What you are experiencing there is called 'springback'.
In metal bending physics, It is IMPOSSIBLE to get a 90 degree angle bend using a 90 degree angle jig. Springback is exactly what it sounds like - the metal 'springs back' after it is bent. Here is a pretty good technical description of how it works (you can even punch in some numbers and watch the metal being bent) :
http://www.steeluniversity.org/conte...eid=2081271346
What most 'real' presses do to make a 90 degree bend on steel is use a 88 degree die - then when the metal is released it 'springs back' to 90 degrees.
I would say your receiver will work fine - the trunnions and pins shoud pull it back in.
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