The reason I say not to use snap caps for function testing is simply because they are NOTdummy ammo, and can give misleading (false) results.
While the dimensions are essentially correct, snap caps do not have the weight of live ammo, and this can lead to errors if using snap caps to check functioning.
One 1911 I know (and a high dollar one, at that) was found to jam solid when you tried to eject a loaded 230gr FMJ. JHP ammo would eject. SNAP CAPS would eject. Loaded 230gr ball ammo would not eject, and would jam the gun requiring dropping the mag, and then using about 3 hands to hold the slide back to reduce tension on the jammed round, and prying it out with a screwdriver. We repeated this several times, and I am convinced it was the lighter weight of the snap cap that let it move just enough differently than a loaded round and so let it eject, while the dimensionally identical FMJ round buried its nose hard against the slide, jamming the gun.
That gun had the wrong ejector installed, at the factory! Worked fine ejecting fired cases, and the shorter than ball ammo JHPs we tried, worked fine ejecting snap caps, jammed when you tried to eject a 230gr FMJ round.
Factory fixed at no cost to owner, with apologies. Works fine, now.
That's why I say don't use snap caps for function testing. The fact that it workd with snap caps is NOT a guarantee it will work with live ammo. It might, but its not a guarantee.