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I'll take a look at this kit probably later tonight.
I have a suggestion already for printing the body in two pieces that can be screwed together (its not gorgeous, but it doesnt disrupt the body lines much) but I'll need to do up a demo so you can see. I printed a carbine kit more than twice this size on an ender 3, you just have to split it up smartly like SRU does ;)
Also, as a suggestion, use self-boring magnesium wood/drywall screws with holes to set the rails in. Theyre strong enough to hold with PETG and even PLA+ for heavy attachments (that's how I did my universal carbine kit and can mount a full size M203 on it)
Good work so far though, I like the overall lines and I love the back-side racking mechanism. I have some suggestions for that part too but I think I want to print it as-is to see where the weak points are first, then work on reinforcing it.