While I have no personal experience with this set up, I did send one to a good friend OCONUS who owns a Winchester 1300. I contacted Calvin at Brightflashlights (they are in the Industry Forum) and he sent me a bracket that mounted to the extended magazine tube and a wide ring that clampoed to the mounting bracket. The 1" ring held the G2 very tightly. I also added a shock-isolated bezel and a "clickie" tailcap, and shipped it. It has been mounted for almost a year now and my buddy says it works great for him. He did fire a few test rounds of SG Buckshot to be sure it would work, but usually removes the lamp and batteries when he shoots it at the range. (No need to tempt fate by constant pounding when it can be avoided)

IIRC the whole shebang was around $90-95 delivered. Calvin will happily advise you on the best set up for what you want and can afford...as will Grant at G&R Tactical...also in the Industry forum. Both are not only strong supporters of our site, withj some of the best prices you will find on high quality lights, but also genuinely nice guys.

I believe several others on this forum, including Hard_Shell and Bigger_Hammer have similar set ups on their 870s that will interest you also.