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That's just bad design. Hear me out though, as I'm not trying to bash your work but rather remind you of the thermodynamics of plastic. If you have to adapt printers to use helpers like brims you didn't design the item at hand specifically for 3D printing. Hell, the print orientations for the shotgun parts are possibly the weakest considering the forces involved and the most likely to introduce friction when racking the foregrip. The idea is solid but the execution is lacking.
Hard angles don't print well, especially on the bed surface. You obviously know this as you point out the use of brims, but it can't be reiterated enough. The parts should be redesigned to have chamfers (or fillets even for wider corners) so the plastic tugs more peripherally when cooling rather then inward (which causes curls). Where two outer lines meet at an angle, the plastic pulls generally into the halfway angular point between said lines. A 90 degree junction will pull toward the innermost 45 degrees of that right angle. Two 45 degree junctions in close proximity will pull at an average of 45 degrees, but with the majority of force spread at opposing 22.5 degree wide angles--more effectively forcing the plastic to shrink into itself (solid walls) rather than the infill (flexible empty space) which is susceptible to curling. Purely rounded fillets, if large enough to maintain integrity of the walls, will pull throughout the curved section of walls almost entirely; this makes it the best option but fillets aren't the best for lots of designs nor are they easy to place sometimes.
As for the shotgun parts, the barrel components as they are can (and in my case HAVE) shear off easily due to torsion of the pump handle. Similarly, without sanding the ripples of the layers add tons of friction that needs to be worked out one way or another. The general rule for sliding pieces is to print them such that the layer ripples run as a sort of track to aid the action. This reduces friction and wear, but also promises more torsion strength as the outer walls are now used structurally. I reprinted these parts such that the layer grooves were running lengthwise on the top and bottom and it has the most strength possible. However I required soluble supports with tight tolerances as the cores of the barrel parts would otherwise sag and break tolerances. If these parts were remade with the "top" portions of the core parts reworked into 45 degree angles they would still serve their purpose but also would be printable without supports in the better orientation.
We all get crazy complex and mechanical when we get this deep into designing things, but sometimes it benefits us to sit back and rethink the simple material physics and reproduction quotient. You're doing a good job and these aren't issues resourceful people like myself couldn't fix ad-hoc, but if the project is for a general audience it's best to consider "fire-and-forget" proofing your prints as well. General 3D printer consumers have lower quality printers than the average and have enough to fix before they even get to the quality of their STL files, so every little fix counts.