True, the bolt & the barrel extension are proprietary; not familiar with Chad Dixon, but I'd expect that KAC would probably either sell you a new barrel or install new one on your upper.Their barrel extension is proprietary, you'd have to have a barrel to work with their extension and swap. I believe Chad Dixon can do that, check with LRI.
I'm pretty sure KAC won't work with you. They only care about their Mil contracts. I have a SR-15 that hasn't worked since i bought it. I've sent it back 4 times now and they can't seem to fix it. They keep replacing the barrel and saying they test fired it, but when I get it back it won't cycle a single round of NATO spec ammo. They won't let you talk to the gunsmith or anyone other than the lady in customer service. She's great, but can only relay a message to them. Very frustrating. I have been a KAC fan boy and currently own 9 of their rifles. I'm done with KAC after this. If it wasn't a factory SBR that I waited a year to get, I'd just cut my losses.
The 2nd time I sent my gun in to be fixed, they sent me an email and said they received a big mil contract and my rifle was pushed to the back of the line.
I would not buy a KAC rifle. Their are better options and their customer service is terrible if your not military.