Ar10 80% build buffer issues

[Ar10bufferblues]

This is my first ar10 build but I am pretty sure I have narrowed down the issue. It's a blitzkreig tactical 80%lower and I'm fairly sure all my cuts are fine since I used a mill and that my issue is an out of spec lower from the factory.

I assembled the rifle and began to get some rounds through it to loosen it up and started getting jams every 2nd or 3rd round. The rounds we're getting jammed up between the bolt and the chamber diagonally and putting huge gouges in the casing. Also had double feeds. Used a Pmag and shot about 40rnds total before I called it quits and accepted there was a problem.

When I took it apart last night, I noticed damage to the buffer threads and tube from what appears to be over travel of the BCG. (Pics below)

This leads me to believe there are 2 possible issues. Either the buffer retaining pin hole is off, or the buffer threads were cut wrong (both of which would be a factory defect). When I installed the buffer tube it was screwed in as far as possible, another turn would have held the retaining pin down under the tube.

So I am thinking a possible remedy to this would be file a notch in the buffer tube and give it another turn. That should prevent the BCG from contacting the threads on the lower. But is that extra turn going to bring it in TOO far and then cause other issues? This 'fix' would also make the buffer tube protrude slightly inside the upper. It does not cause any issues with attaching the upper as I have tested it out,but I just want to bring this up since it concerns me before I attempt this fix(pics below)

Am I on track with my problem analysis or totally off?

[baih777]

same problem. go thru this thread.

http://www.calguns.net/calgunforum/showthread.php?t=1261158&highlight=ar10+buffer

[Ar10bufferblues]

I just read that thread but no root cause was ever identified, just speculation. The upper, buffer/tube/spring were all purchased from the same place at once. The buffer spring does look different from my ar15 (pics below) so pretty sure my parts are good.

Does anyone have a reliable blueprint so I can take some caliper measurements and tell what is off? I really think it's a problem with the lower and not the parts I'm using.

[kcstott]

i think you're using the wrong spring.

Ar 10 spring is longer than a 15. But you're using a carbine buffer tube soo.. I say wait til randal chimes in.

It does appear that the retainer is not correct. Link to prints in my sig.

[Ar10bufferblues]

The buffer i am using is designed for the AR10. Its a GunTec GT-10 (http://www.guntecusa.com/index.php?r...&product_id=93)

I made certain the complete upper, Buffer/spring/buffer tube combo package i got from my supplier was for the DPMS style AR10 lower. (which to my understanding is the style lower i have from blitzkrieg tac).

Although they look the same size, they are not the same strength at all. The AR10 spring is thicker and has less turns. The AR15 one is much weaker, i can jam the entire spring around the buffer using my bare hands, the AR10 spring i cannot push down on it very far its so strong.

[kcstott]

The buffer i am using is designed for the AR10. Its a GunTec GT-10 (http://www.guntecusa.com/index.php?r...&product_id=93)

I made certain the complete upper, Buffer/spring/buffer tube combo package i got from my supplier was for the DPMS style AR10 lower. (which to my understanding is the style lower i have from blitzkrieg tac).

Although they look the same size, they are not the same strength at all. The AR10 spring is thicker and has less turns. The AR15 one is much weaker, i can jam the entire spring around the buffer using my bare hands, the AR10 spring i cannot push down on it very far its so strong.

I meant buffer tube. an AR10 spring is 13.75" long free length and an AR15 spring is shorter by about two inches. I don't know what the configuration is in a carbine length buffer tube. seems like a very light buffer for an Ar10

[baih777]

You need to measure the length of your buffer and Google the dimensions for the AR10 and a DPMS LR-308. See which one you have.

[buellster]

7" internal length is standard AR15 so the shorter buffer would be the correct one to use. However, there is no "mil-spec" for AR-308 rifles. There's always a chance that you'll run into some weird compatibility problem when building them. If it were me I'd throw a couple quarters down the tube, re-assemble and see if your problems go away. That's a quick fix to see if you're on the right track.

[gemini1]

Here's my LR308 and AR15 carbine buffer and spring. The 308 spring has 29 coils and the buffer as you can see, is 2.508" Im using a standard AR15 buffer tube ( 7" measured from the inside) on my 308.

[ar15barrels]

i think you're using the wrong spring.

Ar 10 spring is longer than a 15. But you're using a carbine buffer tube soo.. I say wait til randal chimes in.

It does appear that the retainer is not correct. Link to prints in my sig.

What matters is that the carrier and buffer move back into the tube enough for there to be about 3/16" to 1/4" of clearance between the bolt stop and the bolt lugs while the buffer is bottomed out in the receiver.
If you have MORE travel than that, add quarters inside the buffer tube to reduce the travel.
If you have LESS travel than that, unscrew the buffer tube a turn (will also correct the buffer detent location problem shown above) and then trim the snubber on the buffer until 3/16" of overtravel is achieved.
Obvoiusly, the carrier should not be striking the receiver at this point either.
Clearance the receiver as needed to solve the problem if it still exists after correcting the length/travel issues first.

[kcstott]

What matters is that the carrier and buffer move back into the tube enough for there to be about 3/16" to 1/4" of clearance between the bolt stop and the bolt lugs while the buffer is bottomed out in the receiver.
If you have MORE travel than that, add quarters inside the buffer tube to reduce the travel.
If you have LESS travel than that, unscrew the buffer tube a turn (will also correct the buffer detent location problem shown above) and then trim the snubber on the buffer until 3/16" of overtravel is achieved.
Obvoiusly, the carrier should not be striking the receiver at this point either.
Clearance the receiver as needed to solve the problem if it still exists after correcting the length/travel issues first.

This is why you da man on all things AR related. i just keep learning new stuff.

[Spyder]

Haha, yep. Posts like that make me want to go through every rifle in my safe and check things, and then go read Randall's history. Thanks!

[kcstott]

Haha, yep. Posts like that make me want to go through every rifle in my safe and check things, and then go read Randall's history. Thanks!

Maybe take his history, print it all out, sort it all out, and put that together like Hatchers notebook

[Ar10bufferblues]

I decide to cut a notch in the buffer tube to allow me to screw it in another revolution. The BCG is now making light contact with the buffer like it should when the rifle is assembled, pretty sure this ****ty 80% lower was made out of spec (I will be contacting the company once i confirm this for sure before i blast them to give them a chance to own up to their mistake(s)).

Another trip to the range will confirm this, i will post up to let everyone know if this solved all my issues or not to help others out in the same boat as all the posts i have found online never really gave an "after fix analysis" just a whole lot of speculation.

[kcstott]

I decide to cut a notch in the buffer tube to allow me to screw it in another revolution. The BCG is now making light contact with the buffer like it should when the rifle is assembled, pretty sure this ****ty 80% lower was made out of spec (I will be contacting the company once i confirm this for sure before i blast them to give them a chance to own up to their mistake(s)).

Another trip to the range will confirm this, i will post up to let everyone know if this solved all my issues or not to help others out in the same boat as all the posts i have found online never really gave an "after fix analysis" just a whole lot of speculation.

Pretty sure it's out of spec?? nothing you've done to remedy the situation is going to make it better. Screwing the buffer in further will not fix the issue. You did nothing that was suggested to you. Yet you asked for help and still decided to do it your way...

it's pretty damn easy to measure the receiver extension tower. Follow Randals Suggestion and be done...


For everyone else this is why I hate giving advice to certain people. All the proper procedures in the world will not help if you simply can not and will not follow basic directions.