When playing Chapter 1, part 1: I've escaped the dungeon, and I'm traversing the keep.
I've picked up the MP40 and Kar98k. At some point after the German soldiers start shooting through the floor, opening up a way through the level, the MP40 and Kar98k are no longer in my inventory. When I attempt to pick a new one up from defeated enemies, all I pick up is the ammo. At this point, I have full ammo for my Luger, but it will not reload. Hitting the 'R' key doesn't reload the Luger, even with my key binds properly set. Firing off all the ammo in the pistol doesn't trigger a reload either.
Essentially, the only effective weapons I have are the Knife and the Stick Grenades.
It seems like the newest version of RealRTCW has at least one major bug that needs correcting. The problem is: I do not know what
exactly
causes this issue to occur.

Hello!
I don't think anyone reported something like this so far and I don't think I encountered something like that by myself.
Are you playing on some old incompatible save game? Try to restart the chapter from chapter selection menu.
Cheers.

Seems the issue cannot be repeated. Not sure what triggered it, and at this point I may never know.
Edit: I did not play with an old save file. This was played on cleanly installed and updated version of RealRTCW.

Got same bug, but it fixed by throwing a knife then pick it back and suddenly my inventory back to normal

I catch this issue too.