Audio problem: Right after Bert kills the second graboid he gets on the radio and says "we killed that mother-humper". Val replies "...be advised there are two more mother-humpers". However Val's lips don't seem to be saying "humper". This occurs even in unedited versions of the film. (01:01:40)
Audio problem: When Val gets the Cat running he cheers out loud. It cuts to a shot looking up at him inside the Cat, he cheers again but his mouth is closed. (01:10:00)
Chosen answer: No, there's nothing illegal about any of the weapons they owned. Some of them may have required a Class 3 firearms license, but even this is not difficult (just expensive) for a law-abiding citizen to obtain.
Grumpy Scot