Hi
I'm not in the CF but having researched the C8a3 and C8FTHB, it seems that customization of the weapons depends primarily on unit and location
Basically, in Canada, customization seems pretty uncommon except for special forces. But in Astan, seems that anything that does not require disassemblying the weapon with dedicated tools is possible.
As an example :
http://th02.deviantart.net/fs51/PRE/..._War_Shark.jpg
Best
V