I have a Gen 1 Sig 556 Patrol and a M400 Tread. One of them is getting SBR'd.

If you SBR the M400, what are the rules on swapping another upper to it? I just don't want to reduce the capabilities of the M400 down the road.

The 556 Patrol might be the better option for that.

Advice please.

So far as I know, there are no regulations affecting what you can do to an AR-pattern lower once you pay the excise tax.

Replace the 5.56x45mm upper with one in .458 SOCOM? Okay.

Replace the .458 SOCOM upper with one in .22LR? Knock yourself out.

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M400 Tread

AR pattern gives you limitless flexibility.

It’s the receiver that must be engraved and that’s the part that is regulated...swapping uppers doesn’t come into the equation

You can change uppers as you see fit on your registered lower. As far as I remember, if you were to sell the original upper, the ATF only "recommends" you notify them of a permanent change, but does not require it. If I change uppers on a registered lower, it has always been my practice to keep the original upper that the lower was registered with around, but it probably doesn't matter either way. The only thing you may want to be careful about is having multiple short barreled uppers laying around if you only have one sbr lower and multiple non-sbr lowers. I am probably over thinking it, but I always err on the side of caution with the ATF.

On most (if not all) of the Sig 556 series of firearms, the upper is the serialized part. Check your rifle - if the upper is the serialized part, you're stuck unless you change the barrel. Either way, the AR-15 will be significantly more flexible and have future parts availability.

The upper on the 556 is the serialized part and would need to be engraved, the lower could accept any other 556 upper without problem. You can take a patrol’s 16” barrel and send it to ADCO to be cut down, crowned, and treaded with a shoulder added. Get the stamp before you send it off though!

I appreciate all of the advice. I will probably SBR the Tread. I sure do like both of them.

The only restriction on your SBR’d Tread is that you cannot take it across state lines without notifying the ATF and getting permission from them first.

Such permission takes several weeks, so plan ahead, or just take a rifle that isn’t an NFA SBR.

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Originally posted by esdunbar:
The only restriction on your SBR’d Tread is that you cannot take it across state lines without notifying the ATF and getting permission from them first.

Such permission takes several weeks, so plan ahead, or just take a rifle that isn’t an NFA SBR.

Submit a request listing several states you might want to visit, and list the date of January 01, 2020 through December 31, 2020, inclusive.

BATFE has always approved my form done this way.

I have both - I SBR’d a P556 and I also have three Standard SBR-AR lowers...Go with the standard AR lower because the options related to calibers and barrel lengths are limitless....

Key points to remember - Do NOT register your SBR caliber as “Multi” as it could be rejected by some reviewers - register it as the caliber it is now AND you are required to identify the barrel length on your form...Whatever barrel length you choose to go with as your first system KEEP that upper or an upper of that barrel length...I was told by an BATFE reviewer that whatever barrel length my paperwork reflects I should possess that barreled length upper in my vault so that if I was ever checked I could present a weapon that matched my paperwork...

Again just two tips that I learned while filing for four different SBR’s provided to me by a BATFE staff member...Also go to you tube and watch the videos that many have posted on the right way to fill out your paperwork if you are going to E-file...Those videos will provide you step by step instructions on what to do and what NOT to do....Mark

One last point - Get your lower engraved while you are waiting on your paperwork to clear..Many good companies out there that do a very professional job.This is a requirement and cannot be overlooked...

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One last point - Get your lower engraved while you are waiting on your paperwork to clear..Many good companies out there that do a very professional job.This is a requirement and cannot be overlooked...

To this point, since you’re in Fort Worth, look up Ident Markings over in Rockwall. They do fantastic work and do walk-in appointments on weekday evenings; in and out in less than 30 minutes. Have used them several times with great results.