Gemsbok405
09-21-2014, 05:07 AM
As my favourate "fun rifle", the above gives huge shooting pleasure from can shooting to falling plate to paper target.
But struggling to find the relationship between the SAAMI chamber and actual cut chamber and matching dies...
The SAAMI drawings show a tapered case with a parallel neck profile (similar .40-72 style), while all fired cases come out straight taper like a .38-55...but a shorter case with thicker brass. It would appear that with the tolerances of SAAMI chamber, can just cut the abovementioned straight taper. A visual inspection of sizing die / trim die also have same straight taper...
Then when size cases, come out with an internal mouth ID of about 0.374 (ideal for J-types) but too much constriction for a cast bullet at nominal 0.376+. As mentioned in previous posts re the 405 win, are going to try the 375 win trim die (RCBS) to either fully or partly size the case and see if works the brass less with CB loads.
To date, have been using the RCBS 37-250GC bullet, and has worked well...with the odd flyer. The cause has thought to be the problem of bullet front band (ahead of crimp groove) being too short for throat and bullet lying skew in chamber transition and 'bouncing' into alignment with throat/leade when fired.
Have a new Mountain Mould 275gr GC bullet with an extended front band (no crimp groove) and which will put the front band substantially into the parallel throat upon chambering. Will post pics of this shortly...and hope to create a higher pressure version of .38-72 in this rifle/chambering.
Of course, I could have just seated the RCBS bullet more out to improve situation, but this is a hobby and alway looking for an improvement over factory mould options...
But struggling to find the relationship between the SAAMI chamber and actual cut chamber and matching dies...
The SAAMI drawings show a tapered case with a parallel neck profile (similar .40-72 style), while all fired cases come out straight taper like a .38-55...but a shorter case with thicker brass. It would appear that with the tolerances of SAAMI chamber, can just cut the abovementioned straight taper. A visual inspection of sizing die / trim die also have same straight taper...
Then when size cases, come out with an internal mouth ID of about 0.374 (ideal for J-types) but too much constriction for a cast bullet at nominal 0.376+. As mentioned in previous posts re the 405 win, are going to try the 375 win trim die (RCBS) to either fully or partly size the case and see if works the brass less with CB loads.
To date, have been using the RCBS 37-250GC bullet, and has worked well...with the odd flyer. The cause has thought to be the problem of bullet front band (ahead of crimp groove) being too short for throat and bullet lying skew in chamber transition and 'bouncing' into alignment with throat/leade when fired.
Have a new Mountain Mould 275gr GC bullet with an extended front band (no crimp groove) and which will put the front band substantially into the parallel throat upon chambering. Will post pics of this shortly...and hope to create a higher pressure version of .38-72 in this rifle/chambering.
Of course, I could have just seated the RCBS bullet more out to improve situation, but this is a hobby and alway looking for an improvement over factory mould options...