6BR overpressure, at the bottom of tables

[Hogu]

Hi All,
Currently reloading for 300M ISSF matches I have experienced a fairly weird overpressure occurence during a match
Rifle is a match rifle, LW 26' barrel with 1/8 twist rate

During these matches firing rate is roughly 75 rounds in an hour,
we share the rifle for 3 shooters and we clean the barrel as well as we can due to time constraints between matches.

After the first 30 rounds had a small hard point at the end of the upper bolt stroke, nothing major but the brass started to show some ejector marks, but seldomly, the bolt operation remained smooth between these events. During the second match (roughly 100 rounds in the day) my fellow shooter had a stiff bolt, with evident signs of almost catastrophic overpressure, as you may see on the case shown below



the bolt locking studs show a minimal damage not impairing the bolt action, gladly


Our Load is:

Norma brass, once fired in another firearm
Sized using custom to our chamber dies, .267 bushing with 0,005' shoulder bump
TTL 39,40 mm using RCBS TRIMPRO 3 way cutter
Flashole deburred using K&M tool
No neck turning nor annealing

CCI BR4 Primers

VV N150 powder at 1,81 gram (27,9 gr)
Vihta load data starts at 1,85 g and ends at 2,05g for 105 gr Lapua bullets

107 gr Sierra MK seated 0,1 mm (0,005') off the lands, for COAL 60,5 m, CBTO 46,40 mm

I am at lost regarding the cause of said overpressure
-> Too light load causing secondary explosion effect ? might be weird as i am really slightly below the min load
-> Bullet jump too tight making the bullet contact the lands as the barrel heats up ?
many shooters put the bullet in the land in BR as well in ISSF rifles AFAIK
-> N150 acting due to cold weather ? (5°C during the match)
-> carbon ring ? the barrel is cleaned thoroughly after every session with Eds red and Robla Solo using a BR procedure

Well, we will shoot Manufactured ammo for the upcoming French national championship due to this unwanted halt in our reloading developpment

Many thanks to everyone of you who can help me out

 

[Keefe300]

Are you shooting a turn or no turn neck?

Can you slide a bullet into a fired case? If so you can eliminate clearance as a potential issue.

Carbon ring in front of the case neck can lead to pressure, not allowing the neck to expand and stretch while releasing the bullet.

Just a couple of quick thoughts

 

[Hogu]

hi, thanks for your reply!
unturned neck as i dont have invested yet in a neck turner and wasnt aware it could play in pressure building, but was planning on doing so for accuracy purposes in the long run

I can slide a bullet in a spent case with my fingers, requires a little push but doable without much effort

about the carbon ring, we cleaned the rifle quickly before the second match focusing on carbon removal, but it wasnt a complete procedure.

thanks again for your time

 

[CJ6]

I can slide a bullet in a spent case with my fingers, requires a little push but doable without much effort.

thanks again for your time

 My thoughts. If it requires "effort" your necks are too tight.

 

[fkimble@charter.net]

Carbon removal from the barrel is not the same thing as carbon removal from the throat.

Frank

 

[snert]

^^^ Im thinking along these lines. Someting wong

 

[Capt. Oblivious]

Measure the OD of the Necks on one of your fired cases

 

[ATFIV]

Borescope will tell you when it's clean.

 

[6MMsteve]

is your barrel loose? maybe my eyes r playing tricks on me

 

[6MMsteve]

I think .010 is max tenon to bolt head max I could be wrong though won't be the first or last

 

[JEFFPPC]

I agree with Keefe plus overall case length must not be to long but too short just allows for a bigger carbon ring. Only way I know to set it correctly and also help on carbon ring control is drill out your longest case through the primer hole so you can look through it with your borescope and see exactly where your case neck ends. I like to keep mine as long as possible cutting down room for carbon build up but that does require checking length every reloading.

 

[Coyotefurharvester]

You shoot as a team, do you reload as a team?
1. Since these are reloads, possible reloading error.
2. You didn't say how soon after cleaning the over pressure occurred, so bore or chamber solvent/lube.
3. Did the round before the event sound different, have an out of group impact?
4. Could lube or solvent inadvertently transferred to a round used in the second string of fire.

 

[Hogu]

Hi everyone,
thanks for your replies

Planning to get my hands on a borescope, any recommendations ?


As for reloading Error, my best guess is electronic scale drift as it is a 50$ jewelry scale ... yet it proved reliable so far (being tested with witness weight at every startup) and i dont think a few kernels off may have caused such havoc

I reload alone, one of the shooters helps me for tasks such as depriming, cleaning or trimming but the most crucial parts are dont by myself.

the overpressure occured lets say 40 rounds in the match, with no effect to the group nor any signs prior.

as for the cleaning we thoroughly dried up the bore with dry patched prior to shooting

@Steeve
There is some gunk around the barrel but it is firmly in place, however i have spotted stresss marks and will get the rifle checked prior subsequent shooting

 

[SecondsCount]

I'm leaning toward a powder overcharge. That's a lot of pressure.

 

[Rocketvapor]

Teslong Borecope from the muzzle.
22 Nosler brass.

 

[Hogu]

Teslong Borecope from the muzzle.
22 Nosler brass.
View attachment 1548826

impressive quality, not like my cheap wi-fi borescope, i've ordered one

 

[Hogu]

Measure the OD of the Necks on one of your fired cases

i will get back to my man cave in the end of the week and keep you posted !

 

[rjtfroggy]

I would be more concerned by what appears to be a crack in the lug, right at the burn spot.

 

[Hogu]

I would be more concerned by what appears to be a crack in the lug, right at the burn spot.

Checked it last night as it was unsettling me hard, the metal has stressed a bit, the crack appears more to be a scratch, doesnt catch the finger nail nor the tip of a cleaning pick,
will still get the action out of the stock for measurements and get the rifle checked before the championship

 

[6MMsteve]

chk headspace first, grab barrel and see if it turns....get that out of the way it just don't look right and the case shows swelling for a possible loose barrel.....actually hope it is, easy fix but with same person who headspaced the barrel...hope u get it worked out