Sorry if I’m stating the obvious, but SIG has used two different slide styles over the last few years. I believe the older slides were folded steel while the newer ones are milled stainless steel. I suppose if you had one of each there might be enough difference that the barrels would not fit, but really just speculating.
A major source of objection to a free economy is precisely that it ... gives people what they want instead of what a particular group thinks they ought to want. Underlying most arguments against the free market is a lack of belief in freedom itself.
- Milton Friedman