new brass faster(fps) than necked brass

[jorvin]

i shot a 308 today and with new full sized brass i got 2760 fps. than i shot once fired neck sized brass (same load/bullet) and i got 2675 fps. is this because the new brass is expanding making higher pressures????

[Broz]

Maybe because the barrel was warmer than at first? 15 fps is not a huge spread. What chronograph was used?

Jeff

[jorvin]

spread was 65-70 fps. but you may have something there with barrel temp?

[Broz]

Originally Posted by jorvin

spread was 65-70 fps. but you may have something there with barrel temp?

Oops, sorry, my eyes did some number swaping for me there. I have seen swings like this as a barrel heats up, and also, if you have a shooting chrony as I do this is probably the norm .

Jeff

[J E Custom]

Originally Posted by jorvin

i shot a 308 today and with new full sized brass i got 2760 fps. than i shot once fired neck sized brass (same load/bullet) and i got 2675 fps. is this because the new brass is expanding making higher pressures????

If you did not size the new brass before you loaded it, It sounds like there was more neck tension
than on your reloads.

You may want to look at a different expander ball on your dies(Smaller) to get more neck tension.

Also if your chamber is a little larger than the new brass, after you fire form and neck size only
the case has more volume (This will lower the pressure with the same powder charge).

Just a couple of ideas to check out.

J E CUSTOM

[Mikecr]

Originally Posted by jorvin

is this because the new brass is expanding making higher pressures????

It's because the new brass has not yet expanded to the volume of your fireformed -neck only sized brass. This is the reason for fully fireforming, and loading as you intend to, BEFORE attempting any load development with it.
It's ok. You just can't expect much from 'brand new' brass.

[davkrat]

I like to shoot one of my plinker 110gr VMax loads in my new .308 brass to fireform them to my chamber and then neck size for my hunting loads. Seems the brass is more uniform after firing, sizing and trimming.

I'm moving towards getting 20 or so pieces of brass that I weight sort and weigh bullets for as my primo hunting rounds and then don't do quite as much work on the rest of my practice rounds. Practice rounds get loaded until they're just dead and I don't care so much if I lose one.