Here's a great article for your quest from the 6mm br site. I feel your 23" barrel will have to be a hot load even with these wildcats. If you go to 25" you'd be better suited. Also consider having your firing pin bushed because to get there your pressures will be high might as well do it right.
Hot Rod Versions of the .243 Winchester — BR-K and Super X
Gunsmith Mike Sosenko and long-time 6mmBR.com supporter 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.
Wicked Velocity with Stable Brass
The main advantage of the 243 BR-K is serious velocity in a case that is very stable. Mike’s favorite load is the 95gr Berger VLD pushed by Reloader 22. With a stout load of RL22 and Federal 210m Primers, Sosenko is getting 3450 fps with the 95-grainer, with no bullet blow-ups. This is with a 1:8.5″ twist Broughton 5R barrel finished at 28.5″. The cases are holding up very well. Mike has a half-dozen loads on his brass and he hasn’t had to full-length size yet. Mike runs a .262″ tight neck, but there is also a no-turn version of the case (see illustration). Accuracy is excellent. Mike says the round delivers repeatable 1/4 MOA groups at 100 yards in testing. He has also experimented with N160, but, thus far, Reloader 22 has delivered smaller groups with better ES and SD.
VIEW 243 BR-K REAMER PRINT (No-Turn Neck)
John Adams shoots a no-turn (.274″) neck 243 BR-K with 105gr Berger VLDs. He’s getting about 3230 fps using Reloader 22. John says he can push the 105s faster, but 3220-3240 fps “seems to be the sweet spot.” John notes that “after about five reloadings on a case, it gets a little tight”. John then full-length sizes with a custom Hornady FL bushing die. “The Hornady custom shop dies work great” according to John. Adams also shoots a version of this wildcat necked down to 22-caliber. It has demonstrated outstanding velocity and good accuracy in initial testing with a 9-twist barrel. Using the 80gr Amax bullets, John is getting 3570+ fps speeds. John feels that his 22 BR-K needs some more development work. “The 243 BR-K is proven. We know what works. With the 22 I want to try different seating depths, experiment with a few different bullets, and fine-tune the velocity.”
Whitley’s 6mm Super X
Robert Whitley shoots a variant of the .243 Winchester he calls the 6mm Super X. This features a 30° shoulder, and slightly less body taper. He gains a little case capacity over the standard .243 Win, and he says the cartridge is extremely accurate with both 105-108 grain pills and the heavier 115s: “Here’s a picture of a .243 Win (left), a 6mm Super X (center), and a 6XC (right). All I can say is the 6mm Super X has been good to me and I have shot many a clean in 600-yard High Power matches with it with either DTAC 115s or Berger 115s.”
While Mike Sosenko and John Adams use their BR-Ks to push 95s and 105s at high velocities, Robert takes a different approach with his Super X. He shoots the high-BC 115s and keeps velocities under 3000 fps. A long-range High Power shooter, Robert demands consistency during long shot strings. That means backing off from max attainable speeds, at least with the 115s. Robert writes:
“You can get 3050 fps with H4831SC and the 115s with no problem, I did it in testing multiple times, but to me that also does not mean anything because I shoot loads where they are the most consistent and accurate over a 22+ shot string. I have never found that any of the 6mm cartridges I have used with 115s will stay consistent, tight and accurate the whole way at 3050 fps for 22+ shots straight. I have tried 115s in the .243 Win, the 6CM, the 6mm Super X, the 6XC, the 6-6.5 x 47 Lapua and none of them ever stayed consistent and tight for 22 shots straight with the 115s at that speed. Most of the time with all the 6mm cartridges, if you get the 115s much over 2975 fps, they won’t hold tight for 22+ shots straight. Now if you’re a bench rest shooter and you only need to do a few sighters then 5 or 10 shots for record, you can run 3050 fps or more and the groups will likely hold tight during your string, but not when you need to go 22+ shots straight with no break. I have shot many different 6mm cartridges and done a lot of testing with many different powders, moly and non-moly bullets. I don’t find the ‘consistent accuracy’ (for 22+ shots straight) at those higher velocities.”