What kind of dimensions for a muzzle brake should be used for a 6mm? I have a 243 for long range precision and would like to bring muzzle rise down a little more.

Why not just buy one from Brownell's? They are all good, some better than others. For a 6mm it would not be rocket science. I make my own.

Actually Jim it is rocket science.

Newton's 3rd Law of motion: "For every action, there is an equal and opposite reaction. Rustin, if you want to push the muzzle down, then place a hole, or, series of holes on top. If you want to push the muzzle to the left, then place a hole, or, series of holes on the right side of the muzzle.

Likewise, if you want to push the muzzle to the right, then place a hole, or, series of holes on the left side of the muzzle. If you want to push the muzzle down and forward, then angle the holes up and back, or, provide a wall for the exiting gas to slam into.

The object is to direct the gas in the opposite direction you want the muzzle to move. Rocket science!

Do you want to build a screw on brake, or have it machined integral to the barrel? Depending on the barrels dia you could bore 3/8" holes or 1/4" slots from the side of the barrel with a mill thus making a baffle style integral break. Bore a hole or slot from the top into the baffle. Also counterbore 0.010" over bore dia past the base of the rear most baffle.

we have a machine shop in the high school where I put the weapon together and hee shop teacher and I are interested in building screw-on brakes. Paul H is the the .01" a crown? Even though drilling the hole straight through w/ 4 jaw chuck will there be problems w/ bullet catching edge or interior of brake if drilled to 6mm?

All you need: http://www.brownells.com/aspx/ns/store/ProductDetail.as...title=MUZZLE%20BRAKE

These really work. I have one on my 300WSM. They are 2.50 inches long and require a thread of half-inch be machined onto your barrel. They add a total of two inches to the end of the barrel. The exit hole is best opened up to .020 inches over bullet diameter after the brake has been installed on a barrel that has been indicated to .001 inches off the bore. Then you contour it in and it looks fabulous. I have the Shrewd on four guns.