I posted a couple of times on Burnside topics, and have thought of a novel approach to use conical rounds (.556 diameter are typically available to fit either the brass or nylon cases sold today). These rounds don't fill the full depth of original rifling (.560), so thats why they don't group very well. Up to now I've been modifying brass cases to accept .562 rounds (round or conical) AND soldering a .030 #8 brass washer to the base so the case will chamber properly and not move rearward when firing (which causes case failure). The nylon cases are long enough, but stretch when a full-diameter round is inserted and will not chamber. A proposed solution ?
Have the Burnside re-lined to a .540 nominal bore with .075 deep rifling at 1:60 twist WITHOUT gain twist. That will yield a bottom-of-groove max diameter of .555 - and so a .556 conical round (like many Burnside molds make, as well as the size on pre-packaged alloy Burnside rounds) will be perfect for the bore AND will fit into either the brass or nylon cases available today. There is still the length issue on brass cases, because they can vary considerably as supplied, but nylon cases appear to have only one source and are long enough.
Of course, one can argue that you are no longer shooting a 'real' Burnside. Hmmmm, I'm still working on getting my Burnside to shoot well as manufactured, and have put too much work into modifying my brass cases to consider a re-line. With .562 round ball (37.5 grains 3F) I can get all in the black at 50 yards. It will take additional mold modifications to see if I can get the right conical round length (thus weight also) to work at 50 AND 100 yards. I'm milling off the aluminum mold from Accurate (I started with the longest round offered, and even then had to touch-up grind the base rings with a variable speed 'dremel' to get .562 diameter instead of the .560 that would originally come out of the mold.) a little at a time to slowly bring the length down to see if I can get the group to tighten at some point. The brass case mouth can be hand-reamed with a .562 reamer with care. My adjustable reamer had to have the front adjustment nut ground around the periphery so that it would fit inside the tapered portion of the brass case. A single-purpose reamer (as available form industrial suppliers like MSC) won't have this challenge, but a tap handle is still used to accomplish hand reaming.
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