No. The BREN was a variation of the Czech-made Vz.30, and was very intentionally designed with a top loaded magazine, which actually makes for easier magazine changes.

Did you wind the magazine up with the wheel on the bottom of it? This happened to me too.

The M16 assault rifle can accommodate one (1) STANAG magazine in its magazine well at a time. Some soldiers in the field would tape two magazines together upside down to reduce reloading time, known as "jungle style".

It should have only been disassembled at operator level. You place the barrel into the slide, place the return spring into the slide, mount the slide onto the frame (pulling back the slide into place), flip the slide release up, then perform your functions check.

The upside is ;the good thing about it .