extreme pressure,what happened?

[3-300s]

By this data 38gr should have been ok, but nosler versus federal brass.
I shot over 39 grs before primers started to show signs of pressure.
50 or so rounds fired since cleaning.
necks measure .297-298" on pressure case and several normal ones.
I have two Bartlein barrels chambered with this reamer and smith (m40 and #3 contour) .
500 or so rounds have been fired through the M40 barrel with no problems.
I'll call JGS and get a reamer print, I don't know how to measure the chamber length.
Dedicated trimmer set up for the 260 and I spot check cases while trimming.
The pressure case is a few thousandth's longer. Maybe I didn't trim it properly and it clamped on the bullet like other have said.
Powder smells normal.
I did go the range yesterday and shoot the remaining loaded rounds.
No problems with pressure or accuracy.

Frank, I really appreciate the offer. Bartlein should be a model for others to follow.


Robert

 

[T-shooter]

I have seen 175gn come out of 155.5 boxes for target shooting where pressures are high for the 155.5 pill. Think about that problem if it had not been found. Blow up if fired disqualification if the ammo police found it in the ammo checks. 155.5 max Palma.

Imagine a 7.62 bullet in a box of .308. About .004" larger in diameter.

 

[Bindi2]

The 7.62 x 39 uses 310/311 projectiles but some in there wisdom made 7.62 x 39 barrels 7.62/308 diameter so a big problem.

 

[T-shooter]

I had an overpressure problem several years ago. It wasn't hot outside, maybe 60°. I was working up a load and figured I was safe until I got close to the limit and was going to visually check the 45.0 and 45.5 loads. I loaded 4 each from 42 grains up in 1/2 grain increments. I was using Hornady bullets which are almost identical to the Sierra. Everything fired fine until the 45.0 grain load. The rifle kicked a lot harder, was noticeably louder, the bolt stuck and I had to use a piece of wood to pound it open. No more testing! I got home and noticed all 4 of the 44.5g loads had ejector marks on the cases. The 45.0g had a large brass flow sticking out and the case head expanded so much the primer fell out. And that was well under the printed maximum from Hodgdon's site.

The only differences, I used CBC stamped cases, a 1/10 twist 22" barrel. I checked a couple of the case volumes and the CBC was 3.499cc average and the Winchester was 3.642cc, about 4% less capacity. The Hornady 168g Match are advertised to be .025" longer than the Sierra which would have decreases the case capacity even more. All that brass went into the trash. No need for hot loads, the rifle shoots much better at 41.5 to 42.0 grains of IMR4064.

 

[3-300s]

I just learned of the Sinclair chamber length gauge. Ordering one for every caliber I have.
Starting to feel that case length and or wet chamber was the cause.
With case length the most likely. Anxious to measure this chamber and see.

thanks everyone for your help.
Robert

 

[Bindi2]

Another way to see what is really happening is use a bore scope from the muzzle to see where your brass is at in the chamber. One tool for all rifles.

 

[tommeboy]

Two things you have. Partitions are a much harder bullet and will raise the PSI sometimes ALOT. Then you have a 6.5mm. They are known to blow. How many rounds have been through the barrel. How is the throat?

 

[Webster]

Hello,
Had a problem with high pressure and don't know the cause.

This is a hunting gun. Rem 700 in 260 Remington with a Bartlein 1x8" barrel. It was chambered with a reamer made for my case and bullet combo. Bullet is jumping about .030".
I had a shell blow out the primer and I felt gas on my face when I fired . VERY hard to open the action.Had to remove the bolt to get the shell off the bolt face and primer fell on the floor. 260 rem, 125gr nosler partition (blem), federal case been fired three times, 38gr of IMR 4064, WLR primer. Load has never shown pressure, I had loaded 60 (at one time) of these last year for hunting and shot all of them but 5 or 6.

measurements of high pressure case.
case web .471"
case rim .4755"
case length 2.035

measurements of same load fired case without pressure signs
case web .470"
case rim .471
case length .2.029

measurements of a loaded case
case web .469"
case rim .471
case length 2.032"

No chance of wrong powder or bullet. All charges weighed on RCBS beam scale.
39.3gr is what I consider max load in this gun.
Cases are cleaned with acetone then tumbled after sizing.
Shell was in the magazine when gun was fired 3 times, thought bullet may have slid out the neck.
I tried to make bullet slide out the neck by closing the action HARD ten times on one loaded shell. It didn't move, slim

chance bullet was in the lands. ​

It was raining that day but I carried the gun muzzle down to a covered box stand.
I don't remember if the action was hard to close or not?
Need to get the reamer print and see if case is to long and clamped down on the bullet ?
Have not fired the gun since.

I'm at a loss , no real idea what happened.
My only problem like this in over 40 years of reloading and it scared me.

Robert

Your above max load in 2 of my reloading manuals. Funny how putting less powder in the case solves the problem.