An apparatus for adapting a clamshell-type of gunlock for a shotgun to lock a rifle. The clamshell gunlock can be easily modified using the apparatus without the need for basic changes to the gunlock. The apparatus has a single molded rubber part attached to the gunlock to hold the buffer tube of a rifle; providing a snug fitting constraint to hold the buffer tube of the rifle such that the rifle will not vibrate or move causing rattling sounds or damage to the rifle.