I have been having issues with my mk12 mod 1 with and without the can.  Blown primers and overpressure. Primer gets stuck in the bolt face/behind the bolt and stops function. Bullets in mag very dirty.

relevant information:

barrel 17.9 criterion SPR stainless barrel m4 feed ramps CLE match chamber (don't know gas port size on barrel)
Colt BCG and matched bolt to barrel
Gas block DD mk12 FSB .0750 gas port
A2 stock rifle buffer
suppressed and unsuppressed AEM5 suppressor same issue

ammo shooting

PMC 55 grain 5.56
Federal XM855 62 grain 5.56
IMI OTM 77 grain 5.56

shot 100 rounds today.  blown primers ~10

1 blown primer stuck in bolt lugs, had to sledgehammer the stock and charging handle to remove.

last blown primer got stuck behind the bolt head and stopped the bolt in rearward travel.  had to disassemble stock to retrieve bolt.  Primer located.

Thanks.  My gunsmith is going to check it next week.  

He suggested checking head spacing, which i'm not sure of since it is a matched bolt.

and measuring gas port sizes between the barrel and the gas block.  If this is the issue, i suppose an adjustable gas block might be in order?

Your gunsmith is on the right track about the headspace. Excessive headspace can mimic over pressure by allowing the primers to unseat.

Was it blowing primers with all the ammo or one particular type?

the CLE match chamber is a 5.56 chamber

it was doing it with all 3 types of rounds listed

No, I suppose I should have inquired.  The barrel came from CLE marked as CLE MATCH

also, When I was building the Rifle, the guys in the mk12 thread said the CLE match chamber was fine for all types of ammo.

IMI 77 grain RAZOR ammo is popping primers in lots of rifles. It's loaded HOT! Some chronographs are recording 2850 fps with a 77 grain bullet. That's close to 200 fps faster than the hottest loads I will shoot.

There have been plenty of people having the same problem you are.

Well I learned something today. I did not use Jp bore paste. Never had a rifle that hadn't had that done before. GG. Ugh thanks compass lake for helping me sort this out.

Do what now?

Jb bore paste as part of the break in

What part?

I use JB Bore paste when a barrel stops shooting well, usually when it's nearing the end of it's life. I mix it with Kroil and clean the crap out of the bore.

Usually it will help, it has never hurt. I have never used it to break a barrel in.

Check your spent brass, I would wager the only cases missing primers are the Israeli 77 grain RAZOR brass.

Stop shooting it. It's too hot for your rifle. I think it's too hot for every rifle.