I realize this thread is ancient, but what process did you use to resize the 300 RUM into 375 RUM? Were there any "gotchas", or special prep you had to do? Or, did you just lube the 300 RUM cases and ram them through a 375 RUM full length resizing die? I've contemplated getting a 338 RUM expander ball and running that first, but if that is not needed I don't want the extra steps.
I've been experimenting with something similar, necking up some 7mm to .338. Not quite as drastic a neck up as yours, however I have been pushing the shoulder back as well. I anneal the brass first and it hasn't given me any problems. I would look into that and also running a .308 expander through it as well before the .375. For a jump that big I would probably run it through the .308 expander, then anneal, then the .375...just a thought.
Using a 338 neck sizer is almost a must in order to get the 300 rum to 375. If you don't the brass really takes a beating. So much easier to go up sequencally and have a very good end product. The last thing you want is to wreck that hard to find brass.
bear78, How do your 300 cases look after resizing? Do the shoulders have fairly sharp angles? I'm resizing some 300 brass for a 375 bme, essentially a 375 imp. Some of the cases are tough to chamber and I noticed the shoulders have slightly rounded edges. Just curious if you had the same experience before firing them?
The 375 BME or Box Monkey Express is/was made with 404 Jeffries brass. You can use 375 RUM brass but you are going to struggle a bit. I have talked with Andrew countless times on how to make big bore's like the 375 rum imp really sing. I have just received my reamer from Manson on my biggest 375 wildcat yet. A 378/375 Weatherby imp. Shortened the neck, 40 degree shoulder, took out the taper. Should be quite a contender.
Here is a thread that shows pictures of the 404 transforming into the 375 BME. You will be much happier with the 404 brass...you will gain a little capacity too.
I should have been more clear. It's a 375 Black Mesa Express, it was developed about 6 years before Remington came out with their Ultra Mag cartridges. I know of others who are making their brass out or 375 RUM brass which just needs to be ran through a FL sizing die. Others who have loaded for it said the 404J brass was a lot tougher to form than the RUM brass and they could never get it to eject very well, they ended up with a lot of stuck cases.
I ran a few old once fired 300 rum cases I had lying around just to try them out. 5 nickel plated Federal cases chambered and ejected with no problem. I then tried a new Nosler piece of brass and one once fired piece. Neither of the Nosler pieces would allow the bolt to close all the way. Not sure why one type works perfectly and the other won't.