Frank,
Well, we can agree on most things.
Alignment is the name of the game not only in fixed ammunition but all CB shooting. And tapers align things. It is just done in a different way in breach seating as you note.
I agree that excessive extruding is a bad thing. All successful CBs are extruded a bit unless cast at exactly groove diameter which isn't what we usually do. I guess you would also have to cast the grooves into the bullet to eliminate all extruding but that is picking nits.
We both like full spitzers and mine are a two caliber ogive, not four as you suggest because I didn't want the nose to take up too much length but they are still pretty spitzery as Joe observed. The meplat is about .03”. This is half the diameter of Sierra 53 grain Match Kings and very sharp. However, that is only because David Mos is extraordinarily skillful and other mold makers that will make 22 caliber molds at all, either can't or won't do it. So we probably will have to settle for a bigger meplat if the maker uses lathe boring.
As far as the gap ahead of the gas check, your design could be changed. Is there some reason to minimize the gap? My Mos molds have a gap of about .015” which seems to hold all the lube needed. However, I have no objection to having a small lube groove after all it is traditional to have grooves and it might be needed. If the bullet had both a gap and a small groove it would give more lubing options.
How the bullet seals the bore and aligns itself is where we don't agree. Maybe you are right that the rear band can seal the deal. In breach seating that is true, but with fixed ammunition the rear band is back in the case mouth when the things start to happen and has to move forward before it touches anything solid except case. Maybe this works if things are right but with the rifles I have been shooting unless I get a good seal on the front band with the seated cartridge I have trouble.
It seems to me that with a properly sized bore riding nose the bullet alignment is completed by the nose before the cartridge is fully seated and before the front band touches anything. So I don't think the front band has anything to do with alignment ”€œ at least with either your or my designs. The tapers involved in alignment by the nose are the throat taper and the curve of the ogive just before it reaches full size and both are very gentle. Of course if the nose is undersized the front band is all you have to align the bullet but then the taper involved is the much sharper taper leading to the ball seat. This would work but I don't think either of us is assuming an undersized nose.
The only other area of disagreement I see is whether the bore riding part of the nose should taper or be a cylinder. If the bore diameter were exactly .219” your bullet would be a bore rider for the full length IF it were fairly soft. But if of linotype, or harder, the .221 rear part of the nose would be very difficult to engrave and would probably push the bullet back in the case somewhat ”€œ at least that has happened to me when trying to engrave linotype bullets even less than .002” over bore size. However, if the bore dimension was .220”, as some of my factory rifles are, only half of your bullet's nose would be touching anything and the front half of the nose would be ineffective. I think a cylinder is the right shape for the bore riding part. Of course it may have to be fitted by lapping, alloy adjustments, or even beagling but when fitted it will be guiding with the full length.
I suspect that some of our differences of opinion are because you may use more custom barrels and carefully cut chambers and swage bullets after casting and thus may have more confidence in the SAAMI numbers than I do. My main interest is in getting accuracy with factory chambers and bores with all their warts.
John