I just built a .300BLK upper to hog hunt with. I've assembled some loads that I have no idea what the jump is. They cycle fine, but it bugs me not knowing exactly what's going on and as I experiment with different bullets, it's a PITA to find the proper length so they will function. So here's where I need some of the boards brain power:

There are absolutely no .300blk modified cases available...anywhere! I ordered an extra .223 case, figuring I'd just cut it down and form it to 300blk.
Now I realize that after forming the new case to 300blk, the neck will be too tight for a bullet to freely pass through it.
If I somehow stretch it out, it won't fit the chamber.
I do not have the capability to turn the neck, inside or out.

I know there's a way, but I can't think of it. Any ideas??