A question of head space...

[AzDak42]

I was with a buddy of mine at the range on Friday and he had a massive over pressure round that stopped his testing for the day.

What he was finding was that his loaded ammo was really tight in the chamber. He was having to mash down the bolt handle to get it to close. This wasn't as much of an issue on factory ammo or new brass loads, but all his reloads were really tough to close the chamber on.

He lit one off and it blew the primer out, flattened the lettering on the case head and jammed up the ejector. He weighs each charge on a chargemaster, so I'm fairly confident that he didn't overcharge it.

In walking through this with him, it turns out his seating die was too high and it wasn't getting full length sizing. Once he adjusted the die the case was chambered without force and all was well.

My question for you all is.. His COAL was ok. So I dont' think he was jammed in the lands. What's the mechanic that would cause a primer to blow out if the case is jammed into the chamber?

I don't want to just assume this was because the case was not dimension ally correct and leave some other issue with his reloads for another dangerous incident.

The load he was running was about a 2725fps with a 140g Hdy HPBT. The brass might have been range pickup blended in with his from a match.

Anyone have any thoughts?

 

[ShadeTree]

"What he was finding was that his loaded ammo was really tight in the chamber. He was having to mash down the bolt handle to get it to close. This wasn't as much of an issue on factory ammo or new brass loads, but all his reloads were really tough to close the chamber on."

That there should've been clue number one in my mind that something is way off. If a case is fired in that chamber, even if it is not resized at all, there might be some resistance, but no way should you have to force a bolt closed.

All else being right, then a bolt closing extremely hard is either the bullet is jammed into the lands or the case was too long and the neck was jammed crimping it into the bullet causing a massive pressure spike.

Did he trim before or after sizing?

A harder closing bolt because of not sizing cases that were fired in that chamber even numerous times, is causing some added stress and possible premature wear on the locking lugs and will extract hard because there's no springback gap left for that case in that chamber, but that's not a pressure issue.

 

[ethmoid1999]

Is there a possibility of incorrect bullet diameter? You didn't mention the cartridge but .308 with a .311 comes to mind.

The cases did need full length resizing as indicated by difficulty chambering and case length may have also been an issue. A cartridge with an over-length case may be jamming into the lead and raise pressure.

 

[HeathSexton]

the case was too long and the neck was jammed crimping it into the bullet causing a massive pressure spike.

I was in a gun shop a few years ago when a guy brought a 25/06 in that he had locked up solid. Turns out he didn't trim any cases and the neck was jammed as you said. It was on a Savage and they had to remove the barrel because they couldn't get the bolt open. After they finally got the barrel off the bolt head was toast. The smith asked him why he didn't trim and he said he didn't know he needed to and thought it was just wasting powder space.

 

[ShadeTree]

the case was too long and the neck was jammed crimping it into the bullet causing a massive pressure spike.

I was in a gun shop a few years ago when a guy brought a 25/06 in that he had locked up solid. Turns out he didn't trim any cases and the neck was jammed as you said. It was on a Savage and they had to remove the barrel because they couldn't get the bolt open. After they finally got the barrel off the bolt head was toast. The smith asked him why he didn't trim and he said he didn't know he needed to and thought it was just wasting powder space.

I can see a hard closing bolt if using a case not fired in that chamber and not sized back down, (case too fat or shoulders to far forward) but unless I'm not thinking of something I can't see that making pressure by itself unless either the neck was too long and jammed, or the bullet was jammed in the lands. Both of which would also cause a hard closing bolt and spiked pressure.

The other possibility is all measurements were within spec prior to chambering. Case length, bullet behind the lands, etc. Except for the measurement of a fat case not fired in that chamber, and not resized.

Forcing the bolt closed on a oversized fat case for that chamber, would in essence do the same thing as sizing. That compressed and squeezed brass has to go somewhere and it can only go forward.

 

[SJB358]

Case neck jammed into the rifling will cause big pressure. A case that’s tight in the chamber shouldn’t cause excessive pressure if everything else is good. I’ve had tight cases a few times on WSMs but as long as the necks aren’t jammed you shouldn’t have excessive pressure spikes. It will show some bolt scuffing since it’s rubbing on the bolt face but that’s sorta a false positive of pressure.

 

[Anonymous]

As some others have said-If I'd wager a guess... his brass is over trim length and he's forcing the neck into the leade of the rifling.

Bad things will happen like that. It's not that uncommon either. I know three guys who have presses and none of them own a case trimmer.

 

[BretN]

https://www.midwayusa.com/product/47699 ... -lock-stud

Add a $5 case length gauge for your caliber, chuck it up in your cordless drill and trim away. I have a sporterized Mauser 30-06 with a tight chamber and abrupt leade. I have to watch case length very closely in that gun.

I have only ever used the above Lee trimmers. They are cheap, simple, caveman stupid to operate, but they work.

 

[AzDak42]

According to him, he'd trimmed the case prior to this reloading. But I'm going to ask him to check the trim length and make sure it was correct. I'm guessing if it was on the long end, even after trimming, that the oversized case did flow forward enough to jam in the leade. Should be easy enough for him to measure the case neck at a few points for both chambered and unchambered rounds.