I'm trying to figure out a mystery rechambering job for a Valmet 412 that started life as a 375 Winchester. While I'm awaiting a chamber casting I did rough measurements and it may be the cartridges I mentioned. If so does anyone here have experience or knowledge of forming cases or issues that might be encountered?

I have three calibers that I shoot that are based on the 348 win,the 475 Turnbull,450 Alaskan & the 416/348 improved,if you have the right dies? & know how to anneal? Case forming is not a big deal at all.

as bill said. no big surprises if you can get the brass. over the years I have had a 450AK, 50AK, and a 375-348. in fact I still have the 375 reamer. made up a Siamese mauser for a client, a few ruger #1's and I also know it was used to build a double rifle for a buddy out in MO.

When I lived in Juneau, Alaska, I spotted an M71 (Browning) and was so impressed with the handling that I bought it and rebarrelled it to .375/.348 Improved.
It'll send a 300gr Hornady RN out at 2200-2300fps with about any powder I can lay my hands on.
The M71 jumps quite a bit because of the drop in the stock, so your Valmet, or the above-mentioned double, should be about perfect for self defense against the bruins I was gonna encounter.
If I was going after Elk, Moose, or even Deer, I would slip a cast 270gr or 300gr slug over Alliant 2400 or some other pistol powder into the chamber.
That would produce 1400-1800fps and work just fine on any of the herbivores.
Have fun,
Gene

I just measured one of my ready-to-shoot handloads so your chamber dimensions SHOULD be similar.
length of case, minus the rim-2.128"
diameter in front of rim-.546"
diameter at shoulder-.535"
length of neck-.357"
the chamber casting should be close, if not it COULD still be based on the .348 case.
Have fun,
Gene

Being a rimmed case, fireforming is no problem at all.
If you can't expand the neck enough to get a .375 bullet in, then you can put 15-25gr of a pistol powder like Bullseye, Red Dot or 700x, ad infanutum, then fill the case with COW (Cream of Wheat) uncooked, lol.
Put a bit of parafin on top to hold the COW in, and shoot it up in the air. It'll be a bit noisy, so don't do it in the house, or you'll become persona non grata, lol. don't ask me how I know.
The case won't be fully formed, but will be close enough that you could load it with a load that will impress you, even over the screens.
Have fun,
Gene

348 brass is no longer a problem..Midway has the new Jamison and Hornady brass in a variety of old win. calibers. I just purchased some 25-35, 348 and 250-3000 from them..

If I ever change my Browing .348, it will be to a 375/348 IMP..Thats a good balance and a real hammer in that it will shoot a 300 gr. bullet to within 100 FPS of my 375 H&H handloads, meaning Ive always loaded m 375 H&H to 2450-2500 FPS depending on the gun, as that velocity works best on Cape Buffalo etc in that its kinder to bullet performance on heavy game and elk for that matter.

I am kinda torn between the 375-348 & a 348 Ackley improved,a 250 gr bullet going 2500 fps outta the 348 improved is serious medicine & no need to rebarrel,I have killed lots of African game with a 250 going 2750 out of a 338 Lap & it has never let me down including a one shot kill on a huge giraffe.

The only advantage to a 375/348 IMP and 348 Imp. is cross section of bullet, and I place a lot of pluses on cross secion that seems so little to some on these boards who have a new micrometer , plus the advantage of a 300 gr. bullet and perhaps a 350 gr. bullet...

If I decide to go either route it will be the 375/348 IMP to get that cross section of bullet and its knockdown advantage.

That said I can get 2400 with my Browninng 71 348 and that's a fair hammer on elk, and that's the reason Ive never broke down and had it changed and the 348 IMP does have some advantages such as a better case that lasts longer, and shoots a little faster, and is probably more practical.

quote:

Originally posted by Blaserguy:
I'm trying to figure out a mystery rechambering job for a Valmet 412 that started life as a 375 Winchester. While I'm awaiting a chamber casting I did rough measurements and it may be the cartridges I mentioned. If so does anyone here have experience or knowledge of forming cases or issues that might be encountered?

Blaserguy I have a 375-348 Ackley Improved built on a Siamese mauser. The short ogive on many 375 cal bullets works well in the Siamese action. I picked up a supply of 348 Win headstamped brass at varios gun shows etc. Some of it was old. I would just run the brass through my size die , load and fireform. I lost a couple cases to splits on the first box and not wanting to loss more of a hard to find case I choose to anneal the cases before fireforming. Haven't lost a case since. Gives up very little to the 375 H&H with bullets 270 gr and lighter. With the heavier bullets the bullet robs too much powder capacity. COAL must be held to about 3 inches in the Siamese magazine. It would be almost a dead ringer for the 375 WSM.