why the velocity drop?

[stu_bear2002]

I have just reloaded some new lapua brass with the same load (46.3 of 2208) i was using with winchester cases without changing any other factors and have lost 170fps. I know the lapua cases have a smaller I/D so my load is pretty close to filling the case now, would dropping the charge back to give a little empty space help to build up more pressure to lift the velocity?

[johnk]

No, that would be unlikely to achieve anything.

I'd be looking for other reasons, like were you using the same chronograph & screen distance from muzzle? Was it the same batch of 2208? There are tales of differing burning rates with 2208.

How about loading some of those Winchester cases & test them at the same time?

[stu_bear2002]

John, thats why its got me stumped same canister of 2208 same packet of cci primers same seating depth same batch of dyer's same chrony set same distance of 15ft only difference was the cases. I even set up again at home with the same results.
FWIW this is all with a 26in barrel and my load of 46.1gr gets me a mv of 2887fps using winchester cases, lapua cases only get 2712fps with 46.3gr and every round is hand weighed needless to say im and
Stuey

[Matt P]

Which Lapua cases are you using ??? Large or small primer brass ??
If small that is what I would exspect to see, I had some of the SR primer brass and I sent it back, I estimate the load would need about a grain more to get the same velocity.

Matt P

[AlanF]

Different neck tension (ie less in the Lapuas) would cause differences, but nothing like 170 fps. Or did you drastically FL size the Win cases perhaps?

If you tested at different times then I would try it again interspersing the cases in the same test. In my experience, absolute chronograph velocities can vary widely, particularly in different light conditions.

Alan

[stu_bear2002]

I'm using the Large primer cup cases, I had FLS the win cases as they were from other chambers but i had only necked the lapua's because they were new. All testing was done within 5 minutes of each other when i did it at home and i also fired 3 foulers before commencing the test

[johnk]

The result is inconsistent with general expectation from the cases unless maybe:

It's an arcane issue of your relatively short barrel length compared to the general 30" TR length that most are familiar with.

or

Its a consequence of the different preparation of the cases. Try again with Winnies & Lapuas fireformed for the chamber.

John

[AlanF]

Here's my theory :

Most (but probably not all because it is so large) of the difference will be due to the difference in age of the brass, and the fact that the Win brass is older, and in need of annealing. New brass does not always give higher velocities - it has enough give, that the "smooth" expansion seems to counteract any effects of reduced volume. However hardened older brass that has been FL resized will behave more like a smaller case, and I'm guessing that you may have bumped the shoulder quite drastically.

I agree with John that a better comparison will be between fire-formed cases of both brands. Hopefully you won't need to FL size the Win brass this time - if you do, then try to get it as close as possible to just enough and no more, and this should reduce the velocity difference.

Alan

[stu_bear2002]

thanks guys i will NS 5 of each case and reload at 46.3 and see if there is any difference since they have been fireformed now in my chamber.
Hopefully i will have time to do some testing saturday before work.