I am wanting some input on these conversions. If they're reliable. If it's not super expensive. And who makes them.
Thank you in advancement for all your help
The measurements I didn't see in that link as the width of the two factory barrel hoods and the barrel hood to front of the locking surface (chamber block). I expect the second measurement to be virtually identical. Design differences in hood width can produce issues that the number of rounds noted in the link won't reveal.
Storm Lake makes barrels specifically intended for conversion of the M&P40 to 9mm. They have an excellent reputation to reliability. I expect there are others.
I'll also note the 9mm slides are not exactly the same as the .40. the 9mm slide is lighter and there are other minor differences that can affect function.
The barrel hood is the rearward projecting tang from the top of the chamber area.
Manufacturers frequently change the configuration of that part of the barrel to minimize the use of barrels which may cause problems if used with the wrong slide. An example is the Colt Government and Commander barrels have different tangs, they also have different machining in the lug area.