Originally Posted by jpterp
I would like to keep the length of the barrel around 20" to keep weight down and make it manuverable, (similar to the Remington 700 SPS Tactical) ...
Interesing topic... I'm contemplating a similar project and am intrigued by the 20" guns. The 20" R700 LTR with the fluted barrel is only 7.5# per the catalog. However, I'm not that intested in a 308 and would prefer a 7mm-08 or 260R for the superior ballistic coefficients.
I did some internal volume calculations to compare the different calibers and found that a 7mm would need something like a 23.6" barrel to have the same internal volume as a .308 20". A 6.5mm would need a 27.3".
Certainly I'm not an internal ballistics expert, and there are certainly other variables than specific volume, but I do believe the larger-bore cartridges for a given case capacity will have an easier time achieving their velocity potential with a shorter barrel than the same case necked down to a smaller bore.