Why couldn't you file for a form 1 SBR on a pistol chassis? Thinking something like a Recover Tactical brace or B&T USW or even a chassis for the Ruger PCC. In all three cases the buttstock attaches to the chassis and not directly to the pistol frame. Without the chassis, the frame can't accept the stock.

One argument might be that the pistol frame is serialized, but what if you use an 80% frame or even 3D print the frame?

Another argument could be that the chassis doesn't meet the definition for a firearms, however neither do registered auto sears or for that matter AR15 lowers.

Has anyone tried registering a chassis and been approved?

The form 1 will be denied because the device being registered doesn't meet the legal definition of a SBR.

Registered sears and the like are different because conversion parts are explicitly included in the legal definition of "machinegun"

When marking form 1 suppressors you can engrave the end cap, tube, or mount. When marking your info on an AR15 SBR, you can mark the barrel with your info instead of the receiver. Why wouldn't the chassis be different? Couldn't the frame of the chassis qualify as the frame to be marked?

SBR "conversion parts" are not a NFA item like they are with machineguns, you have to register an actual firearm, be it an existing title 1 firearm you are converting or a firearm you are making from scratch. It'd be like trying to register an AR stock with the belief that you can now make any firearm a SBR just by using that stock.

The engraving requirements are a completely separate thing, just because ATF lets you engrave maker info on the barrel instead of the frame doesn't make the barrel itself a NFA controlled item.

EDIT: I was going to say something but honestly it made me more confused than when I started, lol.

You should read the latest ATF proposal on what will constitutes a firearm frame.  It’s only going to get more confusing.

I also noticed the BATFE doesn't like when people try to get creative, i.e; loophole that people find.

I decided to take the path a least resistance and ordered a completed P80 lower that I'll dedicate for use in a chassis.