Have yet to shoot my CZ550 yet in .375 as I didn't wanna use up the factory ammo that came with it. I've cast up a goodly sized batch of the 264 lyman bullets (375449) and put them on top of 32gr of 5744 but the books say that will only get about 1700 FPS. I doubt these are going to shoot POA but I'll still try them out. Anybody have data for this bullet that gives close to factory specs? Powders on hand 5744, 4198 and IMR 4350 could get some others but these are ones I use for my other belted mags. The old lyman book load of red dot, green dot or unique have my curiosity but those are only going at most 1400 FPS and I got this thing to rock and roll with.

Depending on bullet hardness (alloy) you should be able to reach near full velocity without a leading issue.

"I" would use IMR 4350, for a 270gr load, - 8-10 %, for a starting point.

Keith

Might try asking the guys over at the cast bullet forum. Many of them really know there stuff about cast bullets and big bores.

yeah, I've asked the Q a few times over there in the nightly chats. I'm pretty active over there but only a small handful of guys really seem to shoot the .375 H&H there which is kinda surprising.

I'm kinda excited about redneckdan's 390gr design for the .375 H&H made for the CZ550 but I'm just breaking in my rifle now so that will wait for a bit.

Depending on the alloy you are using,and the fit of your boolit, velocity is not hard to obtain. Accuracy, on the other hand can be a bit more difficult.
Like you I want my cast boolits to do a bit more than the moderae velocities in the usual cast boolit data manuals.
I have used red dot for low velocity loads and blue dot for mid range but they are only plinkers really so when I want to FEEL the shots and have accuracy along with the velocity I go with H4350.

I dont shoot a 375, however, I do cast almost all of the boolits I shoot in my 7x57(160gn at 2415fps) and 404 Jeffery (350gn at 2365fps and up to 2437fps over 88gn H4350). I use H4350 in both cartridges and started much lower than the minus 8-10% suguested above.
A cast boolit gives more velocity for the same powder charge compared to a jacketed bullet and as accuracy is just as important as velocity I would start about 65gn H4350 with a tuft of dacron and work up from there, and forgo the filler when (and if) I reached 65 - 70% of usable case capacity.

My experiences only and posted with usual cautions. YMMV

Von Gruff.