I have been kicking around getting my post Mod. 70 .375 H&H rechambered to an ultra mag. What brass does a guy get, dies, loads. I know this is a proven round but I would like to hear from folks who have had it done.
Remington make brass, it's pretty easy to find. but IMHO there are a couple things you should be aware of. You will loose 1 round of magazine capacity and you could run into feeding problems. I'm a big fan of the ultra mags however Remington made some internal changes to the magazine on the model 700. In addition the magazine area of the stock is slightly different. I never noticed the difference inside the stock until a friend and I recieved 2 stocks for our ultramags from accurate innovations. Niether rifle would feed in the new stocks until some metal was removed from the width of the magazine. both stocks fed fine as they were with our normal magnums (300 & 338). A little work with a dremmel tool solved our problems.
You might consider a 375 weatherby, JRS, or ackley improved. You are much less likely to have feeding problems and you can buy weatherby factory ammo and still shoot factory 375 in a pinch. You will loose 25-40 FPS though.
You get 375 UM cases and reloading dies. I am guessing you would gain maybe 200 fps. If you are needing more power I would simply step up a to 416 Rem with bigger bullet weights. The extra velocity is really insignificant in terms of flatter shooting.
A second vote for the 416 Rem mag as a step up vs the re-chamber.
A 375 is not a round one will tollerate mediocre feeding for, and making a reliably 375 ultra out of your H&H will require work to the feed rails, and really should be coupled with bottom metal sized for the dimensions of the UM.
I've put together several rifles that use larger dia cases than the original chambering, and I retained the original magazine. I've never been happy with the results, and am in the process of putting properly dimensioned magazines in place.
A 416 is a simple barrel, and it is a step up in power. Drive a 350 gr @ 2600 fps and you have about the most perfect serious Alaskan rifle that can be had. Shoots flat and hit's hard.
In my opinion the 375 Ulta Mag and much earlier 378 Weatherby have to take the award for worst ever cartridges...more bark, more recoil, and no more bite. They offer no advantage over the 375HH except they shoot a little flatter. Expanding bullets will not penetrate any deeper or kill any better at any range except perhaps past 300 yards. The extra velocity does not make them better killers at close range, as on a charging bear. In fact the extra velocity will very likely reduce penetration at close range unless you choose you bullets very carefully. Even bullets like the Swift Aframe and North Forks extra heavily constucted soft point, their heaviest wt .375 soft, will penetrate less at 2800fps than 2600fps impact velocity. The only commonly used bullet that might penetrate more at the higher speed is the Barnes TSX. In that case it will only be if it sheds its petals and then acts like a solid. You get a smaller wound channel and it still won't kill any better...but you might make a small hole out his butt!
As the others have stated, if you want more bite to go with all that recoil and muzzle blast, get a 416. I would never rechamber my 375HH to the UMag.
If what a person wants is the ultimate long-range big-game rifle, the 338 ulta Mag makes more sense to me than the 375UM. The 338 caliber can be made to shoot flatter than the 375 and with a little less recoil. This would be for those past 400yd shots on elk, moose, or African plains game. A 338 bullet is adequate for anything up to moose. I guess if a person trains enough at those ranges...well you know the rest...
The 300gr Sierra Matchking bullet can be shot at 2800fps out of a 338 Laupa..This combination losses only about 100fps/100yds. So for those half mile shots it's still nearly 2000fps!!! So what if it's a target bullet, it might expand at 800yds and probably won't blow apart. But it's still not a 375 on those up close shots.
The best thing I've learned over the yrs is that no amount of velocity will turn a 308 bullet into a 338, and nothing will turn a 338 bullet into a 375, etc, etc. But at least with the 338 calbers and below, I can see some use for the extra velocity...Flatter shooting and an increase in effective range.
I still remember those slick Weatherby catalogs in the 50s when I was a kid. I guess he had it half right. Velocity does make it easier to hit at long range, but past a certain point it can even be detrimental to killing performance. Weatherby's cartridges, or rifles for that matter, have never had a good reputation in Africa. Sure would have saved me some money building 300 Super mags etc if I'd never seen anything of Weatherby.
Additional velocity does infact create a more damage to game providing the use of premium bullets that will withstand the additional velocity. In my hunting group we have shot animals side by side with variuos calibers from the 270 to the 416 remington mag. 3 years ago I was the first one in my group to buy an ultra mag (300 ). After several hunts and the observations of wound channels there are now 2 300 RUM and 2 338 RUM going afield between the 6 of us. One guys admits he's sensitive to recoil the other just has not found a left hander a an afordable price yet. The killing power has to be seen to be believed with good bullets and resonable shot placement. You do ruin a few more steaks though. I have harvested more game with the 338 win mag than all other calibers combined, however there is no dought barring spine shots that my 300 rum puts game down quicker than my 338 wm.
No offence intended to anyone. This is just MHO based on our obsevations.
The 300 rum kill quicker than the 30-06, the 06 kills quicker than the 30-30 etc.