Re: AR 260 Load questions
I think it's your barrel length and your barrel is still new. I don't know why but even with lapped in barrels I've found them to speed up after 100 or so rounds.
The 260 really excels when the barrel lengths increase, a 22" is the absolute minimum that I'd consider for a 260 build, 24" makes much more sense for it's overbore and expansion ratios.
The case has similar behavior to a 30-06 and the difference between a 22" and 24" 30-06 is pretty good. The difference between a 22" and a 26-28" is HUGE. I've had all of the above. My 260 was bought with a 24" tube for a reason, I've tried them in other rifles that I loaded for at 20 and 22". My 30-06 runs a 26" tube on it now. When I rebarrel in another 1k or so rounds I'll put a 28" tube on there since it's just a target rifle.
ETA:
Also keep in mind that the 139 Scenars have a long bearing surface, this translates to more frictional drag in the bore. You said you weren't getting pressure signs yet so keep going on the powder and see what you get. The 140 Amax is a middle length bearing surface, the 140 Bergers are short. The 140 Berger (when loaded to the lands) shoots about 100fps faster than I've ever managed to push the Amax. It is a combination of many factors but that bearing surface length is certainly one of them.
