I tried quite a few 6mms over the years. I currently own the extreme ends of performance with a 6BR and a 6mm-284. I own the reamers and do the chambering myself.
There is a wildcat I may have to try one of these days. The 6 BR reamer would be used to cut deeper to headspace as a 243. The shoulder angle would be changed to 30 degrees with less body taper. I used my 30 BR reamer to do exactly that with the 308 win. The neck does require turning but end result is a super accurate cartridge. A BR die can be used for sizing and seating.. I have gotten 10 firings of the lapua 308 brass before primer pockets loosen. (max loading with 215s)
Here is some more information on the 6mm and 22 dubbed the BRK
,October 6th, 2013
Hot .243-Based Wildcats — The .243 BR-K and 6mm Super X
Gunsmith Mike Sosenko and long-time AccurateShooter Forum member John Adams have been using a modified .243 Winchester case with great success in Varmint Silhouette matches at the
Pala Range in Southern California. Officially called the ".243 BR-K" (and informally dubbed the "6BR Long"), the wildcat is basically a .243 Winchester with less body taper and a 30-degree shoulder. The design essentially grafts a 6mmBR Norma "top end" to the .243 Winchester case. After fire-forming, Mike and John can reload this case using normal, unmodified 6BR neck-sizing and seater dies.
Compared to a .243 Winchester, the .243 BR-K's body length is about .006″ longer, and the shoulder is about .0055″ wider. The main difference is the shoulder angle (30° vs. 20°), and the location of the neck-shoulder junction ("NSJ"). Based on reamer prints, the base to NSJ dimension is 1.718″ on the 6BR Long, compared to 1.804″ for the .243 Winchester. Neck length is a bit shorter because "the neck shrinks a little when the shoulder blows out" according to Sosenko. We've provided a mock-up diagram of the .243 BR-K, but you should check with Dave Kiff of
Pacific Tool & Gauge for exact dimensions. Dave created the reamers for both the 6mm and 22-caliber versions of this wildcat. Ask for the "22 BR-K" or ".243 BR-K" reamer designs.