I have just recently taken a 7TCU forester seater and made a new floating chamber to seat bullets for 300x221.
I simply made a piece that replicates the 7TCU seating chamber, but used a 300x221 reamer instead.
The area where the bullet slides up inside is then reamed 0.309"
I have not even made a proper 30 caliber seating stem yet, but the bullets are held so straight in the reamed bore that it does not seem to matter.
The 7mm seating stem simply pushes the bullets down into the case.
Don't take this as an open offer to make more of these, it was a pain-in-the-butt.
If several people were interested, I suppose I could setup and run some...
You can convert any of the smaller forester seater dies if you already own them.
Randall Rausch
www.ar15barrels.com