I am a firm believer in training with what you carry - however - the majority of my rounds downrange are 9mm WWB/Blazer rather than Pow'rball (which I carry) due to the cost of premium ammunition.
So let's say I purchased an M&P40c as a 'training equivilent' of my M&P9c CCW, and then shot cheap(er) .40cal ball at the range to approximate the recoil management of shooting premium 9mm ammunition. Maybe you could benefit from reduced ammunition costs while still replicating the effects of shooting premium 9mm ?
At the moment, I shoot several boxes of 9mm ball, followed by one magazine of premium; they are so different that it sometimes makes me question the value of cheap ball for training.
Opinions on calibre aside - does anyone do this already ?