This is not special to the drum mag. All kind of items that are forced into your inventory without the right amount of item slots with disappear. They need to fix this by forcing items to fall the ground with a little warning message that would look something like this "there's no room for the 75rnd drum mag in your inventory. You dropped it on the gound."

This is the case for most of the guns. At least all the one I've tried. I play with 3 friends and most of the time they can't hear me shooting. Same thing the other way around. Most of the time I can't hear them shooting.

Doesn't have to be a cheater. just some one who happened to be invisible and didn't know it... But I've also stumbled upon many invisible zombies.

I've been lucky so this has never happened to me. But all of my friends have had this problem multiple times when rejoining. :/

Good to hear i am not the only one. I've never seen anything like this in any other game. With comunication being so important in this i find it very odd the a problem like this still exist from arma 2.

I doubt this is a steam issue since no other game have this problem except for arma 2 and 3. I've played A TON of steam games and never stumbled upon this before

I think the reason for it to be in Arma in the first place is because the FOV can not mimic a real pair of eyes and therefore you won't be as aware of your surroundings as you would in real life. 3rd person view almost give you that awereness (with a slight unrealistic awareness level too...)