To use up scads of O.F. brass for a certain application,,I am looking for a .260 Rem. load useing 140gr. match slug @2550,,so as to still be super sonic @ 1000 mtr.,,,but has to operate in the 45,000 to 48,000 C.U.P. range,,,I know I can do it easy enough with larger cap. cases,,,but as I said,,lots of .260 brass to utilize.
THANXX

39-40 grains of H-4350 ought to do it for you

Starting loads with R22, H4831, H4350 etc will run those speeds and be around 50K psi. I don`t know what that figures at in CUP but I`m sure it`s close to your goal.

I like IMR7828ssc in my 260ai. It is a bit slower than RL22, which also works well, but a bit more unifrom from lot to lot. H4831sc would be another good choice. 2550fps would be pretty easy to get to in a 22"bbl or longer bbl.

for that velocity range, and the pressure limits you put on it, I would say that your quest starts with 7828 powder and/or slower...

I frequently use a load of 44 grains of IMR 4350 and a 140 grainer, in a Ruger 260 for an MV of 2750 fps out of a 22 inch barrel..

same load gives 2800 fps out of a 26 inch barrel..
large rifle primer used..

I guess a 26" bbl. would help as far as powder selection is concerned vs.20 or 22" or even 24".With velocity paramiter of 2550 with a 140 VLD slug,,any Idea as to twist that will help get the hit on,w/o suffering too much from rotational drift while staying S/S to 1000mtr.?