Lack of obturation

[MrCetrizine]

I have a problem, I think.

I have a load for .308 using 175gn TMK on top of 38gn of N150. It's very accurate and consistent.

However, I've noticed that the walls of the case are a little bit sooty after firing. Not huge amounts, just a very light dusting.

It's this a problem?

I've tried a heavier power charge and although the sooting disappears, so does the accuracy. I've gone all the way up to cratered primers and never found another node.

I'm using very light neck tension. Essentially, I full length size with no expander in the die then expand with a .307 mandrel. This has given excellent results with 155gn and 167gn bullets and N140 with no sooting.

I'm wondering if turning down a spare mandrel to .306 would create a little more pressure to help the case obturate, while keeping the consistency I'm getting from resizing this way.

If a little bit of sooting isn't a problem then i won't bother.

For info, I'm using Lapua standard large primer brass which gets annealed every 3rd firing.

[DaveT]

I would guess that increasing neck tension as a means of increasing pressure and therefore MV is likely to push you out of your stated accuracy node just as upping the powder load appears to do.

Is there any chance that either your chamber and / or your brass had residual lube contamination which prevents sealing at the neck until overcome by higher pressure loads?

[duey]

How old is the brass? Could it help to anneal if you don't already? 

[Laurie]

It's a very light, low pressure load. For the old 175gn SMK, not that much different in internal ballistics I would imagine, QuickLOAD calculates a mere 32,081 psi chamber pressure and 2,200 fps MV in a 24-inch barrel. Viht's maximum (and its max loads are usually notoriously conservative) range from 42.1gn to 46.3gn for a variety of 175/180gn bullets and that's certainly where I'd look to be loading finding a bullet that works at the higher loads, velocities and pressures. (There is nothing wrong with light loads, but when you're as  low as that, pressures / velocities are rarely consistent especially when using a slower burning propelllant such as N150.)

 

However, if i works for you and you're happy with the results don't worry about mild sooting. It's just a nuisance cleaning the bolt, chamber and brass up afterwards. Changing the primer might alleviate the problem or even cure it. Don't muck around with neck tension for this reason alone.

[MrCetrizine]

Well after waiting for a bout of bronchitis to abate I've finally managed to test a few things.

Essentially I've switched primers to CCI 200 and increased my load to 45.6gn.

I was using federal primers which were cratering above 43gn but switching to CCI has enabled me to find an accuracy node at 45.6gn with no signs of pressure issues.

Thanks all.