I have 8 slabbed 1/2x 28 Mini Magnum Muscle Brakes. they are all .22 cal and need be opened up to what ever caliber you are installing the brake on. They are .850” tall and 1” wide. $50 shipped each
As I understand it, the Muscle Brake was invented by Jim See at Centershot Rifles, at least 10 years ago. His design goal was to produce a brake that provides significant recoil reduction while being (somewhat) pleasant for the shooter via reduced muzzle blast felt by the shooter. The first ports (closest to the receiver) are aimed 90-degrees outward from the brake while the remaining ports are aimed slightly backwards toward the shooter. The sideways blast from the first ports are intended to "catch" the slightly backwards blasts from the remaining ports, thus shielding the shooter from a great deal of blast. I have no idea why he called it a Muscle brake, but it does work as intended.
