Forming Brass into a Different Cartridge How-To, Please.

[03psd]

So I am taking the plunge and getting into .375 Raptor which is .308 brass reformed into .375 and trimmed down. Bullets are from 270gr to 350gr and will feed from slightly modified PMags in DPMS format AR10 rifles.
My question is about opening up the case necks from .308 to .375. I have purchased a 2 die set from CH4D (FL Sizing and Seater) as well as an expander die body and 2 tapered end plugs, .30-.33 and .32-.37. I have never used one type brass to make another caliber so what is the exact procedure? Do you use the end plugs to open up the case mouth first before using the FL sizing die? Doesnt the sizing die have a mandrel with a ball on it that would open up the case neck to the necessary diameter? If so then why are the tapered end plugs needed? One video I watched online shows a guy resizing 1x fired .308 brass in a single step (rockchucker press) into the .375 raptor case and then trimming it in a Giraud trimmer. I get the trimmer part, I am just wondering if I am unnecessarily adding steps with the 2 different tapered end plugs plus the FL sizing die. I dont want to tear up brass with split necks and would prefer to do it right in order to make the brass last as long as possible. Please educate me on this. Thanks

 

[BearNaked]

I make 260 Remington out of 308 so I size down but it should be the same basic process. these are the steps I take to do this and you should follow something similar.

1) Make sure your brass is clean and inspect for any abnormalities. when in doubt, through it out. A $1 piece of brass is not worth a hospital visit.
2) Anneal it so the case necks soften up so they will stretch and not split
3) Lube the shit out of it. this is the one time I may even over due the lube since I am doing a drastic change and there will be a lot of pressure on the expander ball. don't want it to get stuck.
4) you can try the tapered plugs and see what happens. what I do is just run the press several times over the case. I start out just barely going past contact or when you feel resistance and then back off and go a little deeper on the next and so on. just really working my way up to a full pass in the press. it might not be necessary but it works.
5) Trim, chamfer, debur, uniform flash hole, remove crimp
6) I trim the necks since I went down in size and the necks are to thick to fit into my chamber. then I'll full length size again to resize the necks. I highly doubt you will need to do this since you are stretching the necks and that will make them thinner in theory.
7) Clean, load, shoot, repeat.

go slow and inspect throughout the process. I would put your micrometer on the case necks just to make sure you have a somewhat consistent neck thickness and there isn't any spots that are super thin. I like to keep mine at .015" but I wouldn't go below .010" in my unprofessional opinion.

good luck and let us know how it goes and any lessons learned along the way.