Velocity of Full Length Sized Cases v Neck Size Cases

[Reloader270]

Is it possible that full length sized cases would produce higher velocity compared to neck sized cases if all components are the same. Is there anyone who had tested it?

 

[WIL TERRY]

YES.
NO.
OR,
NO
YES
DEPENDING on how you're question is understood.
And so it goes.

 

[Bart B.]

I doubt it.

One would need a fixed mount barreled action shooting a few dozen (hundred?) rounds of each to tell any difference. Shooting off ones shoulder doesn't have the same resistance to recoil while the bullet's still in the barrel for every shot. And each round would have to be removed from its box then handled and loaded exactly the same way for every shot to keep the powder in it positioned the same inside the case for every shot.

While there is a tiny difference in internal case volume (neck sized ones have more room) and the load density for a given charge weight would be a tiny bit different, I don't think the difference is significant.

There are a few people that think there is an easily measurable difference.

 

[243winxb]

Brass volume Fired vs factory

There is a tiny difference in internal case volume. The 5.56 is 4% more for fired brass. http://www.dtic.mil/dtic/tr/fulltext/u2/a456635.pdf page 29

Assuming the brass case does not relax after firing, the case volume increases
by about 4%.

 

[Bart B.]

That'll vary with case wall thickness, case outside and chamber dimensions as well as peak pressure and case metal properties. It's not always 4%.

And all cases relax some amount after their outer surface equals that of the chamber at normal peak pressures.

 

[nemesiss45]

Also, even with a measurable difference in case size before firing, that full length sized case is going to expand to fill the chamber before the bulled has moved much at all, so the space the gas has to expand would be effectively equal.

 

[Bart B.]

Those believing a difference are using the start-up case volume of the cases being different.

 

[Economist]

I have data on this! I ran 8 full sized .270 reloads with 55.5 grains of 4831 under 130 grain bullets over the chrono last fall. On the same day, I ran 12 neck sized reloads with the exact same components.
Full size: 2727 fps with 34 fps std dev
Neck size: 2752 fps with 18 fps std dev
My sample sizes are a little small, but the difference between the two sizing methods was very low.

 

[steve4102]

NO.

Both cases will expand to seal the chamber walls and be of identical size before the bullet is released and peak pressure is reached.

 

[Bart B.]

Economist, the interesting thing about your data is the fact that proper full length sizing fired bottleneck cases has produced better accuracy than neck only sizing for decades. Your velocity numbers seem to contradict that if lower standard deviation's important for accuracy.

Were you holding the rifle against your shoulder when doing those velocity tests?

 

[Economist]

Yes, it was on front and rear bags and against my shoulder. My data set is small enough that I think my confidence intervals would wash away any real difference between the velocities. I would call them equal.