Breechplug
I have never heard of one untill now, should I use one, and if so how tight or loose should one be in the bore?
YES!!!! you definitely should on your range rod - i really do not worry about it on the gun rod because I only use that once or twice during a hunting season.
I really do not care if your rod is brass or aluminum - your rod it self can pick up a contaminate itself like a piece of metal or silca - constant rubbing is a concern.
Check these out..
http://www.trackofthewolf.com/(S(13s...9&partNum=RG-6
The size that you want is detirmined by the rod diameter...
I have one on every range rod and especially cleaning rod.