300 win mag breaking cases

[TRPrecision/se3racer]

Not sure I should post here or in gun smithing, hoping here, but thinking it is a rifle problem...

I am not a rookie reloader, but am still a little green as this is my 1st go with a belted magnum, as well as my 1st "problem"

Rifle----Armalite AR30a1 300 win mag, today put me at around 100 rounds total fired.
Barrel was broken in using Winchester 180g basic hunting ammo, zero issues seen.

Subsequent rounds fired were my baseline starter loads to fire form brass....
-federal pull down demiled cases run lightly through the full length die for piece of mind.
-208 grain Amax over 77 grain H1000, LONG coal of 3.625, .025 off the lands.

Very 1st round of reloads fired produced a broken case just above the belt, split 90% of the way around the shell. This was on a cold clean bore. Case showed no pressure sign other than the obvious break, primer looked perfect, no ejector mark etc. Decision was made to continue to fire and monitor the cases. Fired another 25 rounds of the same batch of reloads with zero problems, figured the case failure was a case defect.

Fast forward to today. Again, cold clean bore, 1st round fired of the same batch of reloads, case splits above the belt all the way around leaving a broken shell in the chamber. As with before, this was the 1st round of the day, and the remnants showed no pressure sign other than the obvious case failure. Fired 3 more of the same batch with zero problems and decent accuracy and decided to proceed with the latter test I had come to perform. Shot my latter and found a VERY sweet spot, 11mm 4shot group at 200yds. Continued to fire throughout the afternoon with various loads of different power, never to see another failure.

Virgin brass is measuring 3.620" fired brass is coming out anywhere from 3.620 to 3.632".
What could be going on here? Thinking about bandsawing a few fired cases down the middle to look for internal stretch wall thinning. Any other ideas.

This rifle is wicked accurate, I don't want the chamber to be the problem, but I fear a long chamber may be to blame.

 

[PARA1977]

Sounds like I would have the chamber looked at sometimes the reamers are wore out are out of spec.

 

[watchingtrace]

armalite chamber very likely in spec. forget the case even has a belt..means nothing.

what are you measuring headspace with?
measure all the demilled cases to be sure there are not excessively short cases from who knows where that might have made their way into the lot of brass you bought. the case head seperation is erratic. the only 2 erratic variables in your equation..your sizeing or somebodys sizeing of the brass previous to your possesion

 

[watchingtrace]

p.s. your right in the sweetspot for your powder proj combo. well done. a 208 at 2890 ish will get alot of good stuff done

 

[Flashover16]

If you had no problems with the factory ammo then its the case of your reloads.
if you had problems with both factory and your reloads then you likely have a head space problem.
as belted magnums head space on the magnum band the place they are likely to separate is the weak point in front of the band.
either way you need t find out the problem before shooting it, going to get a nasty boo-boo if you keep pushing you luck. try looking into the case and see if you can't see the beginning separation right above the case belt.

 

[jglenn]

suspect your cases.

I'll assume those are 2007 FC cases... 8 year old brass from where? ( yes I have older cases)

did you prep the brass or did someone else. brass can get brittle from just sitting around

1st thing to check is for case stretching.. use a long gym clip unfolded then bent 90 degrees on one end to a sharp point.. use this to go inside the case and feeling for a stretched area where you are seeing the separation.. if it has already stretched the hook will catch on the area. check all your cases.

you also need to measure how much your shoulder is moving forward not just your case length

I'll assume you meant 2.620 for your case length...Trim length is 2.610 max is 2.620


betting it's simply old cases

 

[PA4476]

Was there any excess grease or oil in the chamber prior to the firing of the rounds that split?
Just a thought since it was the first round each time.

 

[Twitch2120]

Sounds like a headspace issue. Don't just assume a gun is perfect right out of the factory. The federal cases may be shorter than the new winny brass.

 

[YOTESHOOTER]

How do you have your resizing die setup, are you going down to the belt on the case, if so you may need to back off the resizing die .001 - .003 and do not touch the resizing die on the belt of the case, (belted cases do not like that at all) if this is case then you are working the brass to much thus causing the brass to stretch way to much. Then just wasting your time, primers, powder, cases, and projectiles too...

Could or could not be the situation, however we had this happen to us about 20 years ago on a 700 REMINGTON 300 WIN, MAG. Made the adjustments and have never had to look back again......

 

[jcountry]

Was there any excess grease or oil in the chamber prior to the firing of the rounds that split?
Just a thought since it was the first round each time.

That is definitely a possibility.

The chamber should be dry. A lot of people don't know it