I have a quick question. If I purchased a 10 inch SBR AR. Could I swap out the upper with a 7.5 inch 9mm under the same tax stamp?

Incorrect. You list the primary barrel length that you will be using. You also only list one caliber on the form. You are totally legal with swapping calibers & barrel lengths after your form is approved as long as you maintain the primary barrel length that you listed on the form. There is also no requirement to notify the ATF if you plan on swapping uppers.

This...maintain the capabiity of returning the weapon to the identified length/caliber and you're GTG.

I believe so.  You register the lower as a SBR, not the upper.

Precisely.

You can, but I recommend notifying the BATFE of your intention, and keeping a copy of the response.  That way if some ill informed LE officer starts harrassing you, you can provide proof that the BATFE is okay with it.

Do you specify barrel length on the form?

Barrel length, and overall rifle length with stock completely extended are specified on the form.

I wouldnt know... will one day

It's pretty easy to find online, just google it.

When you fill out your papers put the MIN length upper and OAL that you plan on using, and in the caliber section put both 9mm and 5.56

This will make you GTG on any upper with a length more than or equal to your specified length, and both calibers

Great. Thanks

ok, just makin sure i got this right. i purchase the 10 inch rifle and fill out the paperwork with specs as is. then i can put any upper on it as long as i keep the original? do i always have to have the original upper with me when i go to the range? even if i dont use it while im there? sorry for all the questions i just dont wanna get f'd up over some technicality.

As long as you can change it back to the original config at the end of the day.... thats what i was told.  You dont have to carry around the 10.5" upper with you at all times

Lots of FAIL here.

Your Form 4 should show the barrel length, overall length, and caliber of the gun that was transfered to you.

Now that you own the gun you can change any or all of these descriptors legally.  This is considered by ATF to be a temporary change no matter how long the gun is in the new configuration.

If you transfer the gun at some future point, Martinsburg will only approve the transfer if the gun is in the configuration listed in the registry, so you should keep all the original parts to return the gun to the original configuration.

Now, if you do something like sell the original upper, then you have made a permanent change. ATF asks that you advise Martinsburg if you make a permanent change.   Before you can transfer the gun you will need to write a letter advising ATF of the new configuration so that it can be reflected in the registry.  It takes about 2 years to get a response from ATF if you change configurations.

Don't ask me how I know all of this.

I will never make permanent changes again.  I will always keep the parts to return the gun to the original configuration.  It gets real confusing otherwise.

Now some guy is going to come along and tell you that what I said is BS and that you can simply list the new configuration on the future Form 4 and everything will be fine because it worked for him..

You can believe him or you can learn from my experience after my interesting conversation with a specialist and several more letters of explanation to ATF>

Just Sayin!

This is exactly what I found while researching before I sent off my 1st F1. At a bare minimum, keep a barrel on hand that conforms with your paperwork(cal & length).

i dont plan on getting rid of the rifle or the upper. just wanted to know if i could slap on a shorter 9mm upper from time to time. and everybody pretty much agrees that i can. so thank you everybody i got my answer.