After having 7 CVA inlines. I having had one that was close to the right headspace. I'm sure yours needs to be shimmed.
I'm not concerned with shimming the FP housing if it sticks out a little. As long as the gun still closes the same as it does with no primer in it. In other words i don't want to slam the action closed that crushes the primer. I just want to take up the extra clearance.
Some guys will measure the primer after slamming the action shut for a certain crush fit. They want every little bit of blowback gone. I would rather have a safe gun, and I don't like the idea on crushing the primer on a loaded gun. Wiping off a little bit of fouling is no big deal to me. It's a muzzleloader.
You want blowback/ Shoot my Renegade with the 460gr NoExcuses that i'm using now. Now there's some blowback. Big deal. One swipe with a rag, and it's gone.