It's funny, in the JP movie, Muldoon had the Spas 12. Not bad, particularly for a shotgun, but not my personal choice of weaponry. In the JP book, he was actually using a LAW rocket launcher. They had tranq rockets earlier on in the book to hit the Rex with, and he grabbed the HE from the gun cabinet after the SHTF.
Crichton apparently knows nothing about LAW rockets, or he'd know it's a one-time use launcher. Description wise, it fits the LAW to a T, being described as a compact tube (duh). Function wise, it seems similar to the SMAW, or maybe the IMI made "Shipon", capable of being loaded with different types of rockets.