Chamber pressure: Full Length vs Neck Sized

[mrawesome22]

Something just occurred to me. When I put a charge of 37gr IMR4064 into a FL sized Winchester case, the charge goes about 3/4 the way up the neck. Now after this round has been fired and neck sized, the same charge only goes to the base of the neck.

Would the FL sized case have higher pressure because it has less initial volume? Or would they be they same since the brass expands to the chamber walls no matter the initial volume?

Know what I mean Vern?

 

[amamnn]

I would guess that the FL sized case develops a slightly higher initial pressure which would quickly fall off. The increase would not be worth mentioning, which is why no mention of it is made in load data manuals.

BTW, it sounds like you could use a good drop tube for that load.

 

[Mal H]

I agree, the initial pressure might be a little higher in the FL sized cases, but I seriously doubt that the maximum pressure would be any higher, all other factors being equal.

 

[mrawesome22]

Thanks guys. That's what I figured but it never hurts to ask.

 

[brickeyee]

The brass expands at a much lower pressure than the typical peak pressure.

 

[Dave P]

The fully expanded case is what you care about, pressure wise.

Full case: try holding it on your old electric hair clippers, to pack it down with vibration, Vern.