November 16th, 2007

"243 BR-K" (6BR Long) Wildcat Delivers 3450 fps with 95gr VLDs

Gunsmith Mike Sosenko and long-time supporter John Adams have been using a modified .243 Winchester case with great success in California Varmint Silhouette matches. 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.

6mmBR long .243 Winchester Wildcat

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.


243 BR-K and 22 BR-K wildcat cartridge

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 5 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.”

Pala silhouette range

243 BR-K in Competition
John Adams and Mike Sosenko developed the 243 BR-K Wildcat for use in monthly varmint silhouette matches at the Pala range in Southern California. Shooting prone with front rest (as in F-Class), participants engage metal targets at multiple distances out to 600 yards. It can get very windy at Pala, so you need a cartridge with great inherent accuracy, plus good wind-bucking ability. With its capacity to drive the .514 BC Berger 95s at 3450 fps, the 243 BR-K has what it takes to win at Pala. Shooting his 243 BR-K wildcat, Mike Sosenko has won many relays at Pala, and he is the only shooter to have shot a perfect 50 score, knocking down every target. Interestingly though, John tells us that “Mike’s five best scores this year, including his perfect 50, were shot using my guns with no-turn necks.”

How good are the ballistics for the 243 BR-K? Consider this… launched at 3450 fps, the 95 Bergers drift just 18.57″ in a 10 mph crosswind at 600 yards. By contrast, a 105gr Scenar shot from a standard 6BR at 2900 fps will drift 22.95″ at the same distance. The 243 BR-K exhibits nearly 20% LESS DRIFT at 600 yards–clearly enough to make the difference between a hit and a miss. (Ballistics calculated with Point Blank software for 70° temp, 1000 feet elevation, using manufacturers’ BCs.)

Click HERE for GIANT SIZE Range Photo.