Case head separation.

[Mram10us]

So I've always used rcbs dies, I couldn't find a 300 prc in rcbs so I got a hornady. They want it set till the shell holder just touches. But in a side not it says it can be backed up for different type of chambers. In the rcbs die there isn't any way to get it wrong. Once the shell holder meet the die it can't go any farther, the internals can't be moved other than the de priming pin. I haven't found any type of better instructions on backing it out. But the measurements all seem right so I don't think it's a resizing issue. Just seems like junk brass.

GH, The rcbs die still screws down which is what we are talking about. Again, take a post it note, fold in half, put in on top of the shell holder with the ram up, THEN screw the die down till it touches the paper. See if that case will chamber. Hope you are understanding the instructions. Not always as clear cut as I think when I describe something

 

[Grizzhunt]

Any die set can "get it wrong". Ideally, you want to set the die to move the shoulder back 0.001-0.003" only. Typically, the "screw it in till it touches" method will give excessive head space, as they build the dies to work for the shortest chamber that might be encountered.

My OEM 300 WM barrel had a particularly long chamber, and back before I measured such things, I followed the die instructions. On the third round of reloads, I got separations just above the belt, with Federal brass. I assumed it was bad brass. I then bought a bunch of MAI (Norma) brass.. After 2 reloads, I bought a comparator set, and started measuring things. Using the standard die instructions, I was bumping the shoulder back 0.008"!. I have the RCBS case measuring station, and was able to confirm significant thinning at the web, after only two firings.

Since then, I've been bumping that same MAI brass ~0.002" on resizing, and they've taken another 3 firings. A few are culled each reload for excessively thin webs, but most are still good for at least another firing or two. Had I not put the first two firings with excessive head space on them, I bet they'd have been good for 8-10 firings (I anneal necks).

Anyway, I would strongly suggest measuring your actual headspace with a comparator, on your once fired brass, then again on your sized brass. My guess is you've got excessive headspace (shoulder setback).

I just bought a comparator set at sportsmans. Unfortunately they didn't have any brass or loaded ammo. So I'm out of one fired stuff unless I can't talk another guy in town out of his. I'm not sure if he's reloading or just shooting factory.

 

[Mram10us]

Is all your brass already resized?

 

[Coyote_Hunter]

Case head separation can be caused by a hunber of factors, but full length sizing and too much (but still within spec) headspace can exacerbate the problem. I prefer to set my headspace to about .003". (That may be too little for some factory ammo, don't know - I shoot 99% reloads.)

If you want to measure headspace, I've had good luck with Plastigage, which is available at any auto parts store. When I first tried it I feared it would just smear when the bolt was closed. Instead it just compressed and spread out, as desired.

The SAAMI spec for the .300PRC is .010" maximum headspace. Blue Plastigage will measure from .004" to .009".

Second item - a 1/8 turn on a 7/8x14 threaded die is .0089". Call it .009", which is near the max headspace allowed. I'd use a feeler gage to set the distance between the die body and the shell holder and increase the distance by .002" (equivalent to about 1/36th turn of the die) until a fired/resized case allows the bolt to close with a bit of resistance. Then I'd back off by .002" or .003" and resize the rest. If using Plastigage to check headspace, you can just subtract .003" from the Plastigage measurement and set the die with the remaining number.

 

[Grizzhunt]

Is all your brass already resized?

Unfortunately it is. I threw all of it away that wasn't loaded already. I saved one to try and measure but after getting a comparator kit. I can see it's aways off. I hope that solves my problem. Now just need to see if the FL die will work or if I have to buy just a neck.

 

[Grizzhunt]

Case head separation can be cause by a hunber of factors, but full length sizing and too much (but still within spec) headspace can exacerbate the problem. I prefer to set my headspace to about .003". (That may be too little for some factory ammo, don't know - I shoot 99% reloads.)

If you want to measure headspace, I've had good luck with Plastigage, which is available at any auto parts store. When I first tried it I feared it would just smear when the bolt was closed. Instead it just compressed and spread out, as desired.

The SAAMI spec for the .300PRC is .010" maximum headspace. Blue Plastigage will measure from .004" to .009".

Second item - a 1/8 turn on a 7/8x14 threaded die is .0089". Call it .009", which is near the max headspace allowed. I'd use a feeler gage to set the distance between the die body and the shell holder and increase the distance by .002" (equivalent to about 1/36th turn of the die) until fired/resized case allows the bolt to close with a bit of resistance. Then I'd back off by .002" or .003" and resize the rest. If using Plastigage to check headspace, you can just subtract .003" from the Plastigage measurement and set the die with the remaining number.

Great info and help. I pulled a bullet out of a loaded case and it measures 2.191, I save one fired piece from the rest of the junk it measures 2.197. So I will back it out and try again. Unfortunately the remaining brass I have is pretty much work hardened and most likely going to have a separation mark after this firing.

 

[baldhunter]

I have fired thousands of rounds out of many rifles I load for,but I've had only one rifle that I had issues with partial case head separation like the one pictured.This particular rifle was sent back to the manufacturer for warranty because it grouped poorly.When I got the rifle back,they not only replaced the barrel,but also replaced the receiver.My guess is the receiver must have been out of spec. when they checked it out.Could have been the cause for the occasional partial case head separations I was experiencing.

 

[Grizzhunt]

I have fired thousands of rounds out of many rifles I load for,but I've had only one rifle that I had issues with partial case head separation like the one pictured.This particular rifle was sent back to the manufacturer for warranty because it grouped poorly.When I got the rifle back,they not only replaced the barrel,but also replaced the receiver.My guess is the receiver must have been out of spec. when they checked it out.Could have been the cause for the occasional partial case head separations I was experiencing.

That sounds no good. I don't think this is the case with mine. It groups really well even with the issues I'm having.

 

[piute]

No need for neck sizing dies. Your dies are more than likely fine just set wrong.
Use { TDOT's post #27 suggestion}. Let us know what you find.
Good Luck

 

[baldhunter]

I don't think the receiver was the cause for the rifle to group poorly,I think it was just found to be out of spec when the rifle was checked out.I think the receiver may have been the cause for the case head separation problem.Even after I got it back it still didn't shoot that great.I had this rifle rebored to a 338-06 and I couldn't ask for it to shoot better,so the poor grouping was from the barrel.

 

[hoffbill]

The only time I had head separation issues, the cause turned out to be excessive headspace. It was as it had been set at the factory. When I checked it it failed the no-go guage test. Corrected the headspace on the rifle and the problem went away and accuracy improved dramatically.

I buy and refurbish old rifles as a hobby. Head space check is now one of the first things I do when I start working on one. Surprising how many have excessive headspace when tested with a no-go guage, and in most of them they have not been fired enough for it to be from normal wear.

 

[jerry moore]

measure a new case oal & fire one round & measure it again you could have excessive headspace

 

[DSheetz]

What is the little square piece of card board sent with the RCBS dies used for if not to start setting the dies for full length sizing ?

 

[Kbaker657]

As said already case head separation is almost always a situation of excessive head space on resized cases. You running near or max loads exacerbates the problem. I have found with multiple rifles of the same cartridge and manufacturer the head space can vary quite a bit. The only way to be sure is to measure headspace and set it back minimally as stated- .001 to .003. If the factory loads started out short for your chambers headspace dimension then it may take two firings to form them all the way to your chamber. Had your load been milder it may have been less violent and not caused separation.