[LouBoyd]

It depends on what you mean by 300/.223. There is a 300-223 wildcat cartridge which is a standard 223 case necked up to 30 caliber without shortening it. It's been around a long time. It's longer and has significantly more case capacity than the 300 Whisper cartridge. It's a supersonic cartridge even with heavy bullets. If it's loaded subsonic it has no advantages over the 300 Whisper.

The "generic" 300 Whisper cartridge bears the name 300-221 or 300 Fireball to avoid copyright infringements whether it's made from 221 fireball, 223, or any of several other other cases with the same case head. What differences may exist between the 300 Whisper and the 300-221 or 300 Fireball depends on who makes the chamber reamer more than what was used as the donor case. Case choice does affect the brass thickness which change the case capacity and has an effect on the loads and performance. I don't think a 300 Whisper made from the 221 Fireball or the 223 Rem case is better or worse, but there are small differences. I don't mix brass with different headstamps even if they have the same weight.

You cannot use 300 Whisper dies to form or resize the 300-223 wildcat cartridge. It might be possible to use it for seating but it would be sloppy. I own a Thompson contender pistol with a 300-223 chamber. The loaded cartridges are much too long to fit in a AR-15 magazine when shooting moderately heavy bullets. It's suitable though for a short action bolt action rifle or an XP-100R pistol.

For handgun silhouette or hunting the 300-223 is useful because heavy bullets will remain well supersonic for a couple of hundred yards avoiding stability problems in the transonic range. My 300-223 T/C pistol has a 10" long x 10" twist barrel. The 300 Whisper can have longer useful range because it never goes transonic, but has much less energy and more drop up to the distance where the 300-223 becomes unstable. If you want even more energy in a 30 caliber TC pistol the next common step up is the 30-30 Win. Other necked up 223 cartidges include the 6mm TCU, 257 TCU, 6.5mm TCU, and 7mm TCU. I believe the 300-223 got it's name before The TCU line was started, otherwise it would probably be called the 308 TCU.