That's pretty impressive velocity for cast bullets! I'd love to get mine that high in velocity. Don't you have any issues with leading and therefore accuracy? What do you use to lube those bullets? What's the alloy? So interesting...
I went through quite a learning curve with my 308 win shooting cast!
The lead bullet is limited by its alloy strength, as to what kinds/levels of pressures that can be applied to it before a structural failure.
My bore is a .300" and .308" grooves, as to be expected. The bullets are sized at .310"
My alloy (18 lbs wheel weight lead + 4 feet of lead free solder) Air cooled.
I use Lymans Super moly bullet lube.
I use Hornady,Gator, or my home made brass gas checks for the higher velocity loads.
The lyman 311299 bore riding bullet I cast is seated out to firmly engauge into the lands when the bolt is closed.
I make sure to lube the bullet noses with lee Liquid allox after seating the bullet on the charged cartridge case. I personally lube the noses because I feel that the liquid allox serves as hydraulic support for the nose/bore alignment.
The powder that my rifle shoots this combo the best is IMR4831. This powder seems to burn as the slowest possible rate to produce the results I am targeting. It leaves a little soot/fouling, but produces the velocity and standard deviation required for my purposes. Having the bullet firmly engaged into the lands definitely helps the powder burn more completely! I feel that the slower burn rate applies the pressure and acceleration to the lead bullet over a slower/longer period of time and thus reduces the tendency of the lead bullet surface to strip off, and contribute to the leading of the bore surface. In conjunction with the slower twist rate of the 1:12" twist, it works out fine at these velocity/pressure levels.
If I push the velocity north of 2400, the bore starts to foul over 5-6 shots, then accuracy starts to degrade. For those 5-6 shots, it is more than sufficient for hunting accuracy.
When I keep the velocity around 2350 fps, I can get 40+ rounds through the tube before I feel I need to clean the bore.
This is a system that has worked very well for me so far. I took a mule deer doe at around 275 yards a few years ago with this load and the bullet hit absolutely at point of aim! It was a very rewarding experience after all that load development and trajectory work! My buddy filmed the whole event of me stalking, ranging, and me dialling in the scope solution for the shot!
The barrels 1:12" twist rate is one of the things that seems to help get the lead bullet up to those speeds before imparting too much torque on the alloy and stripping metal from the bullet/ bore interface.
I have tried this technique with a few 1:10 twist barrels, and I just cannot get the cast bullet to those velocities before accuracy starts to become compromised. That is just my experience though.
I hope this information will help someone! This is my personal approach, and may not be for everyone.
