Does anyone know if there is a source for 375 H&H Ackley Improved brass?

Grayback Wildcats,immediately comes to mind(541)884-1072................

I do believe if you buy 416 rem cases all you should have to do is run them through your dies.

Happy hunting

Sizing .416's to .375 is a good idea as there may be an opportunity to take up the headspace for that first shot.

I would use Remington cases. But I say that only from looking into Remington .375 H&H new brass and seeing a much heavier web than Winchester .375 H&H or .300 H&H cases

All above are true. In addition, you could probably make a close-fitting case by expanding 8mm Remington Magnum cases, which may be a bit easier to find than .416 Rems; but necking down the .416's would give you the opportunity to make a case which headspaces perfectly on the shoulder on the first firing.

Another option is to neck up a 375HH case to .40 then back down to 375 to form a false shoulder for a solid headspace. Probably need to aneal after the forming shot.

With this method, you won't get any web stretch and your cases will be properly stamped. Lot of work though.

I have a 375 JRS and I use 375 Wea. brass all you need to do is fire form it

When an "Ackley Improved" shoulder is cut correctly, the shoulder has been "set back" about .040" from the parent cartridge. This is done so that factory ammo will head space via a "crush fit" on the existing shoulder. If one runs the "Ackley Improved" die all the way down on the standard case, headspace problems will develope when fire forming. I place a washer under my sizing die when necking up 30-06 brass for fire forming 8mm-06 Ackley Improved cases. This does not "set back" the shoulder, and allows a "crush fit" on the original shoulder.

I also run my case over the lube pad when fire forming and seat my bullets to normal C.O.L. The case lube allows the case to resist gripping the chamber walls and I get no case stretching this way. As a mater of fact, the cases tend to shorten a few thousands. This is not a problem to me as I then don't have to trim until I get about 6-8 reloads from a case. I normally get about 14 reloads before neck splitting and primer pockets getting loose, with no sign of case head seperation. Remember to clean the chamber before shooting "full house" loads!