So I've been wanting to dip my toe in the RDS on a pistol for a while now, and I thought about picking up an OSP XDM would be an easy entry.

But they don't make one in .40 (yet, or ever?) and since that's what I've consolidated all my pistols in...

I could just get a .40SW barrel and drop in the 9mm?

The breech face, being sized for 9mm, would be too narrow to fit the .40 case, wouldn't it?

This, among other reasons.

@OP - short answer, no.

Long answer, hell no.

1) the breech face likely won't fit.
2) the barrel probably won't fit.
3) the 9mm is not meant to handle 40 pressures

You'd stand a good chance of beating your gun to death real quick, and add in a dash of "possibly explosively destruction." But hey, it'd make a great LiveLeak video I'd bet.

Just run 9mm. It's a better round anyways.

You can convert from a 40 to a 9 generally, but not the other way. Even if the barrel and casing did fit, you'd be subjecting the gun to significantly more abuse than it's meant to handle.