I built a .260 gas gun with a JP Ultra Match barrel with JP BCG and fitted bolt. I'm having one hell of a time developing an accurate load for this gun. I'm using IMR4350 from 40.0 -45.0 gr. with Virgin Lapua brass and five different 6.5 bullets.  I measured the jump with 123 Sierra Match Kings and it it is .057" at magazine length COL. best I've been able to develop is 1 MOA. I'm wondering if .057" jump is way too much?

Thoughts? Ideas?

Anyone have an idea of a better powder for a large frame gasser?

Dunno, but I have some different powders you can try if you would like.  IMR 3031 is a maybe, always had really good luck with A 2495 (but not in a gasser), and if all else fails try some varget.

I think I will try Varget. I also have some 8208XBR

Have you tried any Reloader 17? My 6.5 Creedmoor (bolt gun) loves RL17. The 260 and 6.5CM are supposed to be really close. What twist is your barrel? My 6.5CM is a 1:8" and is partial to the heavier 138gr+ bullets.
I haven't tried that far of a jump yet. Mine seems to really like the 0.015"-0.025" jump. Of course every barrel is different.
If you have a match barrel with a bolt that's been headspaced to that particular barrel, you groups should definitely be rivaling that of a bolt gun.
Here's a group from doing some load testing.

140gr Berger VLD Target
42.0gr RL-17
GM210M
.020” off lands
2784fps (avg.)

Bandit,
thanks for the info. I'm headed to Anchorage for Thanksgiving so I'll look for some R-17. Today I'm going to load up a few loads from 37-40 grains and see what I end up with the lower end of the loading spectrum.