They don't use the H&K style U clip that hangs off the cocking tube and allows the barrel to free float.
Also, despite all the faggots here saying C308's use PTR barrels, heres a video showing the two compared - it's a current non-railed GI model (these are special order now and GI's are usually railed but the presenter doesn't seem to know this) and a C308https://www.youtube.com/watch?v=SLz4A_6XgzA
Basically: PTR GI's use G3 match spec barrels, and whether C308's use PTR barels or not they are cut to a different profile
I mean you can do the mod to swap a u-clip and add HK parts, but at that point you are spending more than what it would cost to buy a PTR.
C308's are however bar none perfect if your plan is to turn one into a HK11, HK21, MSG-90, or PSG-1 clone - you get a G3 reciever and the buy in is less than a PTR