an ammo issue as well. I know match ammo can push the limits on overall length.

 

[MotoFlier]

It’s my understanding that an adjustable gas block is to REDUCE the amount gas to just what is needed to properly cycle the bcg and direct the rest down the barrel. The non adjustable blocks let full pressure through causing excess recoil. While I love adjustable blocks and add them to all my builds since discovering them I’m not sure the block is to blame for these issues. I would try measuring all the ammo and maybe trying another brand or different primer if your loading. Also check that the firing pin channel is free of excessive oil or debris, Oil on the pin can actually slow it down causing light strikes. You can also check the primer dimple if it is not centered that can point to debris in the channel or the bolt not being fully in battery. Did you give the forward assist a good whack before trying to refire?

 

[newguy2k3v2]

My first thought is the bolt and or carrier wasn't all the way seated so you got a light strike. Either from headspace or a very dirty chamber.
Do you have a headspace comparator? Check your fired cases against unfired.

 

[The King]

So 60% of the time it’s the bolt not closing.

If the bolt doesn’t close, 50% is debris like a blown primer cup. 40% is headspace. 10% is a machining flaw in the bolt carrier or carrier extension.

20% of the time it’s the trigger assembly.

10% it’s something wrong with your buffer setup. Ar15 buffet in a 10, or a carbine spring in a rifle buffer tube.

and the last 10% is everything else...i have seen everything else, like a chamber with no throat because the throat part of the reamer snapped off but was retained by the pilot screw so it just spun while the chamber was cut.

I have seen uneven machining of the barrel extension result in partial lockup. An upper with improper cuts in the gas tube area that made the gas key slam to a stop in there. A new company that was on their first run of charging handles and accidentally made one that was ar10 sized but ar15 length. Lol.

 

[LunaCustomGuns]

So 60% of the time it’s the bolt not closing.

If the bolt doesn’t close, 50% is debris like a blown primer cup. 40% is headspace. 10% is a machining flaw in the bolt carrier or carrier extension.

20% of the time it’s the trigger assembly.

10% it’s something wrong with your buffer setup. Ar15 buffet in a 10, or a carbine spring in a rifle buffer tube.

and the last 10% is everything else...i have seen everything else, like a chamber with no throat because the throat part of the reamer snapped off but was retained by the pilot screw so it just spun while the chamber was cut.

I have seen uneven machining of the barrel extension result in partial lockup. An upper with improper cuts in the gas tube area that made the gas key slam to a stop in there. A new company that was on their first run of charging handles and accidentally made one that was ar10 sized but ar15 length. Lol.

Thanks so much for your reply! I have decided to not mess with it and will be dropping it off at the gunsmith this week. He literally said everything you just told me and said he will start with headspace then move along down the list with swapping parts. I am confident he will get me taken care of. I really like the gun and want to run it in some comps but it needs to be able to run flawlessly.