Thread: 7x57 vs 8x57

Ok, so this is doing my head in, maybe I’m overthinking this, but..
ADI start load for 7x57, 175 gr projectile 2208 is 31 gr
Start load for 8x57, 175 gr projectile is 43 gr
Yes, I know the shoulder on the 8mm is slightly forward, apparently it’s worth approximately 3 gr more capacity
But, other than the larger calibre, I thought the cases were pretty much identical
So, why the 12 grain difference ?
What am I missing here ?

the extra size of the long skinny hole = less resistance to push same weight out the end of it..projectile will be shorter with less bearing surface.

7mm-08 150grn with imr4895 max of 40grns //////////// 308 150grn with imr4895 max of BOTH 45 AND 47 depending on projectile

arguably closer sized cases showing similar thing.

Nearly 24% more bore volume.

7mm bore area is 3.5 x 3.5 x 3.141 = 38.5 mm2
8mm is 4 x 4 x 3.141 = 50.25 mm2

Can't escape the maths

7x57 start load of 31gr 2208 is only 2000fps. That is an extremely light start load.
8x57 start load of 43gr 2208 does 2400fps. That is a normal start/moderate velocity for 8x57. It’s actually faster than PMC factory rounds with 170gr bullets.
It’s not necessarily because of bore size difference, it’s because ADI compile their data from other published manuals, which can include a lot of variation, in that case a difference of 400fps.