These are not hollow based bullets. The rebated base expands under pressure from the powder charge behind it. The hollow base isn’t necessary.
In
@Black Jack Shellac case his chambers are significantly under bore size and a hollow base may provide an imperfect solution. (Ask anyone who’s using 38 special hollow base wadcutter ammunition in .36 caliber conversion revolvers.) a far better solution is to ream the chambers to at least .001” larger than groove diameter and use an appropriate bullet or ball. It’s cheap and easy. Molds for such bullets are also cheaper (and easier to use) than those for hollow based bullets.. your mileage may vary.
As for the relationship between cylinder chamber dia and barrel groove dia, Ruger Old Army looks good. Chambers are .002-003" above barrel groove dia. Problems are those Italian clones with undersized cylinder chambers.
Regarding cylinder chamber reaming to achieve larger dia than barrel groove bore dia, that is a big issue for many of us. I've heard about gentleman boring undersized cylinders for 45 Colt SA Ruger revolvers, but seems to me that he closed business. Apparently, he had enough dealing with some folks. As for me, and other folks in my area with Italian clones, that is basically mission impossible. First, we have to get proper reamer. Second, it has to be done on milling machine, with reamer chucked in the collet, while cylinder chamber is properly centered. Third, it should be done by somebody who knows what he is doing. Of all smiths I know here, none is qualified to do this, cylinder will have to be done in Austria. If cylinder is ruined, we will have a big problem, because to get another one fitted, revolver has to be send to Italy, assuming Uberti and Pietta are willing to take it. Considering all that, hollow base bullet mold looks to me as a better solution, despite more expensive and more complex mold and casting procedure.
As for using 38 Special with hollow base bullet in 36 revolver, well, we are talking about .357-.358" bullet in .375 barrel, and .017-.018" difference in diameters. Too much for any reasonable accuracy, even using pure lead. Loading just lead bullets in 36 revolver, a tool to bump 38 bullet on appropriate dia for 36 cylinder, with all grease grooves and hollow base filled with lube to prevent bullet collapsing. Never done it, but some shooters told me it worked for them, albeit with solid base bullets, no hollow base.
It does seem to me that the hollow base is of value only if the chambers and bore are miss-matched. The disadvantage I see with the hollow base is that it makes for a longer bullet, which will reduce the powder charge. Having the chambers reamed/sized to the proper diameter would be a better solution. And, it should improve accuracy more than an undersized hollow base bullet would.
For the same bullet weight and nose shape, powder charge will not be reduced with hollow base bullet. The trick is when bullet is in the chamber, for the final ramming, revolver should be pointed down, and powder will fill hollow base cavity. Air should go upward and exit through the nipple.
