[metroplex]

Make sure that you firmly seat the fully loaded 50rd mag into place (or at least firmly push it down) on a closed breech block, or else it might stick up a little and eventually cause failures to feed.

It's similar to inserting a 30rd AR-15 mag with a closed bolt, or inserting a fully loaded Glock mag on a closed slide. Except in this case, the mag will stay "down" due to gravity but won't be fully seated.

 

[Tomac]

I find that pulling back & down simultaneously seats the mag faster & easier than just pushing down.
Tomac

 

[metroplex]

That just slows it down a bit, especially with a stiff mag catch spring. I suppose one could hold the breech block open and insert the mag thanks to the ambidextrous cocking handle.