[Crosshair]

Here is my experience:

With a blowback action you are not going to get significant velocity increase with a 230 grain no matter what you do. Loading for a locked breech is very different from loading for a blowback action.

With a blowback action, if you try to use a large charge of slow burning powder under a heavy bullet you are going to get a nice fireball out of the side of the gun as the bolt opens prematurely. It is a zero sum game. The only way to get increased velocity is to either lower bullet weight or increase bolt mass.