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YES, Fits Mossberg 500, 590 & 590 Shockwave in 12 and 20 Gauge only. I hope this helps

I remember you asking this question before, James G down there is now telling you it'll work (He read Edwards answer and realized that his 3 second google search last time he answered was way off base), last time he told you it would only fit the 12ga. Edward told you it would work, then he provided a link to the manufacturers website where Crimson Trace said it WILL WORK, Edward also told you he has one on his 20ga and that it includes a shim kit.
Not really sure what else your looking for as an answer my friend because I'd say Edward has provided you with pretty clear and definitive information answering your question, and even supported it with evidence from Crimson Trace, if that's not enough to convince you, I'd imagine there's nothing anyone here can say that will be satisfactory.
So get it of tou want it, if not keep telling yourself you don't know if it'll work

John, yes it does fit the 20 gauge shockwave/590/500. A shim is included that fits under the saddle for the 20 gauge, fits like a glove. I have one on my 20 gauge shockwave.

Yes as long as it is drilled and tapped.

John, yes Sir. The maker, Crimson Trace says yes : crimsontrace.com/products/laser-sights/ls-250-lasersaddle-red-laser-sight-for-mossberg-12-20-gauge-shotguns/01-7810.html