I checked out the drawings and it looks do-able. Chamber to .360" shorter than a regular 223 chamber. In theory the chamber will be about .006 tighter at the big end...not a bad thing. Measuring factory loads looks like they'll fit OK...has anyone done this?
I will be doing it next week, have two new stainless 224" tubes enroute and two .378" bolt face Rem 700's...one will be a Fireball, the other a 223.

I have not done that.
I own a .223 reamer, .224" groove barrels, 221FB and .223 SAAMI drawings, and .221FB dies, but no .221FB guns, so this IS of interest to me.

Per SAAMI the .221FB brass or .223 brass is .3759" -.008" = ..3679 to .3759" at a point .2" forward of the case head.

The .223R has 1.4636" of headspace
- The .221FB has 1.1038" headspace
__________________________________
= The .221FB has .3598" less

The pinch point will be in the .223 chamber at:
.3598"
+ .2"
_______
= .5598" forward of the breech

I the .223 chamber is .3769" +.002" at .2" forward of the breech and .3594"+ .002" at 1" further forward, then the chamber has a slope of

.3769 + .002"
- .3594 +.002"
__________
slope = .0175"/1"


Horizontal change X slope = vertical change = [.3598"] [0175"/1"] = .0062965" vertical change

.3769 + .002"
- .0062965"
________________
223 chamber size at .221FB pinch point = .3706035 +.002" = .3706 to .3726"

If the chamber were at the minimum .3706" and the brass at the max, .3759", then there would be .0053" interference.
If the chamber were at the maximum .3726 and the brass at the min .3679, then there would be .0047" clearance
But all the brass I measure is .372 to .374", which would give only the hope of selecting brass and having the max chamber.

But a combination of selecting brass and sandpaper on a stick technology, would probably make it all work .

I have a box of 221 FB factory loads that measure .373 at .20" from the base....factory 223 loads measure .369/.370 at .560" from the base....and my reamer measures .372 at roughly the same point...it'll be tight alright but I have nothing to lose by trying. I'll try this one first and if it's too tight it'll become a 223. This may be a rare occasion when a bit of runout at the reamer is a good thing. I'll let you know how it works out.

If the reamer were in the tailstock chuck, and were off .001" from center, the chamber would open up .002".

That would be fine at the base, but I would not want that at the throat.

Both cartridge have a slope of .0175"/1".
That means one wants to open it up all the way to the starte of the shoulder and no further.

You polish the chamber anyway, right?
Rub extra hard while the lathe spins.