A weapon with a telescoping bolt (also known as an overhung bolt) is one with a bolt which telescopes over, that is, wraps around and past, the breech end of the barrel. This feature reduces the required length of a weapon such as a submachine gun significantly, and it allows rifle designs to be balanced around the pistol grip in a way that gives "pointability" similar to a pistol's.
A telescoping bolt is bought to balance the rifle.
The text provides information about the benefits of the rifle becoming balance with the bolt, however it does not say that this is the only purpose it serves.
( z1 buy :ARG1 ( z2 bolt :ARG0-of ( z3 telescope ) ) :purpose ( z4 balance :ARG0 z2 :ARG1 ( z5 rifle ) ) )