Bingo!!!!
use a differential thread. Have an in between piece that threads into the action with different threads in the barrel. That way if you turn the barrel .25 turn in, move this same amount out and it gives the same headspace, just a different barrel index spot.
Jerry, you've described it almost to a "T"
Inserted in between the action threads and the barrel tenon is a bushing that can be adjusted fore and aft in the action. This makes it possible for the barrel tenon shoulder to contact the receiver face at any position of the clock. Of course, the barrel tenon must be turned down and threaded to match the inside threads of the bushing. I used .940 as the major diameter of the tenon and threaded 28 tpi. Everything is a class 3 fit.
The bushing was made from an old barrel tenon as it already had the correct threads. After indicating the OD, I used a carbide boring bar to open up the ID and single point threaded 28 tpi. The finished bushing looked forever like a pipe nipple but I was quite proud of the workmanship; it fit perfectly.
Using a small end mill, I cut two slots to accept the tool I made to turn the bushing. It worked great from the get go but I soon ran into a problem. The bushing was turning in the action messing up my carefully planned index positions. There had to be a way to lock the bushing in place so it wouldn't move as the barrel was screwed in and tightened. I agonized over the situation for a couple of hours, tried teflon tape which didn't work, and finally decided the best solution was a set screw.
I said I would not make ANY modifications to the action but I was forced to reconsider. In the bottom of the Cobra action, there is a 1/4-28 threaded hole that goes thru the aluminum body but does not penetrate the steel insert. I drilled on thru the steel insert and continued the 1/4-28 threads all the way thru. A small stainless steel set screw now holds the bushing securely in place. Adjustments are quick, easy and secure.
IT WORKS GREAT !!
Two safety concerns came to mind and I would appreciate your input. One concerns the reduced wall thickness of the barrel tenon over the chamber area and the other, the thickness of the bushing between the two rows of threads. I chose 28 tpi for the threads because they do not take up as much depth.
I spent two days in the tunnel testing the system. The first shots were fired with appropriate shields in place for safeties sake. The rifle is a Cobra sporter chambered in 6 Beggs stocked with one of my Ultralite aluminum stocks. Barrel torque was rechecked each time the barrel was re-indexed and never seemed to loosen up. A full case of H4198 behind a 68 grain bullet was averaging 3325 fps with outstanding accuracy. The sweet spot was found on the second adjustment at the six o'clock position.
After a couple of days of testing, I now have complete confidence in the rifle and indexing system and unless problems arise, we will soon be tooling up and producing kits which will require gunsmith installation.
Isn't this an exciting time to be involved in benchrest?
Later,
Gene Beggs
