Plan to add a Holographic type of red dot sight to an M&P 15/22.
My dumb question is, what is the difference between Picatinny and Weaver mounts, and are they interchangeable?
I am old school and have always used open sights. My grandson will be shooting this gun more than I will and he seems to think it should have one of those sights to be "COOL".

You shouldn't have any problems at all. I believe, from what I've read that the Weaver bases can have some varience but Picatinney rails are evenly spaced. I've tried several different optic set ups on my guns and the only problem I've had is when I wanted to put a very large Burris scope on my PC with a 50mm objective. I needed to put extended length rings on it to get clearance. I do still get a good sight picure with a cheek weld.
Good luck to you and enjoy the leetle friend.

Hobie

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GunTech : Picatinny Rails, Weaver Rails, What?s The Difference? - World's Largest Supplier of Firearm Accessories, Gun Parts and Gunsmithing Tools - BROWNELLS

Thanks, This is exactly what I was looking for and it answers my questions concerning fit.