Note that most
real world sentry removal methods would be difficult to perform with anything longer than a combat knife. This is because grappling the target and preventing it from moving and crying out are an integral part of sentry removal techniques. And using longer blades in Close Combat is penalised.
Of course, GURPS suffers from granularity issues here. Anything up to a Long Knife (up to 23") is +/-0, while anything from a handy shortsword (24") to a woodcutter's axe is -4. More properly, anything longer than 12" or so should probably have a penalty at Reach C, with a -1 for most Long Knives, a -2 for Short Swords, a -3 for some shorter Broadswords or hangers and a -4 for full-sized Reach 1 weapons.