a piston AR-15 will solve alot of those problems...
And create some by not having any of it's proprietary parts available.
reliability to the point of cycling over 2000 rounds without cleaning (firefights are a B!tC#)
Colt can blow up a M4 in way less ammo, and has on video. I don't really see too many people humping 65+ mags in the field.
No matter what you do to protect the rifle, it will get dirty. Since it will get dirty from environmental crud when it's in contact with the ground, getting sprayed with the blast effects of indirect fire, and possibly small body parts of your buddy, expect to clean it more than daily. Gas residue isn't the problem, it's ignorant neglect and laziness.
Being Infantry, I'd rather have the easiest weapon ever designed to clean, the M16. One pin, it's shotgunned, pull the carrier, two more pins the bolt is out, wipe it down, lube it, put it together. It can be done quietly and efficiently in combat during the 50% overwatch in a lull of the action. It will be done before you eat, sleep, or other body function that can be put off a minute or two. If you don't, whatever the world can dish up, will be jammed in the sights, flashhider, buffer tube vent screw, ejection port, mag well, magazine, or splattered on the optic.
This is a great time of the year to prove it to yourself - grab your One Rifle To Rule Them All, walk out to the nearest woodline, and then get some exercise doing three second rushes until you are absolutely exhausted. Wear about 65 pounds of field gear, it will help accelerate that feeling very quickly. Don't forget to low crawl and roll before popping up again. And have your buddy popping his paintball gun at you for motivation.
Then clean your gun. After the first hour it would be surprising if you could see thru the sights at all. Actual combat is like showing up for a football tournament, the winners keep facing off after a half hour break until the winner is decided. Gas residue build up in the gun is the least of your worries - you'll need to clean off a lot more than that.
Don't choose one that will take even more time or effort than needed.