I love my Lee Collet Dies, but my 222 Rem Mag is somewhat obsolete and Lee no longer makes Collet Dies for it. The 223 Remington is just a little shorter than 222 Mag. My question is: could 223 Rem Collet dies be used with a taller (or shimmed) shell holder?

I can't answer this specific question but I use my 25/06 collet die on my 257 Weatherby mag with a shim washer between the shell holder and die collet. Works great. The 25/06 case is 2.484 and the 257 case is 2.540

Yes, and I've been doing it for years. I use a .223 Collet die and simply drop a washer of the right thickness over the .222 Magnum case as it sits in the shellholder, resize, then remove the washer with the case. The washer drops off into my other hand and the process is repeated. With thicker washers appropriate to the caliber you can use a .222 Rem Collet die for both .223 and .222 Magnum.

I also neck size .300 H&H and .300 WBY in my .300 Win Collet die by the same method.

I don't think Lee makes a Collet Die for .221 Fireball, so I went the opposite direction for that cartridge. I simply took the collet piece from a .222 die to the grinding wheel and took enough length off of the bottom to allow a Fireball case to enter the proper amount when the "sqeezing" begins. If I recall, I had to shorten the decapping stem (mandrel) for that adaptation, but it works like a charm.

Love those Collet dies! NO messy lubricant and nice, straight cases.

Thanks for that confirmation. I need a washer of 0.026". That is great info about all those other calibers that this method can be applied to.
Rocky

Don't worry about the exact thickness of the washer. You only need to resize the upper two-thirds to three-quarters of the neck, anyway. So a washer which is a little too thick will work just fine.

I also use a regular .22 Hornet collet die for my .22 K-Hornet. I had to use a drill bit of the proper size to enlarge and lengthen the shoulder area of the collet so that the shoulder wouldn't crush, but even so the same die still resizes an adequate amount of the neck of regular Hornet brass. Leaving a short section of the base of the neck unsized helps keep the round centered in the chamber and seems to enhance accuracy.