Defcon-One

Finally got out to the rage yesterday. Rifle shoots well, but it still misses a few times regarding the bolt hold open. Maybe 1 or 2 out of 10 do not lock the bolt back. This confirms what I have been told here. I am gonna go up a drill size and try it again. I love these .308 caliber ARs and I am getting closer. Everything else works perfect!

OK, my number drills arrived yesterday. Please note; I still have the stock DPMS spring and follower in the rifle for the simple reason that I could not purchase Armalite replacement parts anywhere. All sold out and on back order. Also, this is a DPMS G2, not an Armalite AR10, and I have to assume that DPMS would know what works best in their gun design. I followed my plan and went up from 0.062” to 0.070 (#50 bit) and fired a few test rounds out back. Still no lock back consistency though, one did lock the bolt back. So, I figured that I was getting very close. I went ahead and used the 0.073” (#49 bit) and opened it up a bit more. I then fired five more rounds out back using the “one in the mag at a time” routine and all five shots now locked the bolt back. It looks like I have this fixed (tuned) but I need to test some more. I may need to go up to 0.080” like suggested above, but I am gonna hold that option on reserve for later. It is easy to go up, but hard to go back down. I intend to go out to the range this weekend and give the rifle a good workout. I will report back here once I know the final results. Thanks to 98Z5V for getting me on track!

OK, I’ll find the parts that you recommend, install them, then gradually open up the gas port until it runs like it should! And yes, 0.080” will be my target. That is a pretty simple project, once I find the parts. Thanks for the advice! DC-1

Sorry, I missed the first question that you asked me. Here you go: The gas block diameter is 0.750”, standard on the DPMS guns, except the bull barrel version. I checked further and the buffer and spring that I purchased and am using are both stock DPMS G2/AR10 parts, made for this rifle. The buffer is in their spec at 5.30” long and 5.4 ounces. The buffer spring is in their spec at 12.75” long and 38 coils. Based on that and the fact that my DPMS G2 Recon in 7.62mm x 51 with a 16” barrel (Mid-length gas system) has the same spring and buffer and it runs like a champ with everything I feed it. I have to assume that the real issue here is the gas port size. Also, I could easily trim the buffer down to 5.20” long if need be and the weight is already right! Everybody seems to be out of the Armalite springs (EA1095) right now, so I am gonna have to wait on them. Would you agree that opening up the gas port to at least 0.070” (#50 bit) would be a good idea, or am I getting ahead of myself? Thanks, DC-1

98Z5V: Thanks for the reply! That is why I am here. Buffer is a DPMS, so must be made slightly wrong. It measures 5.303”! Wondering if I can remove enough from the plastic bumper pad to get to 5.20? It would only be about 1/10th of an inch. Weight is right on and that little shouldn’t change it. I bought two of those springs, supposed to be AR10 Rifle, so somebody stuck me. The one in my .308 seems to work fine, though. I will get two new springs that meet the spec and try it out. I might get the buffers too. You really have to watch what you are getting with these AR10 parts! Thanks for the help, DC-1

This is right! It would also adversely affect the alignment pin location on your Barrel extension. You must do it in one complete turn (0.0625 in this example) and then re-cut the chamber.

Gas tube is dead center, maybe a tiny bit longer, probably about perfect! The gun is well lubed, but maybe not Soaking wet. I doubt life is the issue. I figure I’ll start with a #50 (0.070”) and work The hole gradually up to a #46, which I think is around 0.081”. Don’t want to go much over that and hopefully won’t need to go that far. Edit to add: The only other thing I noticed is the buffer, a DPMS factory AR10 rifle buffer, is about 1/32” to 1/16” over the 5.25” standard.

Guys, I am new to this site! Got here by searching on my problem. I have read what I could find and think I get it, but would not mind some advice and mentoring. I just put together a DPMS G2 with 20” rifle length gas, DPMS .260 Rem Hunter barrel in a YHM free float handguard. SPECIFICATIONS: G2 lower with A2 tube (9-11/16” inside) and stock. Standard AR10 rifle buffer (5-5/16” long, weighs 5.4 ounces) Standard AR10 rifle spring (12-3/4” long, 38-39 coils) Upper is all DPMS G2 parts except the barrel nut and free float rail. This blew my mind, but the gas port is 0.0625” possibly a bit larger as a 1/16th bit fits with a touch of wiggle, but not much. (That is the best I can measure with a digital caliper.) No pin gauges! Rifle shoots good, feeds good, but does not lock the bolt back. It is NOT a magazine issue, as bolt locks back fine when manually charged. Also, fired brass and primers look fine. It drops empty rounds at 4:00-4:30. Pretty obvious from what I read above, this is a classic short stroking problem and the port is likely too small! I have number drills on order! Any comments would be appreciated. Unless I get advice to the contrary, I intend to open the port up. Probably go slowly until I get it to lock back reliably. Ammo is Federal Fusion .260 Rem 120 gr. Factory loads. About 50 rounds down the tube so far! Thanks, DC-1

YES, Timney does make a trigger specifically for an AR10! I have many in my 5.56 guns and I liked them enough to go with one in my DPMS G2 rebuild. It was an AR10 version. I loved that one, Too. It was a skeletonized trigger, 4 lb. Item number: 671 Tested and worked with Federal XM80C, IMI 7.62mm, Hornady Match 168 gr. and Federal GMM. No problems at all. (I use the KNS pin set to securely hold these in the receiver.) They are fairly expensive, but I highly recommend them!