WILL get into your spare mags. So REGARDLESS of how good the GUN is at keeping going under such conditions, you'd have to Glad bag your spare mags, or you aint firing many rds out of the spares, successfully, without cleaning them.
Good advice. Now what about those rubber "muzzles" that you can buy to fit over the mag's mouth. Course that wouldn't keep sand/mud from getting into the mag catch notch or the bottom so the bags would still be a good deal in such harsh conditions.
The problem with glad bagging them is that moisture is trapped within the bags and they will eventually develop leaks and what not.
You may be better off simply cleaning your gear frequently. If nothing else, it allows you to inspect the condition of your stuff - i.e. no surprises when you will be SOL.
It will depend on your AO and what you are planning.
Personally, I don't want to have to do more than open a mag pouch grab a fresh mag and insert it into the weapon. Having to remove a spare mag from a glad bag while being shot at is not my idea of fun.
Ypu should carry your spare msgsxines in duality military magazine pouches. They were designed to protect the magazines and they do a fairly good job, Also use quality real Mil Spec magazines. They were designed to function well in bad environments,