RE: why a .17 caliber and not a .223 caliber?
I think some people get upset when I suggest we correctly and specifically identify what cartridge we are talking about. " .17 caliber" is not clear enough.
There is the .17 HMR (Hornady Magnum Rimfire, a .22 mag. necked down to 17 cal.) and the .17 Remington which is basically a .223 case necked down to 17 cal.
Robin