Ahh, one of my favorites and frustrating subject!
Welcome, need some more details. What rifle?
I have a K-31, 1911 and a Savage 110 I re-barreled to 7.5 Swiss.
Regardless of what anyone says, 308 is the right bullet. It all has to do with specs variation but it is a so close to a true 308 that .308 is the right bullet.
There is almost bullet that comes close to matching the GP11 projectile profile. Some reports of Berger RDF. It acualy looks like it might, have to get some to see for sure.
I do have a few rounds of GP11 a very kind person sent me, I have taken one case apart. It truly is an unusual bullet compared to the rest I have.
Seating deep seems to be a key but still playing with that.
Sierra 175 Grn Match often works good in the K-31
Powders that work are R-17 (that is the original, up around 46 to 50 but start low. This one cams over pretty quick. )
4350 (any of them) is a good one, its in the region of R-17 burn rate wise though that is just an approximation of what works and does not.
PPU Brass is readily available, its good stuff and the cost is reasonable. Lapua no longer is or rare, it may come back but had not seen any in a while.
I went with RCBS dies as you can get the side slot loading with the sleeve, I believe its much better seating bullets, I don't know the sizing dies makes any real difference but I got the set.
COAL for bullet seating seems to need a jump to work (still playing with that)
There are some tuning things to be done to the rifle if its a K-31.