Case head separation.

[Tikka300man]

So I just got my 300 prc built a few weeks ago and have been shooting factory hornady match 225s. I wasn't getting the accuracy I wanted so I got a die and started doing some work up. I found a load my gun really liked at 79 grains of retumbo I set oal to the same as the factory stuff. Shot a 3 shot at .20 on center. However when I loaded it a second time testing the speed I got a case head separation. The brass has only been good for 2 firings. Just wondering if anyone has had any issues like this. Maybe a bad batch of brass?

Case head separation is a sign of poor head spacing. You are probably pushing the shoulder a little too far back when resizing. This causes the case to be too short in the chamber then it stretches upon firing. You will need to adjust your dies so they don't push the shoulder back so far. There are different ways to determine the proper setting from adjusting by trial and error to having your chamber actually measured. The OAL is only one of the factors in being able to chamber a loaded round; from the base to the shoulder is another critical measurement not to be overlooked.
You can Google solutions to your problem, but I'm 99% sure this is what's causing it.

 

[Rflshootr]

Take a 44 MAG pistol case, de-prime it, then put the mouth of that case on the shoulder of a once fired rifle case and take a measurement. Then as you adjust your sizing die down, keep taking measurements until you see .002 less then the once fired case. Then try it in your chamber. Many SAAMI specs take the headspace from a datum diameter of .375 give or take. The 300 PRC's datum diameter is .420. A 44 caliber bullet is .429. Close enough for adjusting your die.

https://saami.org/wp-content/uploads/2018/07/300-PRC-Public-Introduction.pdf

 

[Warbird2006]

So I just got my 300 prc built a few weeks ago and have been shooting factory hornady match 225s. I wasn't getting the accuracy I wanted so I got a die and started doing some work up. I found a load my gun really liked at 79 grains of retumbo I set oal to the same as the factory stuff. Shot a 3 shot at .20 on center. However when I loaded it a second time testing the speed I got a case head separation. The brass has only been good for 2 firings. Just wondering if anyone has had any issues like this. Maybe a bad batch of brass?

Hornady brass is crap, it happens all the time. Get Lapua or Norma

 

[cuslinger]

I believe after reading your post, making an assumption, you resized your cases back to SAMMI specifications thus over worked your brass. Most factory rifles have approximately 6-10 thousandths excessive headspace. If you have tools to measure headspace of the factory cartridge before firing and then after you will get an idea how much brass flowed forward to seal the chamber.

If you do not have tools to measure you could try this approach; remove the decapping/expander ball stem, just makes less stress on the case, lub and then smoke the shoulder. Prior to sizing the case set the sizing die per manufacturers directions, die should touch the shell holder, then back off one or 1.5 turns leaving the lock ring loose. Run the case up just until the touches the shoulder; this will take several attempts while adjusting the die down incrementally. Once the should had been set back try it in your chamber; good tun the case back into the die, this will center the cartridge and die in the threads and secure by tighting the lockring down. Using a feeler gauge one could measure from the shell holder to the bottom of the die. Redding makes Competition Shellholders Sets for the .532 head diameter in +.002" increments up to +.010".

 

[PNWdude67]

I don't have a gauge to check it. I've always just set my full length dies and ran with it. I've never had any issues till now. I did try to measure it, it's basically the same. It's hard to see 10 thousandths just eye ballin it. I'm going to get a gauge this afternoon, and possibly a new batch of brass if they have it. If not then just some more boxes of the match ammo.

Too much shoulder bump...if you're winging it it can be easy to go 12-15 K bump and create an issue. Interested to know what you find out after measuring a fired vs unforced case you loaded with the die set to the same bump as the separated case. Good luck figuring it out and glad there was no injury or damage to your new gun.

 

[Triple BB]

I had a case separate in a new 300 RUM. Played around with getting proper headspace and then another case showed similar signs. Ended up getting a set of Redding's Competition Shell Holders. That took care of the problem. Not sure if he's still a member, but Broz makes it simple on how to use them in this video.

 

[Mowgli Terry]

For the sake of finding out, what if you neck sized the cases on the next round of reloads. Not making any bets one way or another. I'd add die problems in this mix.

 

[Lefty G-gear]

So I just got my 300 prc built a few weeks ago and have been shooting factory hornady match 225s. I wasn't getting the accuracy I wanted so I got a die and started doing some work up. I found a load my gun really liked at 79 grains of retumbo I set oal to the same as the factory stuff. Shot a 3 shot at .20 on center. However when I loaded it a second time testing the speed I got a case head separation. The brass has only been good for 2 firings. Just wondering if anyone has had any issues like this. Maybe a bad batch of brass?

I tried to find you a .300 PRC case gauge - no luck. (Get one!)

But here's an alternative:
Headspace gauges.
GO GAUGE
NO-GO GAUGE
FIELD GAUGE

IF the bolt action closes on a Field gauge - you are in danger! Too long!
I have seen new rifles fail here.
Send it back.

And read this whole page.
http://pacifictoolandgauge.com/saam...18228--saami-cfr-300-prc-headspace-gauge.html

Gee. I'd ask the gunsmith to show me the gauges he used, and demonstrate that everything is good, so as not to blow your head or hand off.

 

[cajun]

Had this happen on a 350 rem mag. At the time I was just setting the dies by feel with the bolt closing. This however was a single shot encore. Bought the Hornady comparator and the lee dies touching the shell holder were pushing the shoulder back .014. I now set up every fl die with the comparator. .006 is a lot but not what I call excessive. I suspect the hot load was part of it. What was the velocity?

 

[Lee Goodwin]

Shoulder set back too far..?, info from wiser men than me.
Neck size or what Mram10us said... don't FL size all the way till the die hits the shell holder...back off 1/4 turns.
Or just till the sized brass fits firmly in chamber, before powder and bullet.
My brass of mixed variety did same after 4 to 5 reloads of aggressive loads.

 

[25WSM]

Cuslinger
Where do you get that most factory rifles have .006 to .010 to much headspace. I've been a professional Smith for over 30 yrs and can tell you that's a very false statement. Minimum to maximum headspace on most cartridges is .003. I guarantee every manufacturer has all the headspace gauges for every clambering they make. And they use them. I've been to 3 major rifle manufacturers and they check headspace on every rifle barrel that goes on. I have seen less than a hand full of factory rifles have too much headspace. I have seen excessive headspace in numerous old military rifles. So basically you say most factory rifles have headspace 2 to 3 times longer than spec. Not true. As far as the issue the op is having most have told him it's excessive headspace which it probably is. But most likely excessive by the die to far down. If you don't have the tools to measure your setback of the shoulder it's all a guess. Also the little cardboard piece in the rcbs die box has nothing to do with setting up your dies. It has a rust inhibitor in it.
Shep

 

[antelopedundee]

Regardless of how you're resizing the cases, I expect that the lion's share of the case damage [somebody correct me if this is incorrect] is being done in the initial firing. So you should think about fireforming them as I described earlier. I've been told [and used with great success] that you want your loads to be stout with the bullets jammed a tad. If you're not comfy with this [esp. if you think the loads are too hot as is] then I would suggest to pull the bullets completely and remove a couple grains of powder and reseat them to lightly jam and commence fireforming. If you pull the bullets it likely won't be necessary to resize the necks before reseating the bullets.


GOOD LUCK.

 

[D Scott]

My gun likes 78 grains, 79 is a little warm, what is the measurement from your fired case headspace and what your sizing with your dies ? My hornaday die can size almost size.010 or more less than my chamber size, though admittedly the hornaday FLS die is hard to adjust, Im looking to Whidden sizing die.
WHat was your seating depth ? I have 4 reloads on my brass with no pressure issues except some faint ejector marks. Aim for bumping that shoulder back 2-3 thousanths that should reduce the brass changing so much. Are you measuring each powder charge and ttrickling up ?

 

[D Scott]

Virgin hornaday brass, unfired comes in the bag .005" of an inch less than what comes out of my chamber, so controlling that sizing die bump is pretty important, for brass life and accuracy. Hoping Bertram or Lapua steps up to make better brass