My 357 mag Green Mountain TC barrel had a sloppy (0.363" - 0.364") throat, for reasons I never figured out. Rather than setting the barrel back to rechamber it, I thought I'd try punching it out to 357 Maximum. The maxi happened to be just long enough to clean up the sloppy throat.
This time I made the throating reamer seperate from the chambering reamer, so I could tweak the throat independently of the chamber. The throat has 1/2 degree taper per side. While the reamer does include a short freebore section, for now I only stuck it in deep enough to cut the tapered lead, but not the freebore. The taper begins immediately after the case mouth at about 0.3585".
I'm not sure whether freebore is good and necessary for cast bullets, but one thing I do know is that I can always re-cut a throat fatter and longer, but I can't re-cut it skinnier and shorter.
So I am starting out with no freebore, and keeping the option to add freebore later.
Homemade reamers. The dimensions turned out well and they cut well. There were no unpleasant surprises.
OK, so now the barrel has a PERFECT throat. It should be a tackdriver, right? I loaded up some of the 160 gr. plain base spitzers that had been the best performers in the 357 mag, except this time I sized them down to 0.359" rather than 0.360", and seated them about 0.015" off the rifling for easy chambering, which worked out to be 1.994" COL. I walked the charge of WW296 up until I encountered sticky extraction, then walked the charge back until they extracted easily, which turned out to be 20.6 grains. I used Federal 200 primers because these cases happened to be already primed with them, leftover from a late 80's project. It so happened that the bullet could not be crimped at that COL.
So how did the PERFECT throat shoot? Not so great. There was some light leading in the barrel -- mostly in the middle -- and accuracy was worse than the sloppy throated magnum.
-- 4 MOA for 10 shots at 100 yards (vs 1.5" for the magnum)
-- 1.29" mean radius (vs. 0.55" for the magnum)
-- 1.69" radial standard deviation
-- 2167 fps (vs. 2000 for the magnum)
-- 0.98% standard deviation (vs. 0.8% for the magnum)Why Does The PERFECT Throat Shoot Worse Than the Sloppy Throat?
-- the plain base bullet may be crapping out at the higher velocity.
-- the bullet may be shaving lead in the PERFECT throat.
-- the 0.359" bullets may be too big for the PERFECT throat.
The reason it may be shaving lead is because I am cramming a 0.359" bullet into throat that may be only 0.3585" or so (I haven't made a throat impression yet, only some quick checks with pin gages). There is a 45 degree chamfer between the chamber and the entrance to the throat, but it's possible that it's still too abrupt to avoid shaving lead.Things To Try Next Time (or the time after that)
-- reduce the velocity to 2000 fps, same as the magnum. If it still leads the barrel and still groups poorly, then we'll know the root problem is the PERFECT throat, not the higher velocity.
-- try 0.358" diameter.
-- if neither of those things makes it perk up, then I'll probably conclude that this 160 plain base does not like the PERFECT THROAT.
-- At which point there will be other things to try -- gas check, or tweaking the throat to make it even more PERFECT.
Also, arranging for a crimp groove, and optimizing the COL. But for now my main concern is figuring out if the throat is optimal.