Are you saying you have a stamp for the barrel? If so, you gave the government an extra $200 for no reason. The only thing I have heard of with regard to barrels is discussion around engraving the barrel.
Well, if the barrel is separately registered, it's not too much of a waste. It can now be its own SBR on any unregistered lower. You engrave the registered part, assuming we are talking about the engraving done to identify manufacturer and location when making one.
From what I understand in the case of a SBR or SBS the ATF requires you to register and engrave the lower receiver not the barrel or upper.
On a suppressor the registered part is the outer sleeve not a firearm receiver. Even in the case of a integrated can the numbers will be on the outer sleeve not the receiver.
At a shop I worked at we had a full auto 10/22 with an integrated suppressor it required two stamps on for the machine gun the other for the suppressor.
If you have a suppressed SBR they both must have a stamp also.
As mentioned above a machine gun can have any length barrel and in the case of a registered "Auto sear" you can drop it into any AR lower and use with what ever barrel combination you want but, if the sear is not installed and it is configured as a SBR you will need a SBR stamp.
lot's of 'freedom' and 'murica in this here thread.
SBR's are cool and hip, sexy even. The AK one i did years ago was a mistake. I regretted converting that. The AR can always be un-NFA'd. If i could go back and do it again, i would not have done it. It created such a hassle when i had to pick up and move years back.
Just bought the Springfield Armory Saint 5.56 pistol last week and put a Sig Sauer Romeo 5x optic on it , I have really been on the fence about an AR pistol but finally took the plunge and after getting the optic sighted in I can honestly say that it is a fun gun to shoot ,