The "m16 carbine" has been added since patch 1.1 if memory serves but upon doing some research i think it may be anachronistic if Reforger takes place in 1989. IF 1989 is used as technology and /or military issue cutoff date then i think the M16 Carbine which currently appears to be based on a Colt model 727 should be remodeled to an earlier Colt model 723 carbine.

the Reforger 3d model of the Carbine.

https://static.wikia.nocookie.net/armedassault/images/8/83/Reforger-render-m16carbineblack.png/revision/latest/scale-to-width-down/1000?cb=20240317120955

Compared to A Colt 727

https://photos.imageevent.com/willyp/firearmsalbums/uscoltarmalitem16ar15556/coltmodel727carbine/727%20R%20door%20open.jpg

The earlier Colt 723 was something of a hybrid carbine . It had a diemaco colt C7 upper receiver ( which was basically m16A1 upper with some M16A2 features like thre Brass deflector) early production version had a pencil barrels but later versions had the same m4 profile barrel as the colt 727. So specifically in Delta force squadrons you see in 1989 many Colt 723's that have this barrel, although other Special operations units like Rangers (some NCO's) and ODA 793 from 7th Special forces group that deployed to Panama in 1989 ( Operation Just Cause) using what appear to be COlt 723's in either pencil barrel or M4 profile barrel. I have not seen any photographic evidence of Colt 727's being employed in this conflict that took place in real life in the same time frame that Reforger is set to take place.

Colt 723 comparison images

https://i.imgur.com/BDyGy2t.png

https://i.imgur.com/zx7dk3g.jpeg

So to summarize To remodel Colt 727 to earlier colt 723 ( late configuration) only requires the carry handle sight to be replaced with M16A1 style sighting system. Everything else can stay the same in the existing 3d model in reforger, except maybe the handguard. I think looking at photos it is a bit slimmer, but that could just be an error due to personal artist interpretation on visual eyeballing?