Hmmm... re: air space in cartridge guns... IMHO, it much ado about nada! Feel free to flame away.
IMHO, it doesn't matter as long as the case is at least 75% filled. The prohibition against leaving an air space in BP guns is more geared to non-cartridge guns, especially those used in hunting with lubed conicals... In a cartridge gun the brass case acts like a "gasket" between the "explosive" force of the charge and the barrel. In addition, the seal of the bullet to case, is generally far greater than that of a lubed conical to barrel wall. But, to the point... A "ringed" barrel is thought to develop from two main causes, shooting an obstructed bore, or a secondary detonation. An obstructed bore can happen with the preceding charge doesn't clear the projectile from the barrel. Your "air space" is now wholly independent of any density of load in the next cartridge. A secondary detonation is described as when part of the charge is detonated and the ball & patch move partially down the barrel... then the rest of the powder charge is detonated.
So, when will a less than full volume charge cause a separation of the powder charge & create a situation where the powder charge becomes a primary and secondary detonation? Especially in a cartridge case with a crimped in bullet. Enter your theory here: ____________________________________________. Mine is that it can possibly happen when the powder in the case has enough room to separate into two wholly lumps in the case... something less than 50% load density. Ergo, my limit of not less then 75% of case volume.
I've been shooting between 28 & 30 grain of Goex "Cartridge" powder in my 45 Colt cases since it was introduced... and still have nearly a ½ case of the last one I purchased. I'm now loading APP in the same manner in the 3F granulation and shot these in my 45 Colt rifle at both EOT and CAC this year. I don't shoot BP in my suppository revolvers, but if I did it would still be with the C45S case, APP & a 160 bullet. I also only shoot about 43 grains of the same in my 12 ga hulls with 1 oz shot. I must admit I don't know if that constitutes a "no air space" column with the 12R (or equivalent) wad I use, but it makes for a better crimp than my 38 grain loads...