best I've looked through so far is the holosun eps green mrs, I just put it on a shotgun really close to back of receiver and it's wonderful...has all the features, good window size for wide fov mounted closer to face like that, enclosed emitter, solar w/battery back up, 3 reticle choices, super low deck height, easy to find dot/keep dot, feathery lightweight...like 1 oz, most minimal astigmatisms of any so far I've looked through, it's almost like my eyes have no issues lol....
problem will likely be how to find the right base and mount to get it on a rifle, it's a direct slap into the cut out on the new mossberg 940 shotguns though
to me would be worth knowing a machinist like Henry Rempel to take it and any project rifle to..to figure out best way to mount it, whether he has to create a base that meets rifle on bottom side and dot sight on top side or whatever....or make the rifle itself directly take it....incredible little sight imo