Pressure issues please help

[Grizzgup]

Hello,

I have a BCM AR with a Rainier 18” barrel. I was loading 24.0g of N540 with 75g Hornady BTHP at 1.847 CBTOG with CCI primers, and it was great. Due to some unrelated light primer strike issues I switched out the bolt and BCG, and all of a sudden I’m getting signs of over pressure on the primers. Can switching the BCG cause that? It’s also getting warmer in PA, but I think N540 is supposed to pretty temp stable. Any thoughts would be appreciated! Thanks.

 

[oldrifleman]

You need to check your headspace with the new bolt. I am betting that is your issue.

 

[Cascade Hemi]

How did you work that load up?

 

[camocorvette]

How did you work that load up?

That i don't know. But 25.8 is max load from their website. But I'm assuming with a bolt gun.

I think your issue is the bolt. You swapped a new bolt in. Measure the newly fired cases and the old fired cases from the old bolt.

Headspace probably decreased.

 

[BCP]

Hello,

I have a BCM AR with a Rainier 18” barrel. I was loading 24.0g of N540 with 75g Hornady BTHP at 1.847 CBTOG with CCI primers, and it was great. Due to some unrelated light primer strike issues I switched out the bolt and BCG, and all of a sudden I’m getting signs of over pressure on the primers. Can switching the BCG cause that? It’s also getting warmer in PA, but I think N540 is supposed to pretty temp stable. Any thoughts would be appreciated! Thanks.

That looks like an oversized firing pin hole

Light primer strike issues shouldn't be caused by the bolt, were the cases sticking or not going into battery?

I don't know what 1.847 amounts to but the 75gr bthp hornady shoots great at 2.25" which is about where the cannelure usually lines up

You did check headspace...right?

 

[Grizzgup]

I checked HS with the factory BCM BCG, but not the other one. I’ve definitely learned my lesson that AR’s are not plug n play.

I worked up the load starting much lower- .5G above book minimum, and worked up in .3g increments until I found the node and good velocity, then I tweaked the bullet length until I got the best group possible. (I know there’s a million ways to develop a load, and mine might not be the best, but it what I do). CBTOG stands for cartridge base to ogive. I saw that used before and it’s a lot easier than typing the whole thing out lol.

I’m going to pull the extractor/ejector from the new bolt and have my gun smith check with his go no go gauges.

As for the light strikes…. This is an interesting problem. It does it on factory M193’s and hand loads. With the BCM BCG the headspace is definitely good. The only other thing it could be is the JP captured buffer and spring. Though I don’t know why. When I switch to the factory H1 buffer and spring the problem goes away. I thought maybe it wasn’t going into battery with the JP, so I put a quarter behind the unit (as they recommended) and the problem still persisted, though I’m 100% sure it’s going into battery. I should also mention the light strikes with the factory trigger, CMC drop in, and now I’m running a Larue trigger. So I doubt it’s the hammer spring. It’s something with that JP buffer, like I said I’m not sure why though….. Thoughts?

 

[mtrmn]

I checked HS with the factory BCM BCG, but not the other one. I’ve definitely learned my lesson that AR’s are not plug n play.

I worked up the load starting much lower- .5G above book minimum, and worked up in .3g increments until I found the node and good velocity, then I tweaked the bullet length until I got the best group possible. (I know there’s a million ways to develop a load, and mine might not be the best, but it what I do). CBTOG stands for cartridge base to ogive. I saw that used before and it’s a lot easier than typing the whole thing out lol.

I’m going to pull the extractor/ejector from the new bolt and have my gun smith check with his go no go gauges.

As for the light strikes…. This is an interesting problem. It does it on factory M193’s and hand loads. With the BCM BCG the headspace is definitely good. The only other thing it could be is the JP captured buffer and spring. Though I don’t know why. When I switch to the factory H1 buffer and spring the problem goes away. I thought maybe it wasn’t going into battery with the JP, so I put a quarter behind the unit (as they recommended) and the problem still persisted, though I’m 100% sure it’s going into battery. I should also mention the light strikes with the factory trigger, CMC drop in, and now I’m running a Larue trigger. So I doubt it’s the hammer spring. It’s something with that JP buffer, like I said I’m not sure why though….. Thoughts?

Get rid of the Gucci stuff and run the factory spring and buffer.

 

[BCP]

When I switch to the factory H1 buffer and spring the problem goes away.

If the bolt doesn't go fully into battery the hammer will hit the firing pin but the firing pin won't go all the way forwards and you'll get a light strike, it's a design feature to keep it from firing out of battery

Excessive headspace is really easy to see, your cases will be ballooned out and hard to resize, length after firing will be atrocious and if you're really unlucky they will look like footballs when you roll them across a flat surface

Always stick to milspec bolt carrier, spring and buffer unless you are a running a special wildcat caliber or a suppressor that creates excessive backpressure

 

[Potsy]

Looks like the new bolt has a bigger (sloppier) firing pin hole.