What if you mounted a Full Auto MAC 10 under a registered SBR? Does this constitute a new weapon or do the two cancel each other out? May be a dumb question but I was bored and had this thought

They definitely wouldn't "cancel each other out" if you mean the new combined item would now be a Title I firearm.

An SBR with a mounted silencer is still two item unless the silencer is permanently mounted and brings the barrel length up to 16"+ inches. But I think that's more that the silencer is now part of the barrel just like a muzzle break on a 14.5" barrel. The silencer is still an NFR item, the rifle is not not an SBR because of the other NFA item, it is not an SBR because the barrel is longer than 16 inches. If you permanently mounted a 6" silencer to a 9" barrel, it would still be an SBR if it had a rifle.

One way to find a reasonable approximation of an answer is to see if buying an automatic M-16 or AK-47 with an underbarrel grenade launcher would be 1 NFA item or 2. That's basically your idea, one NFA item mounted to another.

PS. All of this is my opinion based on pure speculation.

It would be 2 stamps. One stamp for the MG and one stamp for the DD.

As for the original question, nothing really would change. You would still have a SBR and still have a machine gun, they would just be in one package. Unlike a SBS/AOW/Firearm stuff, the MG doens't have many defining features other than being a MG, so adding it to an already stocked firearm isn't really an issue.

That's what I figured, but thanks for the confirmation.