depends how tight it is. You do not want to open it up to point you get side to side play but it is possible to reshape the mag well. Or polish inside of the mag well. I have not some it to a MP5 but have to my G3 clones and a GASP Century CETME. On the G3 clones I have 2 blocks of steel that together are close to a perfect fit inside the mag well. I use a rubber mallet to push the blocks which are clamped together to front of mag well and then to rear. After removing the mag latch assembly. Check mag fit. Then subsequent use of blocks add shim material between blocks, the clamp also acts as a counter lever to expand the blocks. Checking frequently for mag fit and be careful not to booger the reciever rails in the process.