M852 Match 308 chamber

[mac86951]

Ok, New Barrel (40 rnds) 24" long chambered by Accuracy Systems Inc.

Passes a Go Gauge, does not pass a no-go, so headspace is fine.

Distance to lands with 168gr Sierra is 2.775"

I loaded up a "pressure" test with 168gr Sierras with IMR-4094 at 2.765" COL. I'm using Nosler brass fired Once, resizing/loading using Forster Benchrest dies. Federal GMM 210M primers

41.5gr 4094 was where I started, but alas the beginnings of a cratered primer

42.0gr was a serious cratered primer that was almost pierced.

today was 70°F

I'm guessing that a chrono would help verify velocity, but it didn't seem like starting with hot loads.

I'm guessing I'll need a throat reamer, but I'm hesitant to do so since I'm undecided between the 155-175 bullets.

What are your thoughts, and is there anything brass prep or brass measurements I can check? I'd again make another guess that if the necks of the brass are too thick that I'd have pressure problems too? I have not tried other brass.

edit: I've got my reamer print right in front of me, and it has .343 neck diameter, .309 freebore diameter, and .075 freebore length.
Best I can tell from the standard 308Win print is that the freebore length of standard is .090

-Mac

 

[chkunz]

I may be able to help but need more info. Is this an M14/M1A? More detail on the gun is critical.

 

[chkunz]

You say "M852 Match 308 chamber", what does that mean? Do you have any M852 ammo to try in this gun? M118 would be the same dimensions to use as a test. If the gun will not work with M852 or M118 ammo then you have a gun problem. If it works with M852 or M118 then you have an ammo problem.

 

[rminut]

If your measurements are correct you do not have an 852 chamber. It was designed to shoot 168's at 2.80" with an additional clearance to the lands.

2.775 to the lands is what my Palma '95 measures, only .050" freebore.

 

[chkunz]

If your measurements are correct you do not have an 852 chamber. It was designed to shoot 168's at 2.80" with an additional clearance to the lands.

2.775 to the lands is what my Palma '95 measures, only .050" freebore.

Good information but we need a lot more info to properly diagnose the problem.

 

[mac86951]

Ar10

Dave Manson M852 reamer. In my research its supposed to be same as 95palma match freebore which is printed at .075". That said I would expect a reduction of COL to increase pressure, but anyone have experience with a 168 seated to 2.765"


Any way to check freebore without cerrosafe?

I suspect it might need a little finishing with a throating reamer, but want to know more first.

Ive only fired 40 rounds through it. Nosler 175gr custom competition is loaded to 2.800", too hot a load for this chamber, but is listed as 2550fps, I'm planning on hand loading, but was surprised to reach pressure signs early. I'm not worried about the chamber, more curious as to what is going on, followed by where to go from here. I'll try some 155 palma sierras, but my 168s at 2.765" and 42.0gr imr4094 is what I'll call near as makes no diff maximum loading.

 

[rminut]

AR10's can do this to primers. Buy some factory ammo and it will show the same primer results.

here is a link with Al Warner talking about the 95 reamer. http://benchrest.com/showthread.php?66956-308-Palma-chamber-reamer

the 852 and the Palma are not the same. you might have a small difference in freebore but the primers looking overpressure are from the AR10 bolt.

 

[mac86951]

AR10's can do this to primers. Buy some factory ammo and it will show the same primer results.

here is a link with Al Warner talking about the 95 reamer. http://benchrest.com/showthread.php?66956-308-Palma-chamber-reamer

the 852 and the Palma are not the same. you might have a small difference in freebore but the primers looking overpressure are from the AR10 bolt.

Thank you,
Odd but I can work with that. As stated earlier I did see it on the factory ammo, but was surprised to see it with what I thought were "mild" loads.

In theory then, a bushed AR10 bolt would resolve this issue, or turning down the gas port cycling, I'll try the latter first.

-Mac

 

[jonbearman]

You could try the CCI34 primers. I use them in my armalite AR10. They don't seem to flatten at all with moderate loads. If you read the manual they suggest using Federal gold medal match. They also note that they discourage reloads and it voids the warranty. Now we both know that they do this to cover their behinds. Just be careful how hot you make the loads as it wasn't designed for what we can use in a bolt gun. You may already know this but I just wanted to bring it up as a cautionary thing.

 

[mac86951]

You could try the CCI34 primers. I use them in my armalite AR10. They don't seem to flatten at all with moderate loads. If you read the manual they suggest using Federal gold medal match. They also note that they discourage reloads and it voids the warranty. Now we both know that they do this to cover their behinds. Just be careful how hot you make the loads as it wasn't designed for what we can use in a bolt gun. You may already know this but I just wanted to bring it up as a cautionary thing.

Thanks Jon, I'm on a rebarrel anyway, so beyond anything but the "I fixed my own brakes" warranty.

-Mac

 

[Rtheurer]

Are you getting signs of pressure or are you just getting primer cup flow from the excessive clearance of the firing pin hole to firing pin outside diameter? Not unusual to have the AR10 based rifles with larger than normal clearance in that area which can give the appearance of excessive pressure.
Just thinking out load...

I have a Match M852 Chamber on a bolt gun that shoots the 175gr SMK real real well at 2.80" OAL and varget. I get a lot of Velocity and can really push the loads if I so choose to. Gas gun however may add more issues than good ole bolt gun.

RT

 

[mac86951]

Are you getting signs of pressure or are you just getting primer cup flow from the excessive clearance of the firing pin hole to firing pin outside diameter? Not unusual to have the AR10 based rifles with larger than normal clearance in that area which can give the appearance of excessive pressure.
Just thinking out load...

I have a Match M852 Chamber on a bolt gun that shoots the 175gr SMK real real well at 2.80" OAL and varget. I get a lot of Velocity and can really push the loads if I so choose to. Gas gun however may add more issues than good ole bolt gun.

RT

I'm getting pressure signs. Standard AR swipe suggest a timing issue, extractor flow/marks can be either in an AR, protruding primer craters, and a primer that fell out tells me the factory ammo won't work, and handloads show cratered primers at even starting loads.

I'm thinking a cerrosafe will be needed to see what actual chamber dimensions are.

Another question for everyone: given a standard COL and bullet weight (ie 168gr at 2.800") what chamber dimensions would make a min stating load show pressure signs vs. Published maximum?

Distance to lands?
Lead angle?
Neck size?

-Mac

 

[rminut]

I have an M110 configured Armalite and went through some of this with mine. I had swipe marks and firing pin craters at starting loads. I ended up shifting the front sight base a little forward and this minimized the swipe. I use LC brass, 42.5gr 4064, and Hornady 178bthp and it shoots excellent now. the firing pin hole is still flat and that's why I say it's just trait of the bolt design. My barrel was done by Holiger at White Oak Armament and he used the Obermeyer chamber. 2.83" for the 178, .020" short of the lands.

 

[mac86951]

I have an M110 configured Armalite and went through some of this with mine. I had swipe marks and firing pin craters at starting loads. I ended up shifting the front sight base a little forward and this minimized the swipe. I use LC brass, 42.5gr 4064, and Hornady 178bthp and it shoots excellent now. the firing pin hole is still flat and that's why I say it's just trait of the bolt design. My barrel was done by Holiger at White Oak Armament and he used the Obermeyer chamber. 2.83" for the 178, .020" short of the lands.

Thank you, I've had lots of AR15 experience and an adjustable gas block can reduce the swipe. I'll get that going on my AR10 too, thanks too also for the load suggestion, that's where I want to try in the future, but so far it looks as though I'll be doing 155 testing with the had turned down.

-Mac

 

[mac86951]

Not to drag up an old thread, but really to close out a problem incase someone does a search:

My finding was that the Nosler Custom Competition factory ammo loaded had a brass length near maximum measurement. I've researched and found that maximum length 308Win brass can be upwards of 2.015" which is what my Nosler CC ammo measured post firing, but pre-sizing.

Upon comparing this to my reamer prints (Dave Manson provided me the M852 and his standard 308Win). The standard has an allowance for neck length of .321, vs. the M852 match allowance of .312

That reduction in neck length means I should be running brass at or below 2.010" length.

Hodgdon and my Lyman reloading manual has trim-to length of 2.005" which would indicate a maximum of 2.010" case length before trimming, but somehow Nosler is using slightly altered information. No problem, now I know and I'll be watching my brass closely.

Goes to show, that with a custom chamber do your homework and check everything! Even if factory ammo is supposed to work, check it too.

-Mac

 

[mac86951]

Final status in this old zombie thread. I was compounded by LR308 bolt firing pin slop. A JP bolt with matched firing pin and headspace check found that the accurate loads that were previously cratering the primers are now fully functional and still accurate.