Visited Sportsman's Warehouse in Rocklin CA to look at some Eberlestock packs. Store dangerous to the wallet and packs very nicey and pricey.

I use one of these and love it:

Gunslinger's Corral

Of course, if you are not using a bolt rifle, this probably will not work. I've been able to alter it a little and use it with a pack, as well as without and find it to take away most of the fatigue of carrying a gun.

As far as plugging the barrel to keep stuff out, I stick one of those foam ear plugs in. I've always remembered to remove if prior to shooting the gun. Not sure if it would cause an over pressure issue but assume the pressure would just blow it clear. I've never used tape because I don't want the adhesive residue on my barrel.

A bit of Velcro on the sling and on the shoulder area of jacket or shirt will prevent the sling from constantly sliding off of the shoulder. When crawling, best to just hold the rifle in hands, or across the elbows, or grab the sling at the forend, lay rifle across body, and and low crawl in that manner.