Seating depth and speed relationship?

[Sweet 6.5]

Hi all

I reload for my 96 6.5x55. I took my Crony to the range and was supprised by the results. I loaded 120gr and 140gr Speer
to the same over all lenth, and using the same powder. For the 140gr I used 42gr and for the 120gr I used 43gr. The powder is our local made
slow burning powder. (South Africa) My 140's gave me an ave. of 2540fps and the 120's an ave. of just 2200fps.
According to the manual I should have had about 2600fps from the 120s. Could this be because I didn't seat them that deep?
I tought the 120grs would be a bit faster than the 140grs or at least as fast.
The manual gives 42.5 as the max for the 140 with 2613fps, for the 120 it is 46 as max and 2888fps. This is from 24" test barrel and
mine is only 22", but this should have the same impact on both bullets?

[LaOtto222]

You are below max on the 140 by .5 grains, you are below max for the 120 by 3 grains. You have seated the bullet out further with the 120 effectively giving you more case capacity. What you have done is drop the pressure on the 120 giving you less velocity. Work your way up from 43 grains to the 46 grains in .5 grain increments. With the velocity you are getting from the 140 grain load you are near max pressure. I think you will find that the 120 will surpass the 140 at about 44.5 or 45 grains. When you get to 2700 -2750 fps, you will have reached the max pressure point.

[Sweet 6.5]

Thanks for the reply

I was not sure if the case capacity would make such a difference.

[Catfish]

With a slow burning powder you will also lose some velosity with the 22 in. barrel, but it should not be much over 100 fps., if that.

[LaOtto222]

Thanks for the reply

I was not sure if the case capacity would make such a difference.

Case capacity makes a difference, but so does pressure. There will not be as much pressure build up in the 120 grain bullet, because it is easier to push than a 140. That is why it takes more powder to get the same velocity. On the other hand pressure is not as high so you can put more powder in and still not reach unsafe levels of pressure. This gains your velocity advantage in the lighter bullet.