Re: SBR reciever question
None of that matters. The receiver can be marked any caliber you want. Normally, people put the primary caliber on the registration form, like .223. Caliber, barrel length, and other such non permanent changes are not required to be recorded on the form. This has been ruled by BATFE. So for example, if you have an SBR registered with a Form 1 and the caliber on the form is .223 and the barrel length on the form is 14.5", you can legally put a 9mm upper on it or a 7.5" barrel. These are considered non permanant changes by the BATFE and do not require notification to the BATFE. You will hear all sorts of varying opinion on this by mis informed or ignorant people. In the end, you must do what you feel is best for you, and if you are really that concerned about it, I suggest you contact an attorney who is well acquainted with NFA law. Or, you can do what I always do and read the regulations for yourself. They are written in plain English and it's not rocket science.
Hope this helps, and drop me a PM if you have any questions.
-Patrick
J2T