Both of my semi auto 6.5 Creedmoors (GAP10 and JP LRP-07) have a strong preference for the 123 Scenar and 130 VLD over any of the 139-142 grain bullets. After experiencing lots of pierced primers running 140s in the GAP and seeing some slight cratering with 140s in the JP I made some phone calls; both GAP and JP recommended leaving the 140gr bullets for the bolt rifles and suggested trying the 130s and 123s.
While the 139-142s were accurate in each rifle it was hard to achieve reasonable velocity with them as even very moderate loads showed pressure signs and had primer issues. Both rifles are more accurate, more reliable, and show no pressure signs with the 123 Scenars and 130 VLDs. Additionally, the 123 and 130 loads are actually flatter to 1K compared to a 140 load that showed moderate primer cratering & ejector swipes.
This is my experience as well, being a .260 Rem guy since 2007. I have yet to see a factory .260 Rem load that is worth shooting in the gas guns. A buddy of mine recently bought a .260 Rem factory Armalite AR-10, and all his primers looked horrible. He bought every .260 Rem factory load he could find, and they all seemed to run outside of optimum for a gas gun, while not grouping where you would expect.
I have never shot a factory load in both .260 Rem AR10's that I have owned. The 123gr Scenar and 130gr VLD run fast for me from my 22" GAP gas gun that George and Jeff built for me (before the GAP-10 existed). My 130gr VLD load is too easy to shoot with out to 100yds on man-sized sils, and beats the 140gr for drop with same drift.
H4350 has given me a lot of speed, with good gas system operation. I don't know what port diameter GAP used on mine, but it works well. I have also had good results with 140gr pills for accuracy, but I think 2700fps is where the limit is for me.
H4831 under the 139gr Scenar was super-accurate from the start load (.6" for 5rds then shrank down to .4" 5rds). RL17 also works well behind the 142gr SMK, but I like the 130gr VLD the most. I enjoy shooting the .260 Rem rapidly on small targets at distance for demos or in competition, especially explosive targets like we do at BoomerShoot.
If I had a Creedmoor, I would try some 123gr A-MAX's, 123gr Scenars, 129gr SST's, 130gr Norma's, and 130gr VLD's. The VLD's are expensive as crap, but they shoot so dang well at distance. You also might want to try the new Lapua Scenar-L's in 120gr and 136gr.