If one had a fns40, and a competent machinist, could one ream out the 40 s&w to 10mm? By could I mean would it actually work, not yes you could but would be to thin and kaboom. That and stronger recoil spring and you'd be golden yes?

I do not think the mags are big enough for the 10mm.

I don't think this conversion will work in nearly any circumstance.  The .40 cal was developed (in part) to be larger caliber in a 9mm size frame.  The key dimension to make this work is cartridge length.  The 10mm is a longer cartridge, so I'm gonna say it's a no go.

Biggest issue is cartridge length. Mag demention front to back to short.
10 to .40, 40 to 9.

May not be enough chamber length to go 10mm without a kaboom. Maybe if no full boat rounds... But then why bother you can get 500ft/lbs plus out of .40. Full boat 10 is only about 35% more energy and depending on barrel length you may not see that much.

No. I don't believe it would work for the reasons listed above, most importantly the overall length of the cartridge. Also, why? Pistols generally fall into one of a couple of categories as far as their intended use. Some are designed for hunting, some for sport shooting competition, and the majority are designed for defensive use against people. I don't know about sport shooting, but I can assure you that .40 S&W is plenty sufficient for defense against people. If you need a pistol for hunting or defense against 4 legged predators, I'd skip the 10mm Auto and go straight for a .44 Magnum or bigger. The 10mm isn't going to drop a human any more effectively than the .40 S&W. I've owned several of both.

I bought a 10mm 1911...miss 45 :(