Both these responses are . . . .
Hmm, how to put this tactfully? . . . a procedure with which I have little experience or interest.
How about - "ignore those two posts"
They are totally wrong in every regard - very bad advise.
Always install the front and rear trunion before the trigger guard. The front trunion has ZERO to do with magazine fit (well, except when converting a SAIGA to take standard mags . . .). All magazine fit is always done with the selector stop plate (SSP). That's why it is an "adjustable" part, and the others are not.
If, without the trigger guard in place, the mag doesn't fit, then the opening to the mag well is undersize. On old US receivers built off the NDS-3 template, the mag well is 1mm short. It is a lot easier to open the mag well before installing the trigger guard.
The selector stop plate adjusts for mag height, by its thickness; and for front-to-back mag fit, by the tab on the front edge.
On a 5.45, the mag should bottom out on the rails. reduce thickness of SSP until it does. The clearance cut on the mag feedlip for the ejector indicates it needs to insert that far.
On a 7.62, you want the mag to bottom out on the rails, or as close to doing so as possible without completely removing the center of the SSP.
The center cut on the SSP is at an angle of 4-5 degrees.
If both of these are good and mag still binds with mag catch depressed, check center support.
On many new ((10)), they will kiss the center support. This is one place where it is ok to take a couple file strokes on the back of the mag, or just slam it in a few times and the plastic will scrape.
On a 5.45 or if there is solid contact with the center support, cut a flat on the front of the center support to clear the top rear of the mag.
ONLY AFTER ALL THIS
insert mag with mag catch at rest. If it doesn't latch, file the top of the mag catch, maintain the 98 degree angle.
If it does latch, press the mag up and pencil a line on the side of the mag at mag well
Pull mag down and pencil a line on the side
measure distance between two lines. If > 0.5mm, the mag catch is too short. Weld up and refit. This is less critical on the large base 7.62 than small base 5.45 and 5.56.
The mag catch should protrude forward of the leading edge of the SSP by 0.5- 1.5mm.
If, upon grasping the mag and pulling to the rear (while thumbing the catch) the mag catch locks up, AND by thumbing the mag catch w/o touching the mag, the mag drops free, your front to rear is too long. Remove the SSP and weld up the leading edge until the mag doesn't fit. Then file it back until it just clears.
You can test-fit the plate initially by affixing it with smallbolts and nuts. After verifying everything, then rivet. After a few builds, you can just hold the plate in place while testing mag clearance. Even after a thousand builds, I sometimes need some final fitting after riveting, but it is rare.