I found an exploit for reloading the magnum almost instantly. Reloading the weapon while looking at an npc will reload your weapon almost instantly and allow you to fire instantly as well. It’s not really game breaking, but it might be in team deathmatch or something when reloading while looking at friendlies (haven’t tested that).
This bug is actually present even in the HL2s. However, I don’t think it affects MP though.
I believe it happens only in SP.