I'll probably have the local fun shoppe order me a P320 this week, 40 cal OD Green compacts are on sale from distributor. Ideally, I'd want a 9mm to be another host, but there's  no threaded barrels to speak of in .40, but there are some in 9mm.

Currently, we have a 9mm and 40, both compacts in the store, the 9mm barrel fits in the 40 slide, but not vice versa. The gap around the 9mm barrel in the 40 slide is much greater due to the thicker barrel of the 40. This is normal in all semi auto Browning designs, but is that safe to shoot? Other brand pistols this is just fine; is it ok with the P320?

I've heard yes and no regarding the barrel compatibility of P320 and P250 parts. I'm interested in threaded barrels, and since 250 has been out for several years, more options are available for it. The lugs look the same, are they compatible or is Sig trying to sell more conversion kits by telling us they are noncompatible?

I'm not too keen on getting a $400 conversion kit if all I really need is a $150-200 barrel.

You need a CONVERSION barrel, you don't just drop a stock 9mm barrel in a .40 slide and go to town. A conversion barrel is thicker so you don't get the slop you saw playing around