Actually its a modded HK SL-8 (at least the Jetpack Hawk's gun is) with an adjustable stock and cheek-rest.

Its interesting to see that the ROC joes use a lot of current weapons.
There are Tavors

Singapores SAR 21s, HK Mp7s, HK Mp5Ks, HK XM8s (finally), HK G36s, HK G11s, Colt M4s and FN F2000s . Plus Desert Eagles, HK Mk23s, horribly mis-scaled Pancor Jackhammers and Milcor MGL 140s
The Baroness (Paris Pursuit) has a Bushmaster M17 with Red dot (originally I thought it was a cross of a Chicom type 95 and Russian Groza.)
The Cobras seem to have more "fantasy" weapons, although as mentioned before they've got HKs, Grozas, etc.
And I totally agree with you VL, the original filecards always mentioned the weapons they came with, sometimes gave you an idea of what the weapon could do, and in the case of Flash noted that his weapon was experimental.When I was a kid I liked it, as it kind of gave your imagination a starting off point. It also helped me relate the joes to the Saturday Afternoon movies I'd watch, like the Green Berets or Delta Force or whatever.
And I think we should thank MJ for proving to Hasbro that they could radically improve their weapon sculpts, even though they still have a ways to go with materials.