Sized .308 case neck OD vs fired case neck OD

[P_Mo80]

On a F/L sized .308 case, I get a neck OD of .334. After firing, this is repeated over many rounds, I get a fired case OD of .344. The neck is expanding .010 ... is that normal? It seems like a lot based on what I'm reading. It's a factory Remi 700 VTR Barrel from 2008.

 

[PJT]

Not a problem, similar with mine. The size it expands too on firing will potentially be different for different rifles, depends upon the reamer used in manufacture.

 

[MtnCreek]

Is your .334 with a bullet seated?

 

[P_Mo80]

Is your .334 with a bullet seated?

No, the .334 is OD without a bullet seated. Just the brass after F/L sizing.

 

[918v]

It’s normal

 

[WilburW]

Doesn’t seem excessive.

 

[PinesAndProjectiles]

What's the neck diameter with a loaded round? Just out of curiousity

 

[wil_pt]

That’s the same fired case dimension as my factory Howa.

 

[Supersubes]

It is excessive, but that’s what you get with factory rifles. Shoot it!

 

[Doom]

SAAMI minimum chamber dimension for neck is 0.3442”. Your chamber is fine. The FLS case dimension has no correlation to the cartridge dimension until the bullet is seated.

 

[P_Mo80]

By way of explanation:

Im going to guess that your f/l sized case measurement is taken after having used an over-the-counter, standard full length sizer as opposed to a bushing die or some boutique die you had made. What that means is that your die is squeezing the neck down to a minimum diameter, perhaps more than is absolutely necessary, to meet a specification that makes the die manufacturer’s lawyers happy and ensures that brass sized in that die will work in any rifle chambered for .308 win.

Now, in the office of Remington’s lawyer, they decided they want any .308 win cartridge, manufactured anywhere in the world, to chamber, fire and extract from your VTR chamber. So they made the chamber as big as they could get away with while still calling it .308 Win.

What you are noticing is that there’s a lot of difference between minimum cartridge neck size and maximum chamber neck size. Your sizer is probably squeezing the neck down a little more than needed (often remedied by the use of a bushing type resizing die) and your chamber is allowing the case neck to expand a long way in The firing process (often remedied by custom chamber specifications in custom rifles)

The penalty to you is shortened brass life as the brass is worked from small to large and back in every cycle and eventually, especially if you don’t anneal, hardens and cracks. Depending on a lot of other factors, there could be a small accuracy penalty too but if you aren’t shooting specifically for groups in competition, it’s pretty far down the list of accuracy killers.

sorry to be long winded.

Great explanation, makes sense!

 

[Doom]

No, the .334 is OD without a bullet seated. Just the brass after F/L sizing.

Your brass has been sized to an inside diameter between .304 and .306 inches. The case neck thickness is likely between.012 and .015 inches. That makes a outside diameter between .328 and .336 inches. With a bullet inserted the diameter is between .332 and .338 inches. In your case I suspect you are closer to the .338 value.

Based on the SAAMI MAX Cartridge/MIN Chamber the cartridge would grow .009”.

What you are seeing is normal. It’s engineering not lawyering.