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It looks like my previous comment got flagged because I edited it too much. Oops!
Anyway, I printed out the shotgun rail so far and it mostly worked but it came out a little bit warped at the end of the slide, maybe due to not enough support? It seems to work though so I think I'll just use it rather than trying another print.
That being said, I did have a suggestion that I think would make the design a lot easier to print. With the rails what I'd suggest is moving the holes for the pins so that instead of sticking out the bottom they were instead in the bottom of the rail (inside the main body), or maybe the middle of the rail. Then you could flatten the bottom of the rails and it would allow them to be printed sitting on their bottom side. In that orientation it looks to me like the only support you'd need would be for the male end of the joint mechanism. Overall I think it would make for a cleaner/smoother print with a lot less support required, and a lot less post-print clean-up.
Just something for you to consider.