So. Would this .177 upper fit on my M&P 15-22? Anyone out there tried this out?

Would be mighty neat if it did.

Here's a link to it, if you want to check it out.

Crosman | Airguns | Rifles | PCP | MAR177 PCP Conversion Kit

"The MAR177 conversion kit replaces your existing AR/M4 style upper and converts it into a PCP .177 caliber competition air rifle. Designed to support 10-meter match air rifle competition, the MAR177 is just as well suited for target shooters who are interested in maintaining their same shooting platform."

I'm not entirely sure but if the AR upper is not compatible with the 15-22 lower I would highly doubt it. At a cost of $650 I'd be hard pressed to not just get a dedicated target air rifle and save a little money to boot.

subbed id be very interested to know if anyone tries this.

I read this:

"The S&W 15-22 lowers are slightly smaller to prevent centerfire uppers from being used on the polymer lowers."

here:

Crosman MAR177 test report: Part 3 | Air gun blog - Pyramyd Air Report

Actually, 15-22 lowers are slightly WIDER than MilSpec. While a centerfire upper can be made to fit into the 15-22 lower, it's impossible to make one work because there is no buffer tube on the lower, so the BCG will not function.

As for the Crosman, I have no idea how it operates, but seems to be another case of "not can I do it, but SHOULD I do it".