The process I use to make .300 Blacout brass from .223 Rem. is as follows:
(1) Cut the neck of the case off at the base of the shoulder. I use a dremel tool because that's all I have.
(2) I clean up the rim of the case by trimming and then chamferring.
(3) Take the expander assembly out of the sizing die and then size the case with plenty of lube to make the effort easier and reduce the stress on the bras as much as possible.
(4) Put the expander assembly back into the sizing die and re-size the case to exact dimensions.
(5) Trim the case to minimum OAL and re-chamfer.
Like I said, it laborious. If you had a Dillon trimmer and trim die set-up, it would make life a whole lot easier. I'm going to order those items in the next couple of weeks. I can use the trimmer for other cartridges I shoot for so I see it as an investment well worth the money.
I've even thought of importing a large amount of brass myself and re-sell it to individuals like ourselves who are shooting the Blackout. I'm truly pissed that Gravels is not going to support a Remington product.