I have a MDM break-action Muzzleloader. Its very similiar to a NEF/H&R gun. Its all Stainless. I like it's design and have had a good experience with their customer service. One concern I have is the fit between the barrel and reciever. While there is no play when its locked, if you hold it up to the light,you can see a hair line amount of light between the bottom of the barrel and the reciever. Several times when shooting I can feel the blowback hit my face. If this was sealed too tight would there be no place for the blowback from the primer to go? There seems to be alot of fouling down inside the reciever. MDM customer service says that most of the fouling is caused by the primer. What have others experienced with fit and 'blowback' with their break-action muzzleloaders?.

Sounds like blowback although it is likely to be primer fouling as the pressure of your ignited charge is creater than the pressure of the primer thus there will be some of the charge's pressure that finds it's way back through the breech plug and around the primer.

I doubt there is any way of 100% getting around this phenomena.

A tighter fit or seal of the primer into the breech plug will minimize much of this.
Perhaps your barrel to receiver fitment is not quite right.
You may wish to send it back to the MFG to have them verify or have them tell you what the clearance is supposed to be and you can check it yourself or have it checked locally.
Good Luck!