Re: .260 Rem or 6mm BR
How about the 6.5x55? Over a 100 years old, and still hangs with the big boys.
Over 250 000 match shooters here in scandinavia use it with a Sauer match rifle. Very similar to a SIG 3000
140 grains easy at 2700fps, but can be pushed faster. Barrel life is around 4-5000 if shooting at 2700fps.
Or a 95grain V-max at 3300 fps with a B.C of 0.36
Lapua brass, and excellent bullets available.
I would either use 6mm BR or 6.5x55.