Here in IL we can now own SBRs with a stamp but they must be at least 26" in overall length to comply with the ATF's interpretation of the Illinois law (at least this is my understanding of it).
I'd really like to build a SBR using a CAA Micro Roni chassis on a Glock 17 but I think the OAL is only about 23".  The Micro Roni has a steel barrel shroud that extends about an inch from the front of the chassis.  I'm hoping that I could weld a 3" extension to this shroud to bring the OAL up to 26".  
But since the actual gun is removable from the chassis I'm guessing that this design causes some strangeness in regards to the acceptability of a modification like this.  Does anyone here have any insight into the matter?  It might be a question for the tech branch but I thought I'd check here first.

I'm not familiar with IL law, but I don't see why it wouldn't be acceptable.  When the "gun" is removed from the chasis it is no longer an sbr.  It is a handgun.

No idea on Illinois law. Food for thought though, the time and cost of SBRing my Glock would have been better spent on one of the less expensive full size MP5s (like an Omega). Full size MP5s are ~27" OAL.

When you look at the overall cost and what you get out of the gun, the MP5 is marginally more expensive and is much more fun to shoot. As someone with MP5s and a Glock SBR, that's my advice.

You're probably right, ShooterPatriot.  At least about a welded-on extension being acceptable.  Whether the gun turns back into a handgun or remains (legally speaking) a "short barreled rifle with no stock" when it's removed from the chassis is an ongoing debate.  I'm of the opinion that it remains a rifle (again, purely from a legal standpoint.  And that doesn't mean I agree with it) but that's been discussed in other threads.  
The more I think about the extension the more I believe that it would be acceptable as long as it met the ATF's requirements for being permanently installed.
I'm not sure that I'm going to go this route, though.  I'm sure that a Micro Roni isn't the most practical SBR or the most value-added use of SBR funds.  I just think they're cool.  Therefore I want one.  Plus, I already have a Glock and a lot of mags.  But welding a 3" extension onto it begins to lower it's "cool factor" enough that its limitations begin to overshadow its appeal, IMO.  Thanks for the advice on the MP5s, GeneralPurpose.  I've always admired those.  I just wish that Illinois didn't have the 26" OAL limit.  The way the law reads to me, <26" should be allowed but I guess the ATF is erring on the side of restriction.  But we couldn't have SBRs at all until recently so I guess I'll be happy with that for now.

I've never heard of any debate on if it legally remains an SBR or not after it is removed from the stock.

It's no different than putting an Arm Brace or 16"+ upper on an AR SBR so you can travel interstate.

If it doesn't have a stock, or the barrel is 16"+ and OAL 26"+ it's not an SBR.

The debate arises from the ATF's stance that once a gun is a rifle it can't then be made into a pistol.  That's why when guys build an AR pistol it's so important to start with a lower that has never been a rifle.  
Since you're "manufacturing" a short barreled rifle when you Form 1 an SBR for a pistol that you want to add a stock to, the thought is that said pistol then becomes a rifle and as such can't be made back into a pistol in the eyes of the ATF.
This gets debated from time to time since then you'd run into potential trouble carrying your SBR/pistol as a CCW piece with the stock removed since most places don't allow CCW of a rifle.  
I think that you could still take it across state lines when it was removed from the stock but it would still be a "rifle" and would need to be treated as such to avoid running afoul of the law.  
This is just my understanding of the situation based on reading that I've done by guys who know a lot more about NFA stuff than I do.  I might be off base on certain points but it all makes sense to me.  Even though it actually doesn't make a bit of sense from a purely logical standpoint.

Bold part is incorrect.   A rifle can't be made into a pistol, this would be considered an NFA gun and an SBR, as it is "A firearm made from a rifle".   However you can make a pistol out of a rifle, as long as it did not start life as a rifle.  It either had to be built as a pistol, or "firearm" (No stock, OAL 26"+) first.  Then you can go back and forth between pistol and rifle as many times as you wish.  This was always the case since the 1992 TC case, but was clarified by an ATF ruling in 2011.

Same applies with an SBR.  Some may claim you are manufacturing/making a rifle when you form-1 your pistol, however a "Short Barrel Rifle", and "Rifle" are two different specific legal definitions.

Interesting.  Thanks for the info.  I'm no expert when it comes to NFA stuff but I'm trying to learn as much as I can.  There's a lot of info (and bad info) out there.  I'm not sure how old the archived threads that I read were that led me to my conclusion.  Maybe they were from before the clarification that you sited.