...interesting...they never said anything that I remember about not being able to shoot .38s or .357s...however...
I had two .357's modified. As both are fairly rare guns I bought extra cylinders off ebay that fortunately dropped right in and timed fine with no gunsmith modifications. They were a 60-10 J-frame 3" with adjustable sights and a 66-3 3" that is the first batch of 3" 66s made.
So far from those two guns I have only shot them with .38 Supers in the modified cylinders.
The third gun is a 649 2" .38 Special +P. This gun just had the original cylinder rechambered and milled for the moon clips. I have fired all three rounds from this gun including some +P .38s and had no problem with ejection or bulged .38s.
Next trip to the range I'll bring all three guns and a variety of ammo and see what happens...
...and yes I do reload...close to 50 years...
I had the same concerns as you about the enlargement of the chambers which is one of the reasons I bought new cylinders for the two Magnums...
Also remember there was The Medusa...it shot everything from .380 ACP to .357 Magnum... I think these conversions will do the same.. I was told specifically NOT TO SHOOT 9x23 Winchester as the pressures were WAY too high.
Bob
