I think I received SG15-A13 NAC Tribarrel Autocannon Teams as a freebie with a larger order from GZG during one of their special offers.
To start with, I based the figures but left the weapon separate (having made sure it would still fit!). When I covered the base in putty, I waited until it was well on the way to hardening before moistening the base of the gun and pressing it onto the base in the correct position. This left a good clear impression showing where to stick it afterwards. I avoided that area during texturing & painting the base, then it was easy to glue the tribarrel on after painting. Made it much easier to paint the figures without the gun mount getting in the way!
They'll make a handy support unit for defending key positions.