Yes, Flans plus allows you to use .objs files instead of java models, you can ask others about how to do it.
anyway, blocky models huh?
so three rifles here,
the bottom one: Origin model style, which only takes a few minutes to make(it's a few shapeboxes and boxes) it looks 'Minecraft' and it's simple.
the middle one: composite of shapeboxes, it took me about 10 minutes to make(mostly fine tuning the grip, a pistol grip is not my thing). Looks much more 'gun' instead of rectangles.
the upper one: lots of shapeboxes as the foregrip has been modified a lot, but it takes only 5minutes for me the modify it from the middle one. Simply a slightly improved version of the middle one.
you'll saw quite a difference here, but everybody has a different perspective.
for me, making detailed models are fun, damn fun.
okay I guess it's a bit offtopic, so here's a new blocky M16A2 I made in 20minutes
'blocky yet finger lickin' good'
these models won't take you lots of time to make(simply stack boxes into a shape of a gun),
but it takes effort.
An easy way doesn't exist, and Rome wasn't built overnight.
here're the M16A2s for ya if you are interested.