depends on what you need. if you need a sniper enemy and he wont change the gun during the game its better to add as much models to one as you can.
if you would like to be able to exchange the guns you should create all basic animations without the guns.
then you can either
1.) animate the gun in all needed poses and cycles and attach it via script
2.) or use placement dummies and just add the unanimated gun to this invisible dummy. The benefit are way much smaller filesizes because the guns dont need anims and this can help a lot if you have lots of anims or lots of guns (rpg).
in this case one or more invisible simple dummies (i use 8 sided dieamonds for this) are placed in the characters hands and belt, back aso. i use their orientation to place the weapons via script. pretty much the same method as you would work in your model editor. You just export this dummies as real models now.

hope this helps
cheers