if you start out with a factory 223 chamber, you won't be able to have it rechambered to Ackley Improved and have the correct headspace. An Ackley chamber is "supposed" to be at least 0.004" shorter than the standard chamber because the Ackley chamber will contact standard cases ONLY on the point at the bottom of the neck. Some gunsmiths WILL do this chambering,,but it really is not a correct chamber. Factory ammo cases will stretch a few thousanths more than usual. There are some techniques that will allow you to fireform Ackley cases ok [bullets seated out hard into throat, lightly oiled cases, etc] but still, with the inability to "set the barrel BACK" on a Contender,,,you need a custom barrel origionally chambered for the 223 Ackley to be set up with a nice "correct" headspaced chamber.
alan in ga > a fan of Ackley's stuff.