First time at range, issues afterwords

[pnutty]

Hello everyone, I finally taken my first large frame 6.5cm build to the range and everything worked great. Put 70 rounds through it and no issues functioning but I was puncturing primers. (Indoor ranges aren't ideal but it was good enough for a function test and scope zero)

When I got home and started to clean it, I couldn't remove the cotter pin at all. I had to clamp onto the round part of the cotter pin with the corner of the vise and yank it out that way. Once I got it out I found the pin was bent to hell, I also noticed a few pieces of primers that came out when I finally got the bolt apart. (Primer pieces are at very top of the photo)

From my research, the primer issue is because of excessive clearance between the firing pin and hole. I measured my firing pin hole at .082” and the firing pin tip at .077”, I found that cmmg makes a .08” firing pin for their mk3 model rifles. 

My questions are if the cmmg firing pin will work in my dpms bcg and if the primer cups getting lodged in the bcg are the reason the cotter pin bent.

 

Specs are m5 receiver sets

cmmg lpk

armalite recoil setup

dpms bcg

ba 6.5cm 20” barrel rifle gas length

 

Edited Wednesday at 11:35 PM by pnutty

[98Z5V]

What ammo are / were you shooting?

I blew the pic up, and can see it's Hornady, but what load?

Edited Thursday at 02:18 AM by 98Z5V

[pnutty]

8 minutes ago, 98Z5V said:

What ammo are / were you shooting?

I blew the pic up, and can see it's Hornady, but what load?

It was a handload, 40.2g of imr4451 over 140g sst. Primers are cci #34

[98Z5V]

High pressure signs, my man.

Try some 140gr Hornady 6.5C Match. Just get a box and try it.  

[pnutty]

8 hours ago, 98Z5V said:

High pressure signs, my man.

Try some 140gr Hornady 6.5C Match. Just get a box and try it.  

I will, I ran this load in a bolt gun with no problems but that’s a different animal.

[pnutty]

7 hours ago, burntgoose2 said:

 

 

Yep, exactly. A load that looks perfectly happy in a bolt gun can get spicy fast in a gas gun, especially a large-frame AR where timing, chamber, gas, bolt unlock, and firing pin hole all get a vote. Punctured primers and primer bits in the bolt are the rifle waving a big red flag, not just being dramatic. I’d back off that load and try known factory ammo like Tom suggested, just to separate ammo pressure from rifle setup. Also check the firing pin tip and bolt face for damage before you run it again, those little primer cookies can turn into gremlins quick.

I did put a box of S&B 140g fmj through it and no pressure signs I saw except some but not all of the primers are flattened. 
 

One thing I noticed is compared to my bolt 6.5CM (I know apples to oranges) is that the firing pin indent is much deeper, I don’t know what the spec is for large frame ar’s but I measured using a depth micrometer and from front of bolt to bolt face is .125” and front to the tip of the firing pin when engaged is .076” so that’s a .049” protrusion. Possibly this could be an issue as well?

One last thing is that I do have srp 6.5 brass for my other gun as well, would that have any advantages to use with a gas gun?