Howdy all,
  I'm new to the board.  My grandfather gave me a Colt Ar15 SP1 over the holidays, and now I've taken quite an interest in firearms (waiting for my Concealed Handgun License as I type this).  I've got (probably a stupid) question: what is the jag for?  I'm familiar with all the cleaning/maintenance procedures practiced in the Marine Corps, but I've never used one.  What the hell is it for?

The jag is used to push a patch through the barrel.

Roger.  So it can be used vice the loop to pull/push a patch through?  Is there any advantage?

Depending on the jag type, there are some that are slotted which are the most common, and some that are barbed, the barbed are best for just pushing. The best way to clean it is fo completely remove the upper from the lower, and clean from the chamber out, otherwuse you can push the crap back into the gun, not good. And I know the perpetual wait on the permit, I paied for mine and they said "see you in 2 to 4 months".

I find that i can get the gun cleaner quicker using a jag than a loop as the patch better matches the bore.