My original SBR length was 10.5" and I've now changed it to 11.5" and plan to keep that length as the original. Is there a form I need to send to ATF or just a letter? Also do I send it to the WV address listed in the sticky listed for all correspondence except Form 4's?

Do you have to wait for a reply, or just send a letter and keep a copy?

Something like:

Dear ATF

I have a Tax Stamp for a SBR, tax number, for the, serial number of rifle, and I would like to permanently change the length of the barrel to length effective immediately. You can reach me at... with any questions.

Respectfully

Me

"tax number"? The ATF only references the serial number of the weapon, if you look closely that is what is written across your stamp.

"To Whom It May Concern,
I hereby provide notice that I am permanently changing the barrel length of my registered short barreled rifle, model _____, serial number ______, from ___ to ___ inches.

Respectfully,
John Doe"

Kharn

your gun is an SBR and you can change the configuration at will.  you don't even have to notify them legally.

As tony_k has noted in the past, the ATF has a legal responsibility to keep the NFA database as accurate as possible.  If you permanently change your barrel length by selling off or otherwise disposing of the original barrel, it's a good idea to jhelp the people who issue our tax stamps by sending them a letter.

You can have umpty hundred different uppers with different barrel lengths, but if you dispose of the original barrel (or dispose of all barrels that length) it's a good idea to let ATF know-by letter, as tony noted.

And no, you don't have to wait for a reply (which could take a very long time).  Just let 'em know.  The definition of an SBR is a rifle with a barrel "less than 16" in length," so any short barrel is fine.  But your "default" configuration is supposed to be accurate, even if they don't twist your arm to make you let them know about it.

Thanks guys. It's no big deal for me to spend 5 minutes to write the letter, stamp it and throw it in the mailbox. I sent in my application thinking I was gonna have a LMT 10.5" upper, but I've fallen in love with my 6933 upper and have no plans to get rid of it.

I actually came here to verify a similar question. My SBR was registered as a 9mm so if I plan to permanently change it to another caliber, I need to send a letter or are they only worried about length?

"Original configuration" includes both barrel length and caliber.  Tell 'em about both if/when you change things permanently.  

STILL NOT REQUIRED .. but do so if you want to be all fuzzy warm and feel good about it.  

I  built my SBR as described on my F1 and have changed configurations plenty of times and no longer own the parts to revert it back to F1 status and have no intentions of notifying the ATF as its not required by law.  The SN is still the same and it qualifies as NFA in its current status and I'm good with that.

...and your SBR's entry in the NFA database will be incorrect.  So if they do an audit and decide to ask you questions, you can simply say "I didn't tell you because it's not a mandatory report."  I don't know if this would ever happen, but we aren't talking about the "enforcement" folks at BATFE, we're talking about the NFA branch, which is pretty much "our friend" in that they bust their rumps to do all the processes to get us our tax stamps, and the law DOES require THEM to keep the NFA database accurate.  I'm all for keeping THESE guys happy.  They're on our side.

I'm not going to go out of my way to give a sh*t about something that I don't have to .. YMMV