25-06 and 75 Vmax
I have both 25-06 improved and 25-08 improved. The 25-08 improved has 1:14 twist and shoots the 75Vmax very accurately at almost 3900 fps. A friend of mine also has 25-06 Improved and claims over 4100 fps with it I dont know the twist. Another friend of mine has 25-06 Rem Sendero 1:10 twist and claims at at top velocity the bullets can break up. Dont know it it is correct or not. I have not tried the 75 Vmax from my 25-06 Improved as I feel it is a bit to large for that light bullet or the bullet is a bit to light for that large a case. I shoot the 100gr Sierra Matchking from it at 3600 fps with good accuracy and better wind bucking than the 75 Vmax. If I was to try lighter bullet in it I would rather go for some around 85 gr weight. But if you try the 75 Vmax and it doesnt break up in flight it will waporice smaller varmints judging by my experience from the 25-08 IMP.
Regarding custom bullets, at one time (some 10-15 years ago) I was trying to shoot .25BR Improved in both benchrest and score. Due to unavailibility of bullets the same quality as the 6mm bullets it never shot as well as a 6PPC.
I was later told that the .25 bullets from Fowler were up to the task. I never tried them though. Does anyone know if they are still available or if the Bibs are as good ? At that time the 82 and 87 gr bullets from Berger did not work for me at a Benchrest level, I dont know if they have changed or not.