Start out with a Glock 17, which is listed in the Production Gun List, as required in D7 19 to be legal for CO. Remove the slide and barrel, and replace them with a 6" barrel and corresponding-length optic-cut slide, as allowed in D7 21.3.  D4 7 requires that a gun fit in the "box" in order to be listed on the Production Gun List, but the equivalent requirement for Carry Optics, D7 7 ("Maximum size Handgun") is "No".  D7 21.3 just says "SLIDES: You may replace the slide with an OFM or aftermarket slide.  BARRELS:  You may replace the barrel with an OFM or aftermarket barrel."  Neither D7 21.3 or any other section I could find requires that the replacement slide and/or barrel length be the same as the factory slide and/or barrel length on the factory gun when it was approved to be on the Production Gun List.  D7 22 "Specifically prohibited modifications and features" doesn't apply, because D7 21.3 specifically allows replacing the barrel and/or slide.

This scenario seems to be against the intent of the rules, but allowed according to the text.  Agree?

Note that I'm not saying that shooting a 6" 17L-length gun in CO would necessarily be any better than shooting a 4.5" 17-length gun, just that it appears to be allowed as long as you modify the gun correctly.  Starting out with a factory Glock 17L would not be OK, because the 17L is specifically called out in the Glock section of the Production Gun List as not being approved.

Technically, the reverse appears to be true too.  If you took a Glock 17L and put a 4.5" barrel & slide on it, it would still have started out life as a Glock 17L, and therefore would not be on the Production Gun List, and not legal for CO.  It would be nearly impossible to detect though, and I doubt anyone would care.

FWIW NROI replied to an email I sent to them and I'm correct, it's legal.  I asked:

"Must a replacement slide and/or barrel on a Carry Optic-legal handgun be short enough to where the handgun would still fit in the USPSA box if that gun were shot in Production, or is the requirement for a Carry Optic-legal handgun only that it come from the factory in a configuration that would fit in the USPSA box, and it is legal to modify the gun with a longer slide and barrel that wouldn't fit?"

Troy McManus replied:

"The answer is the latter:  the originally approved gun must meet Production standards, but can be modified under the CO rules, so it may not fit the box."

This is not the answer I expected. That said, why would you want to shoot a 6" gun in CO?

Also, I fear those opens up all sorts of craziness for the gamers, but thankfully I don't dive into that stuff too much.  For example, I saw some aftermarket P320 grip modules that have some LARGE integrated magwells.  You could theoretically make those as big as open!

Someone else on another board asked the question and I thought "Of course that wouldn't be legal".  So I read the rules to make sure and it seemed as if my initial impression was incorrect, so I posted here and emailed NROI.

I do have a G17L, but it came from the factory as a G17L, so it would be illegal to use in CO unless I swapped the slide and barrel onto another Gen 3 frame.  Plus it only has irons and I don't feel like getting it milled for an optic.  The 17L seems to be a little softer shooting than a normal 17, but you would have a longer draw stroke since there's no race holsters allowed.  It's probably not worth it, but if someone knows of an aftermarket 17L-length barrel and optics-ready slide for Gen 5 frames, let me know.  My current CO gun is a Gen 5 G17.