It stands for Modified choke.
Degree of choke
If you are looking at a Browning shotgun, the two asterisks indicat Modified Choke.
Focus on the target, ignore the barrels.
If you mean the two barrels are separating from each other, then the solder joining them has failed. A good gunsmith can repair this. If you mean the barrels are "unraveling" then you must have an old twist-steel barrel that has been over-stressed and it cannot be repaired.
It could be the maker of the shotgun, the name of the store that sold it, the name of the person who made the barrels, etc..
One or two, depending on how many barrels the shotgun has.
both barrels are not choked
Store brand shotgun with two barrels, side by side.
The expression refers to a double-barreled shotgun. To "give (something) both barrels" is to make the maximum effort at the task. ie. use both shotgun barrels. It is similar to the expressions, "give it your all," "go all-out," "one-hundred-and-ten percent," and to give something "...the whole nine yards."