Pressure Signs - Too Hot?

[dtrostel]

Cal: 300 WM
Powder: 69.5 gr of IMR 4350
Bullet: 200 gr TSX Barnes
Velocity: 3050 fps

Concerns:
After doing load workups for 3 different bullets (BT, FB, and the TSX)I liked how the TSX was grouping (about 1.5" at 100 yds with my hunting rifle on a front rest) I shot 15 rounds in my Browning A-Bolt, 26" barrel and most of the brass showed no signs of too much presure. So I loaded 50 more rounds.

I had 3 cases that were silver (or nickle or something I am not sure) that I have had around for about 5 years that I considered my lucky cases cause they were shiny and I always got an elk with it. After shooting them, all three had a crack around the case about .5 inch up from the bottom of the case. One broke off during ejection and the casing remained in the chamber. The cracks are right about on the spot where the full length sizer stops on the way down on a sizing pass. None of the other 15 brass rounds showed any signs of cracking. And I never anneal my brass.

My concern is obviously loading them too hot and having something bad happen (explosion etc). So should I pull the bullets out of the other 50 rnds and reload or assume its ok cause the silver brass was probably overworked? Has anyone else experienced this problem?

I have never wanted to load a hunting rifle to its max just for the safety factor, and I plan to cut down to 68 grains of powder on ensuing loads. I have been loading for about 15 years and have never had this problem.

Thanks in advance and any opinions/facts will be appreciated,
DT

 

[RonAKA]

A few issues to consider:

1. The crack just up from the head is not uncommon. It is a sign that your headspace is allowing the case to stretch to fill the chamber on each fired shot. There is a stress concentration right up from the head so it tends to stretch most in that spot. When you have enough resizings pushing the case back and then stretching on firing, it finally cracks from fatigue. A friend has a 7mm RM that has this issue. He made a little wire rod with a right angle hook at the end. Before he reloads each case he uses the hook to check for the thin spot inside the case. It is quite easy to detect. Not sure of his criteria, but at some point they go in the scrap bin. Another solution if you can get away with it is to just neck resize instead of full length. This avoids the restretching on each firing. The root cause may be excessive headspace, so you could have that checked too.

2. Silver "brass" - No experience with these. Not sure they are brass, and I've always avoided them for reloading. They may not be as ductile as brass, and the reason why they would be the first to fail.

3. According to Hodgdon data you are right at the maximum for IMR4350. The one issue with the IMR powders is that they are more temperature sensitive. So while you may get away with that maximum load in cold weather, you may not in hot weather. The Hodgdon line is less temperature sensitive, and my preferred choice especially for a hunting gun used in a wide range of temperatures. I would pick Retumbo or H1000 for that caliber and bullet size. See this link:

http://www.hodgdon.com/extreme.html

 

[dtrostel]

What head space do you use?

I full length size each case because I am loading for 7 different chambers. I dont want any tight fits for some and a "just right" fit for others, I want the cartridge to fit all the rifles.

Also, my question is, even though it may be harder on the brass, is it dangerous to shoot the ammo I have reloaded already?

Thanks,
DT

 

[rocketron]

i'm not 100% sure but i think case head seperation is a common problem with the belted cases.

Ron

 

[RonAKA]

My understanding is that belted cartridges are "headspaced" based on the distance to the muzzle end of the belt, instead of to the actual shoulder. I've never really got my head around it, but there is criticism of the belted cartridges not providing the accuracy of non belted. Potentially that is because you have to allow some minimum "headspace" to the belt dimension, and also allow another clearance to the shoulder. So possibly two tolerances that have to be positive add up.

However, while a belted may end up with more headspace, I have seen the separated head on a .308 too. I have a belted case in two guns and mine all die eventually (20 loads without annealing) with split necks. I've never had the separated head issue. I think the bottom line is that too much headspace catches up to you.

To the OP I know I avoided your question on whether or not the cases are safe. I've not heard of any serious accidents due to head separation. Perhaps others have? One less serious but ugly part is losing a gun for a hunt if the case stays in the gun, and you can't get it out.

If you have good records by the type of case you have, I would pull all the cases of the same type and # of loadings as the ones that have failed, and unload them. then do the check with the wire hook. Perhaps others have a more sophisticated method?

 

[Glocksig]

Check this page out, they make a die specifically for belted magnums, may be of some use to you:

http://www.larrywillis.com/windex.html

 

[Laurie]

As noted by RonAKA, belted magnums 'headspace on the belt', that is the belt has exactly the same function as the case-rim in cartridges like .30-30 Win, .30-40 Krag, 0.303 Brit etc., except that instead of it being a flat rim that butts up against the rear face of the barrel or bottom of a flat rim-dia. recess, it's a little further forward and the front of the case-belt butts up against the front end of its recess in the chamber.

Belted cartridges were devised by Holland & Holland c. 1912 for dangerous game numbers used in bolt-action African rifles to give the positive headspacing of a rimmed case, and the smooth and reliable feed from the magazine into the chamber that you get from true rimless rounds.

Rimless rounds 'headspace on the shoulder', ie when the firin g pin hits the primer, the entire round is pushed forward in the chamber until the case-shoulder is stopped by the matching front end of the chamber. It obviosuly requires a relatively small case-chamber clearance. It was thought by many at the time that this requirement was undesirable in a dangerous game rifle as it needed a closer case-chamber fit than is desirable for 100% ease of chambering and extraction.

There are two downsides to the case-belt apart from any extra expense in forming the brass. First, the belt is right above the case web that sees maximum pressure effects on the brass, so the belt dimensions change marginally on each shot and reloading. Since that belt is crucial to cartridge fit in the chamber, it means that you never get 100% consistency with a maximum loaded cartridge over its life. This is of little interest to hunters who don't reload or who get more than good enough accuracy. It makes belted cartridges inferior to rimless numbers for long-rnage precision shooting, hence their displacement in long-range competition by (rimless) short magnums. The brass also expands just above the belt and full-length sizing dies don't reach there, so cases can become a tighter fit over a few loadings. That is what Larry Willis' collet die is for - it reduces the body diameter just above the belt.

Secondly, and this is where your problem lies, because the key case-chamber fit area is the belt and its chamber recess, case and rifle manufacturers don't worry over much about the fit at the front end of the chamber as it's not important to headspacing. In fact, they make the case deliberately loose here to facilitate relaible feeding. Resize the case with the die in the default position hard against the shellholder, and you usually push the case shoulder back too far causing case stretch on each reload / firing with eventual separation at the weakest point / area of maximum stretch on the body, usually just above the belt. The answer is to treat your cases as if they were rimless on resizing, backing the die out in the press so that the resized case only has a small amount of clearance with the front end of the chamber - just enough that every resized case chambers reliably in your rifle.

The 'silver' finish is nickel plating over a brass case. It weakens the brass and makes it more brittle, hence giving a shorter life. This may not have made all that much difference in your rifle. There is a recent post on this forum in which Sleepygator confirms nickel plated cases are a bad thing technically, for the handloader at any rate.

http://www.accurateshooter.com/forum/index.php/topic,3749024.0.html

Your ammo in the other cases may or may not be OK for use depending on the number of firings etc. Examine their bodies very closely on the outside at the point where your nickel plated brass separated looking for a ring (often asymmetric) of metal that shows signs of stretching - this is called an incipient separation. Do you remember breaking a piece of soft metal when you were a kid by bending it backwards and forwards along a line until it broke? First the metal along the fold changed colour and showed signs of stress, then a thin crack appeared, then finally the sheet broke into two pieces along the fold. You're looking for those early symptoms. The other test is with a fired case using the hook (a straightened paperclip with short bent over bit at one end is fine) to feel for the 'valley' inside the case body at the point where it's stretching and the brass is thin as already outlined by RonAKA.

Laurie,
York, England