It also may work just fine without any shims or new locking collar. It all depends on the initial threading of the barrel and how that resulted in the flash hider being timed. We ordered six Mod 2 guns and six PRT suppressors. Five of the six attached as they should without touching the rail. The sixth barely bottomed out on the rail but still attached. We still sent all six uppers and suppressors to KAC and asked them to replace the URX 4 with the URX 6, which guaranteed no fitment issues. All was good with the world. The only issue you will have if you choose to replace the collar is that you will lose the ability to use the suppressor shroud once it’s released, if that’s something you want to do. So if you do have a fitment issue, I’d probably recommend the shimming route so you have the option of using the shroud. Plus, the collar on the PRT looks way cooler, and we all know how important that is when choosing our weapons and accessories.