This is based on Insurgency's AK modifications, where the AK rifles doesn't have the option to equip Heavy Barrel.
I don't know if AK rifles have heavy barrels on by default or that there is no option to equip heavy barrels, but this could be a balancing opprotunity for the AK rifles in INS:S. Both AK rifles have very little recoil, and with heavy barrel equiped, AK rifles are a bit too useful at close-to-middle range. To indicate that the Insurgent faction doesn't have much funding compared with the Security faction, there should be limitations on the Insurgent side that shows that they can't have everything that is a one-to-one counter with Security's equipment.
[EDIT: I've also noticed that there are other weapons where Heavy Barrel can be equiped: M249, SVD, M24, G3, FAL, MOSIN, etc. Based on @CHONMA-HO's comment, I believe that there should be weapons where most weapons should be fine by themselves without having heavy barrel as an option.
Heavy Barrel doesn't have much downside to equip, and paired with most weapons, it'll be a modification that people choose by default. Also, I believe that it makes sense that the AR-15 based rifles should be the only weapons in INS:S that Heavy Barrel is a modification option since AR-15 rifles are made to be highly modifiable easily. Either go with this, or base what weapons should get Heavy Barrel by Insurgency's weapons.]
[EDIT2: Thanks to @Jarple, I realized that the "Heavy Barrel" I'm referring to is the LONG Barrel. Changing the title for that, but really, the long barrel attachment is very similar with the effects of appling the Heavy Barrel back from Insurgency, so this suggestion still stands.]