You can get heavy base turned cases from MRC in any length.
For a 750 gr that fast you have too high pressure for double.
Using 3.5 inch plastic I got a 600gr to over 2000 in heavy barrel
NEF, and cases stick and won't extract when I went higher.
You can probably get to 1800 depending on extraction, with 750gr.
If you can find drawn cases without balloon heads that is best.
Personally after all research and getting cases around, if
I do a gun without going the full 3.85" length, I will do them
in 3.5" plastic, it is just so much easier and saves money,
and 3.5 inch plastic wil lkeep up with 3" brass. Ed