Pressure Signs in 308 PA10

[DMW1116]

I’m trying to work up a load in my PA10 using 168 grain Hornady Amax bullets and IMR 4064 powder. I used the starting load for 308 Service Rifle from Hodgdens web site. I increased charge weights in 0.5 grain increments from 41.5 to 42.5 and saw some pretty big burrs on brass around the ejector hole. There were no other signs like flattened primers or cratering. My current plan is to use 40.5 grains as the top of the next batch and work down from there.

I’m puzzled by the charge weight causing problems as it is the same charge weight I use for my 175 grain SMK load. I’m concerned there were signs there I missed and now I have a box of ammo I’m not really thrilled about shooting again. Is it common for a powder charge to cause trouble when moving to a lighter bullet?

 

[243winxb]

The bullers weights are close. I would measure the bearing surface of both. Longer bearing surface, more pressure. Jacket thickness matters, as does tne cores alloy.

 

[edwardware]

. . . and saw some pretty big burrs on brass around the ejector hole. There were no other signs

I pick up a lot of semi 7.62 NATO brass, with ejector smearing, and crimped in primers. . . so not reloads. I wouldn't be too worried about it if that's all.

 

[DMW1116]

Oh right. I forgot to say these are using Winchester cases on both loads.

 

[DMW1116]

I pick up a lot of semi 7.62 NATO brass, with ejector smearing, and crimped in primers. . . so not reloads. I wouldn't be too worried about it if that's all.

These were a little more than smears. All had lines and a couple had raised burrs. FWIW this charge weight is about a grain over the maximum in the Hornady Service Rifle data.

 

[Blue68f100]

Which direction is the smear. I've seen AR-10 set the shoulder back 0.002" when the bolt slams home. So if the brass is not sized properly this can happen during feeding

 

[DMW1116]

I haven’t had anything like this before using the same brass and the same sizing dies. Ultimately I’m going to have to fire a couple of the 175 grain loads and see how they look. FWIW the same bullets in the same cases and same primers look fine when loaded with CFE 223. I just wanted to try the IMR 4064 to see if I could wrong a little more accuracy out of these. It’s given me the most consistently accurate loads over 2 calibers and 3 different bullet weights.

 

[Charlie98]

IMR4064 and either the 168's or 175's go together like macaroni and cheese... it's what I use, exclusively.

Hoary internet lore says Federal used 43.5grn IMR4064 and the 168grn SMK for their FGMM load, but that would not be with canister powder, and certainly with pressure testing equipment.

I load 168's for my M1a, I've compiled quite a bit of data, including personal data, for suitable loads, most data levels off at 41.5grn IMR4064 with a 168grn bullet, with 42.0grn being the top end... and I'm likely to agree.

As 243 suggests... there might be more to it to look for... is your neck tension enough to not allow bullet setback during chambering? The AR platform has a fairly violent cycle, and if those bullets are piledriving the front of the mag well on the way to the chamber, there might be a dash of bullet setback. Also, make sure you aren't jamming the rifling with those particular bullets.

 

[243winxb]

a grain over the maximum

Is most likely the cause of brass flow into the ejector hole.

 

[IALoder]

Do you have an adjustable gas block? If so try turning your gas down. My LR308 was putting some smears/burrs on even light loads until I got the gas adjusted/turned down.

 

[Charlie98]

Do you have an adjustable gas block?

I've found an adjustable gas block is almost mandatory on a big block AR.

 

[Otto]

The Gen 1 and Gen 2 PA10 doesn’t have an adjustable gas block…only Gen 3s.
Either way, he shouldn’t exceed published load values.

 

[IALoder]

At 42.5gr he's 1.4 gr below Hodgdon's max load for a 168 over 4064 in their .308 service rifle data. Large frame AR's have a tendency to beat up brass and give ejector swipes for fun, an adjustable gas block usually does a pretty good job of remedying that.

 

[Blue68f100]

That and/or go to a H3 Buffer. The extra weight will help to slow it down.

 

[DMW1116]

It does have an adjustable gas block. The current setting cycles most everything though if I end up loading the rest with CFE 223 I will lower it a notch or 2. I picked up a box of M80 Winchester to see how it treats the brass on that for reference.

 

[DMW1116]

I did not test the lower charge loads but I did test some PMC and Winchester M80. 147-150 FMJ and heaven knows what powder or primer. They showed slight ejector marks but nothing like before. The Winchester also shot 1 ragged hole at 50 yards, so I’ll be grabbing some more.

I also tested my 175 grain SMK/IMR 4064 load. 3 shots and one case had a noticeable smear from the ejector, but again nothing like the others. I found/was given a load of 308 brass so I called it a good day and went to the pistol range to miss some more.