Mine was a bit tight too on my first build. I looked closely at the blue prints and saw that the back of the side plate end has a few radius. These were not done on any of the sideplates I have purchased.
I made my RSP fit the back plate. The reasoning here was the back plate was fine something else must be wrong. Once I knocked off the sharp edges as shown on the prints (available here http://1919a4.com/blueprints/Browning1919A4Blueprint.pdf) it slid right on. Also by not modifying the back plate all my back plates fit all my other guns. As posted above you don't want it loose but a rubber mallet should not be necessary to assemble/disassemble the gun. Save it to smash your top cover back in place if you have a kaboom.
HTH
I made my RSP fit the back plate. The reasoning here was the back plate was fine something else must be wrong. Once I knocked off the sharp edges as shown on the prints (available here http://1919a4.com/blueprints/Browning1919A4Blueprint.pdf) it slid right on. Also by not modifying the back plate all my back plates fit all my other guns. As posted above you don't want it loose but a rubber mallet should not be necessary to assemble/disassemble the gun. Save it to smash your top cover back in place if you have a kaboom.
HTH
