I saw a documentary in the 90s that followed a family of terrorists, IIRC PIJ, and the local cell tgey brlonged to. Most of their weapons were Israeli, CAR-15 and M-16A1. Could be captured ones, but they had the serials ground off and why bother to do that unless you're trying to hide the precise origin of the weapons. It's not like they're trying to abide by Israeli law ...
@m208 How you make weapons dissappear? You wait until they're slated for disposal so they're removed from inventory and then it's a matter of bribing or coercing whoever is meant to destroy the equipment.
The guns are in poor condition, but it's better than a slingshot and as long as the lower receiver is in working order every other part can be acquired through regular channels (especially the US) or on the black market and smuggled in.
Then there's the more basic parts like springs which might be worn out, but that's an easy fix for an experienced machine shop.