eric harris used the Hi_Point 995 Carbine Rifle as seen here:
as well as the sawed off shotgun appearing in my sig.
dylan used this tec-9:
and this shotgun:
and they carried these, but did not use them:
And yes, they were bad at detonating the bombs, it was the wiring actually, but if they were wired correctly, they would've killed like 800 ppl.
As far as rounds go, dylan fired 55 rounds total with the 9mm tec-9, and 12 with the sawn off. eric fired 96- 9mm rounds, and 25 shotgun rounds..(eric seemed to be more trigger-happy )
here's a screenshot from a video showing the guys testing their weapons out in a forest. This pic shows what looks to be Eric Harris holding a tec-9.
dylans car, as he had set it to blow at 11am, detonated at 11pm.