308 W neck dimension?

[johned]

My Sierra reloading manual says the loaded cartridge has a neck diameter of 343". My loaded PMC cart. has a neck diameter of 335.5". My fired brass is 346". I know it must spring back from the full diameter of the chamber and that amt. is known.....but not by me. What is the neck size my chamber is reamed to?

I am told that my neck diameter for a no turn neck should be .002" inches larger than the chamber neck for a bolt action and .003 for a auto loader. my arithmetic says that the neck should be reamed to .3355 plus .002......or, 338 to round it out. Correct?

Won't this current condition overwork the brass in the neck?

Thanks for the advice,


John

[memilanuk]

Hello John,

If your gun has a factory chamber (non-match), then .344-346" for fired cases is normal. Even a 'match' chamber, in .308 Win, tends to be a little 'looser' than a lot of other cartridges - .341-343", depending on which 'match' reamer is used. Most of my factory 'match' chambers (12 F/TR, BR, Palma) run about .343"... and thats with 'Palma' and 'Obermeyer' match chambers (very common even in *custom* chambered rifles (they may have tighter freebore diameters, though). A lot of what you may see or read about in smaller calibers like a 6mm BR, with say a .272" chamber and fired cases measuring .2715" and loaded rounds measuring .2705"... is not directly applicable to .308 Win, simply because 'we' don't run things that tight - and the dang things shoot just fine anyways

If you dig around looking at custom reamer specs, you may find a few with smaller neck dimensions such as .335 or .337(for tight-neck custom-fit with Lapua brass) or .330 (same, but for Winchester brass).

Since you most likely don't have a custom chamber (given that .346" value), some might argue that neck turning is of *no* value. I would agree it is of perhaps 'less' value, but I think it still helps (or at least sure doesn't hurt) to get a consistent thickness all the way around to help ensure even release of the bullet. Neck clearance in the chamber is *not* a concern in your situation. Take a ball or tubing micrometer, measure some of your cases at three or four spots around the neck to find what the thin spots measure. Set your neck turner to just clean up enough of the neck that you get *most* of the thin spots - 80-90% clean-up is plenty.

In theory it might over-work things... but in practice, I generally lose cases due to loose primer pockets before any concerns with the necks, especially in this modern day-n-age where annealing is more accessible and commonplace.

HTH,

Monte

[johned]

Monte,

Thank you for a most excellent response.

"simply because 'we' don't run things that tight - and the dang things shoot just fine anyways"..... that is where I want to go and easy is in my vocabulary. I guess I will leave all that obsessing to the 6 and do so happily. Your advice will get me to the range more often working on loads and having fun. So long as I have a fighting chance at sub 1/2 MOA with a light and a heavy bullet.

Thank you very much for the advice and info.

John

[stangfish]

Since you most likely don't have a custom chamber (given that .346" value), some might argue that neck turning is of *no* value. I would agree it is of perhaps 'less' value, but I think it still helps (or at least sure doesn't hurt) to get a consistent thickness all the way around to help ensure even release of the bullet. Neck clearance in the chamber is *not* a concern in your situation. Take a ball or tubing micrometer, measure some of your cases at three or four spots around the neck to find what the thin spots measure. Set your neck turner to just clean up enough of the neck that you get *most* of the thin spots - 80-90% clean-up is plenty.

In theory it might over-work things... but in practice, I generally lose cases due to loose primer pockets before any concerns with the necks, especially in this modern day-n-age where annealing is more accessible and commonplace.

HTH,

Monte

Yup.