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So far I've managed to print the shotgun rail and it came out alright, although the end of the slide was a little bit warped maybe due to not enough support. It's not the prettiest, but it seems to work, so I think I'll just use it rather than trying another print.
With that being said, I did have a suggestion of a relatively minor change which I believe would make the design a lot easier to print. With the rails, what I'd suggest is moving the hole for the pin up so that it isn't sticking out of the bottom of the rail. Instead put it in the bottom of the rail (inside the main body) or maybe in the middle. Then if you flatten the bottom of the rails they could be printed sitting on their bottoms with only minimal support (mostly only the male end of the joint mechanism looks like it would need support in that orientation).
I think that would allow for overall a nicer/smoother print with less clean-up needed afterwards. It would help especially for the shotgun rail since it has the spherical slide which doesn't work as well with support as a flat surface. Anyway, just something for you to consider.