I built a SBR with a 10.5" upper.  Is it legal for me to put a 11.5inch upper on it, or does the barrel length have to stay the same as what is stated on my form1?  

no it doesnt have to stay the same, its technically good to go if you still have the original barrel  or a barrel in your household of the same length


so if you put 10.5 and want an 11.5  just get make sure you have both, but i don't think it will really matter unless you are trying to sell your sbr

You can use any barrel length you wish on registered SBR and you do not need to keep the original Form 1 barrel, contrary to PO.

Once you've "made" your SBR per the approved Form 1, the regulated and approved action is complete and you may move on to any other configuration you wish.

People keep the original barrel to lessen LE confusion, but there is no legal requirement to retain that Form 1 configuration or the barrel itself.

So if my form1 was for a 5.56 7.5 inch upper. After getting the stamp I could run a 5.56 10.5 inch upper or a 9mm 7.5 inch upper or a 5.7x28mm 11.5 inch upper?

An SBR is an SBR  - yes you can do whatever you want with it once the stamp is approved.

I've probably had my SBR in 4 different barrel lengths and calibers since I got my stamp.  I originally built it per F1 and then started changing things.  Mine was registered as a 7.5" 223 .. its currently an 8.5" 300 Whisper.  I do not have the 7.5" upper anymore .. and no I havent sent a letter to the ATF telling them of a permanent change in configuration.  They request it but it is NOT required.

Thanks for the info. It makes a SBR lower much more fun now.