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Why would you want to go through the hassle?
Who would care if you used the lower for 5.56mm, 6.5mm, and 6.8mm uppers?
I don't think it's worth the time or the potential for being disapproved just because it's different than ever other permit request they see.
Same thing goes for different barrel lengths. As long as it's my SBR lower I don't worry about it.
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My lower is marked multi & I listed 5.56/.223 on my Form 1's.
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I did a Form 1 about a year/ year and a half ago and listed the lower as .22lr, 9mm, and 5.56mm; also listed 3 different barrel lenghts and it got approved. I think it depends a lot on the examiner.
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The caliber/barrel length/OAL descriptors on the Form 1 are for ID purposes.
They do not limit or restrict future changes of caliber/barrel length/OAL in any way.
ATF requests that any change in physical description be noted to ATF in writing, so that the registry may be amended and the description of your firearm contained there accurately reflect its actual configuration. This request has been further clarified in at least two letters I've seen to only permanent changes in configuration.
And this request has no statutory basis beyond ATF's charter to maintain teh accuracy of the NFRTR (LOL!).