is the width of the weaver rail on the M15A4 the same width as the low scope mount for the MP5s?
They all use Weaver rails, which is 20mm. The one you've gotta watch out for is the Picatinny rail. Not sure about the specs on that, but I know that it's different because I can't seem to use Weaver rings on my DTP P90 triple rail.
So, the Leupold scope mounts sold at walmart would fit on the M15's weavers, right?
Yes, standard scope rings are designed to fit on Weaver style mounts.
Air gun weavers are 3/4in not 7/8in so if you see a scope or red dot at Walmart that says fits airguns it will not fit (I did that in the past and had to return it.)
[quote="Cl"]Picatinny rail is 22mm[/quote]