Buddy just got a custom 260. He has 139 Lapua heads

I suspect the same powder as 6.5 Creed will work.

I loaded my Daughters with 120gr Barns TTSX.

Got a ton of I4895, not much listed with that powder, but QuickLoad has it.

4895 is a little too fast burning for the 260 .A powder in the burning range of W760 or IMR 4350 would be much better. For 139 gr bullets IMR 4831 might be better yet. just because you have a bunch of a certain powder doesnt make it a good choice in some cartridges

Ruger M77 45.5 H4350 with 120 Sierra SPBT Same powder for my 6.5 creed and my .243 with 100 grain

Rlr 19 works well if you can find it. I intend to try some staball 6.5 in mine if I ever get some shootin time.