WHAT DOES COLT 20 ROUND MAGAZINE MEAN?

Note:  Inventory photographs will not present the bolt carrier group and charging handle, but both are integrated.Which is my usual luck. No dollars when one thing I want is for sale instead of for sale Once i've received some cash to spend.  I am going to retain hunting. Thanks. -Ken I purchased one of many 18" barrels, and while I haven't asse