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bolt will lock/seat! updated w shrtstrking issue

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hope this hasnt been answered 100 times. ive been doing searches for days with no answers.  (how i found this forum)

i have a DPMS classic upper with 24" bull. bought it lightly used from a local who went to a shorter/lighter setup. i bought a phosphate coated BCG from JSE.  when i drop it in, the lugs dont line up. if i pull the cam pin, allowing the bolt to rotate a tiny bit more, it drops in fine. i had a few people eyeball the upper and they said it looked fine, so i sent the BCG back to JSE (who have been great btw) they tested it in two custom uppers they had in house and said it worked fine, so sent it back. brought the BCG and the rifle to a local shop to see if they could figure it out.  they pulled the BCG from a factory new LR-308 on the wall, and it dropped right in my upper. the JSE BCG didnt fit in the factory rifle they had. any ideas here?? im dying to get this rifle out to the desert.

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Or just the cam pin . Try a different cam pin .

If said BCG seated in other rifles , ,there is nothing wrong with its machining , you are just getting that tolerance stacking we talk so much of here.

Different manufacturers don't have to keep to a mil spec tolerances(as with the M16/15 family) , they seem to make all there own.

It just may take some lapping in or just a good working in .

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I had the same problem a week ago, I managed to fix it. The bolt would fully engage without a round in the chamber but would stop a 1/4" short with one. I took  the bullet out the brass broke the gun in half dropped the case in the chamber lubed the lugs and bolt up and pushed the carrier forward by hand. This is where the gun guys are gonna cringe , :-)  I tapped the carrier forward with a brass hammer and it closed fully. I then had to lightly pry the carrier back open,  after repeating this 20 or so times I cleaned and reoiled. I put the lower back on chambered the empty case normaly and with it 2 bumps of the forward assist it was in firing position.    I repeated with a live round and it fired. I have put 40 round thru with lots of cleaning and oiling all seems to be fine.  All I can figure is the parts needed a little lapping to work in. This non-typical approach worked but before you do it inspect any brass that won't chamber for an obvious mechanical interference in the chamber.  .  See I told everybody years ago a hammer can fix anything!!!!  ;D

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Since you are dealing with parts & not a manufactured rifle (they do have problems like this also with production rifles ) , you have to make your self ,over night , into a gunsmith.

Just another good reason to have a set of Head space gages around .

I can tell you over the years , I have had to lap bolts to seat properly .

Not often, but it does come up when you do this full time .

I can tell you ,I have never hammered on one , to get it to seat.

Sure glad you did it with an empty case.

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well..minor progress. left the rifle with a friend who has built several very nice AR15s. he did a little work with a needle file and has it closing/locking but its very tight and hard to crack it back open. think im just going to spend some time actuating the bolt to break it in a bit more and then spend a day running some cheap ammo through it. im sure my wife will be so fed up with hearing CLICK-CLACK...CLICK-CLACK.....that she will beg me to spend a day out in the desert  :P

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  • 2 weeks later...

we are functional! just need a firing pin retainer as we bent mine up during some aggressive bolt "fitting" lol. might need a cam pin also as we filed a little metal off of it to get the bolt head to rotate a little more, but it feels ok and seems to be functioning normally with the modded part. not a very high end build but im probably only out of pocket $500ish so it will do for now. (hooray for trades and gift cards) im floored by how heavy this monster is, but i guess shell be nice and steady on the bipod. bit different from my RRA AR with carbon tube and ace skelly stock. anyway...

its a girl!


ill do a proper intro/specs when i snap a cooler pic.

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Well maybe its, "his" play pen ! <laughs>

Glad to see you got her to function.

Have you looked at the POF roller cam pin replacement ?

I have one in my 16" & it seems to work fine .

Not that ya need something other than the std. cam pin ,but it may help. Since you plan on buying another cam pin , any way.

Just another option.

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  • 1 month later...


most of the bugs worked out. rifle ran fine through 20rds of match ammo but didnt hold open on the last round. was a new p-mag so wasnt able to verify weather it was the mag or the rifle.  switched over to some russian/tula cheapo stuff and it went back to single shot. was ejecting about half the time  but not chambering or holding open  with the p-mag or the stock mag.  did that for about 30 more rounds and got bored and went back to playing with my AR and my new P99. all the spent casings had two parallel scratches on them (barrel extension?) gas rings on the bolt are aligned right. noticed some crud on the barrel streaking out from around the gas block. seems like a gas problem. think it just needs more ammo through it or should i rip it apart and look for gas leak? i know other DPMS rifles can eat the tula stuff without issue. was running it really wet.

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That gas block will seal itself, as you shoot a few more - doesn't take much, in normal circumstances, but it is common to see the carbon signs like that around the tube, at the block.  Worry not. 

Once that is sealed, it might solve the last-round hold open.  If it doesn't, let's dig into it some more.  <thumbsup>

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