AKA 375 Whelen Ackley Improved AKA 375-06 w/40 degree shoulder and shoulder blown out.

I got my 98 Mauser 375 WAI built. ER Shaw no. 2 contour 22”, I rented a reamer and cut the chamber by hand. CH4D dies. I used 35 Whelen Hornady Brass - opened it up to 416 and set the die down with the CH4D dies to meet the ram on the press, thereby getting my ‘go’ chamber.
So far 55 grains TAC and 270 gr. RN Hornady bullets (shoots well) and 31 gr H4895 with 233 grain cast (no gc) are the two loads I’ve used. The cast load is very low pressure and doesn’t leave a sharp shoulder, but that’s my fire forming load. I’ve ordered/received a NOE 270 grain mold (gc) but it’s way below zero outside and I cast in my shed, there’s a wood burner in there but I’ve been busy.

I have a boat load of those 270 grain Hornady bullets.

I’m planning on trying IMR4166 powder though I’ve not seen any loads with that powder. I’ll also be looking at powders with similar burn characteristics.

My 98 Mauser (action built between the wars) has a steel Lyman Aperature sight and a post front sight. I had bought what might be a Brown Precision synthetic stock here on the fire, but that would work well with a scope, not iron sights - too high a stock. The stock I bought and am using is from flea bay - laminated. It took a lot of material removal to get it to fit. I also bought a plain walnut stock - same story lots of extra wood.

If anyone has more to add regarding this cartridge, it would be appreciated. I’m thinking I will be using mainly the NOE bullets and for deer. If I scrape up enough coin I might take this bear hunting in Alaska and I think jacketed bullets. It should work in Africa on Cape???

I killed a bull elk with a 375 Rimless (9.5x57) last year and it worked nicely. I was using factory Kynoch that uses a 270 Woodleigh RN at around 2200 fps.

Sounds like a really cool rifle. I am curious about you chambering by hand. Did you drill it first? How difficult was it to do?

I am building a 338-06 on a Mauser and ordered a short chambered Shaw and will rent a reamer from 4D and head space by hand.
Seems to me cutting the full chamber by hand would be difficult?

The barrel came un-chambered from ER Shaw. I only used a finishing reamer, which I rented from 4Dreamers out of Montana.

The method I used:

1. Bought a can of Crisco.
2. Stuck the reamer into the Crisco (filling it to capture the cuttings.)
3. Turned a few times, pulled the reamer out and cleaned out the cuttings and the Crisco
4. repeated 2 and 3 until the chamber was close. cleaned the chamber and tried chambering the resized cartridge case.
5. Continued until the case would just enter the chamber.

I loaded a case and test fired. I resized the case and checked if it would enter the chamber. It did. I repeated test firing - loading up to 55 grains of TAC behind 270 grain bullet. (started at 51 grains, I think.) 56 grains seemed too warm.

Reloaded the same case multiple times (10+ perhaps) at full power and I believe I have a safe full power load with good longevity. I'll try more loads after fire forming with the above cast bullet load.

If your time is worth more than mine or if you decide that you don't have to do all the work on your wildcat, go to a gunsmith that has the proper tools. I sold my lathe in 1999 and I am only a hobby guy, not a trained Gunsmith - though I've worked with gunsmiths.

It took me a few hours to cut the chamber this way.

It's the middle of winter and I shot these loads from my deck. The range is not that far away, maybe 75 yards, but it is cold and windy. I won't check velocity until Spring.