I was wondering if anybody knows if a .22 conversion kit would work on my M&P 15. Also, if it does, what would work better. The quick bolt kit or a dedicated upper? Also, would it have an adverse effect of the warranty?

I looked into for my LMT and did research. Your accuracy will be off cause the rifling of the barrel is made for the 5.55 or .223. There will be a gap before the .22LR. That's why opted for the M&P15-22

A couple of places offer full .22LR uppers.

Ar - 15 .22lr Dedicated Conversion Upper, Shooting Accessories, Velocity Firearms, Ar15 Ar 15 Rifle 22lr 22 .22 .22lr Lr Dedicated Conversion Upper 193198 at Sportsman's Guide

If you want one and one half rifles buy a complete dedicated upper.
Or you can get a complete rifle that does everything you want and you don't foul your AR bore in the M&P 15-22.

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Buy cheap, pay twice.

"Will a 22 conversion kit work on my M&P 15? "
Yes. Which kind works best for you, I don't know.

- Conversion Kit = Bolt+Black Dog type Mags. These are the cheapest option, and many what don't need accuracy (lead in and bore diameter as mentioned above) enjoy these. One issue is that the "dirty" .22 can fowl/plug the gas port/gas tube. Many recommend switching back to the 5.56 bolt and fire a few rounds after using the conversion to clear out the gas system. These can be found at times for as low as just over $120.
- Dedicated .22 AR upper. There are many of these. I’ve been looking at CMMG as they are the only vendor that impliments "last round bolt hold open". CMMG .22 uppers start at around $350.
- Dedicated .22 AR platforms, such as the 15-22. Of all of these I like the 15-22, as it implements the full function (except for forward assist) including last round bolt hold open. We have a 15-22 and really like it.
- A complete AR, that happens to be chambered for .22. Many manufactures, if going this route I'd look at CMMG for the reasons listed above.

There's much more to be said for each option, but that's a start.