308 Winchester .340 Neck Diameter Barrel

[kft101]

Hi, all.

I recently picked up an older custom built Remington 700 with a Hart barrel installed.

The barrel is marked “308-Win 340ND”, which should mean it is chambered with a .340” neck diameter and not the SAAMI spec .344” neck diameter? I mainly shoot FGMM out of my 308s, and having randomly measured some ammo I have on hand, they measure between .3385” and .3395” at the neck.

My question: is it safe to shoot the factory ammo I have through this rifle? A knowledgeable source says anything with less than .003” spacing between the round and chamber at the neck may result in pressure and safety issues.

Thanks in advance!

Pic of the barrel markings:

 

[Dolomite]

If your loaded ammo neck diameter is that close to .340 i wouldn't shoot factory ammo it will pressure out immediately.

 

[PelletTrap]

Interesting I've never heard of "340ND", wonder why it was cut that way?

Dolomite, when you say "it will pressure out immediately", could you explain please? I'm not familiar and trying to learn...

 

[XTR]

I made the mistake of shooting factory loads in a tight neck 308 a long time ago. I’ll just say it was hard to get the bolt open. I would want 2 or 3 thou clearance.

@PelletTrap , it’s a custom chamber for someone who turned his brass.

It’s one of those bleed overs from BR. It may make a difference there (not sure it really does) but the belief is that having a chamber neck that is close to the brass neck improves precision. I believe all my 308 reamers are .342 necks. It might make a difference in BR, form the rest of us I think it’s just another thing to worry about that doesn’t help.

@kft101 , you could have it rechambered, or reload for it with WW brass. Winchester brass is typically 12 to 12.5 neck thickness. Unturned Lapua brass might be too tight, been a while since I measured new stuff but I seem to recall it’s about 15 thou., that is getting into your margin for error.

 

[Will Henry]

I would be inclined to not shoot unturned necks in this. In general, if you have any clearance at all, you should be safe. The 308 operates at pretty high pressures though, so you don't want to risk it. If that was mine, I think I would ream the neck out to .342.
For many years, it was commonplace to minimize neck clearance, in search of increased precision. Tight necks, which mandated turning, were SOP. I still chamber my BR rifles with tight necks, but just because I always have. Reducing the working of the neck is a good thing, I suppose, but I'm not so sure accuracy is enhanced at all. Concentricity and alignment are what matters. WH