I have done several searches and have not been able to come up with a definitive guide. I have found bits and pieces but not step-by-step instructions.

I have about 100 pieces of .375 RUM brass that I would like to convert to .470 AR brass. I am trying to get a step-by-step description of how to get there. Like I say, I have seen descriptions like trim the .375 brass, form the shoulder and fireform, but I need something that breaks it down a little more, e.g., how much do you trim the RUM brass, how do you form the shoulder, etc. When I get to fireforming I think I understand the process, 15-20 grains of Bullseye, corn meal to the shoulder, a piece of toilet paper, some wax or something to hold the corn meal in place, fire (preferably at an angle) and trim. But I need to understand how to get the brass to the point of being ready to fireform it. Thanks in advance.

have you talked to Jeffe about it?

Jeff has sort of disappeared. Last I looked he has not posted on AR since April or May.

Been experimenting.

1. Took the .375 RUM brass and liberally coated the inside of the neck with sizing wax and ran the case through a .416 Rigby expander die.
2. Then took the case and ran it through the .470 AR expander die.
3. Trimmed the case to 2.55" and ran it through the .470 AR sizing die.
4. Then ran the case through the expander die again.
5. The cases will chamber in the rifle with a little pressure on the bolt, presumably as a result of the rough shoulder from the sizing die.

I am not sure what I gain by fireforming at this stage. The cases are 2.55" in length, same as the .470 AR. The cases have been run through the sizing die. The shoulder is not as sharp as fired rounds but that should take care of itself when I fire it. I guess my inclination is to load the rounds up as normal and shoot them. Am I missing something?

Hello MJines,
I make 416 Taylor out of 300 win mag and 458 Accrel out of 300 and 375 RUM. Here's what I do:
1. trim case to about .010" longer than what finished length will be. I grind on a coarse 2" belt grinder for knife making to remove the bulk of the length. You can trim it off on a regular case trim set up if you have the pilot. Remove inside and outside burrs.
2. I expand mouth in a two step process. Generously lube inside of neck. Put FL die in press. Push case onto expander ball until you get some good resistance. Lower ram. Mouth should be funneled out. Re-lube and push onto expander ball again. I can usually do it in two steps although three might be required. Use lots of lube.
3. Pull the expander ball out of the FL die, lube outside of case and size just enough so the bolt will close with some resistance. This will take some trial and error turning the die down until the bolt will close. I start with the die about 1/16" above the shell holder. This will give a zero headspace condition so you won't get head separations.
4. Put expander ball back in the FL die and expand the mouth enough to seat a bullet.
5. Load and fire with at least a medium load to expand case fully. Light light loads will not fully expand the case.
6. Trim to desired length.
In the next reloading session you can adjust the die to a true FL condition if you want. I use a Stoney Point headspace tool and push the shoulder back to zero headspace or back .001". I am on the 10th reload of my current 416 Taylor brass. It has a small shoulder like the 470 AR. My 458R brass is on the 7th reload.
Ken Howell recommends annealing the neck before forming operations. I tried it and followed all the recommended techniques and it made the brass easy to work but I crushed a couple of shoulders when I crimped the case mouth.

I just looked at your procedure. It looks like you are finished. They might shrink a little upon the first firing but probably not much.

Thanks for the feedback Dan. While I am familiar with reloading I am not that facile with wildcats. Certainly makes you appreciate buying factory brass and going through the simple size and seat process.

For others that might be curious, after forming the cases as described above, I loaded them with 82 grains of H335 and 500 grain Speer Bear Claw softs. Fired them at the range today and they worked perfectly. The shoulders came out nice and sharp. Feeding and extraction was perfect. Velocity was right at 2100 fps. I have never fireformed cases, so I cannot compare this process to that, but other than being a little time consuming it served the purpose of working quite well.

[FYI, I used a Dremel tool cutting wheel to trim the original .375 RUM brass to approximately 2.60-65" after opening up the neck to .474 diameter. Then used my Forester Classic to take off the last little bit down to 2.55".]

Mike, sounds like you've got it figured out....glad I could help :-)

Howdy Mike ...
The way you are doing it works great, and saves you a separate "just fireforming" step -- i usually set my dies where the gun will BARELY close on an empty case.. and i mean barely .. which gives my great fireformed cases, if I forego the pistol powder fireforming....

you still have my cell number, right?

Hi Jeff,

Hope all is well. If the number is still xxx-xxx-xx28, yes.

I have 60 pieces of formed brass now. That should get me past an upcoming trip. I am getting right at 2200 fps with 82 grains of H335 and a 500 grain Woodleigh solid. Rifle is 9.75 pounds and recoil is extremely manageable. I have enjoyed playing around with it. Only negative is that the magazine on my rifle will only hold two down, plus one up the spout.