Anything on the .225 Win case in a Contender works very well when the taper is blown out of the case, but you can get essentially the same velocities with the .222 Rem. Mag. Imp. case with less risk of damage to the frame when/if you let pressures get away from you, which can and will happen to all of us sooner or later.
Awhile back, we, my bud Howard in Missouri and I, did an experiment with .224 Weatherby in a Contender. The case is essentially the same diameter as the .225 Win, and both cases are quite strong, plus the Weatherby is about as straight sided as the JDJ case. Because of the diameter of the case, permissible velocities were not quite as high with the Weatherby case as they were from the .222 Rem. Mag. Imp. AND...... this was done in the SAME BARREL. It started out as a .222 Rem. Mag. Imp. for which Howard had good chronographed data to start with, so the results from the same barrel rechambered to the Weatherby were quite meaningful. It eliminated the variations found from barrel to barrel by using the same barrel for both tests.
Dies for the .222 Mag. Imp. are much easier to come by, circumventing the necessity of using just SSK as the source of chamber work and dies. Relatively common .223 Rem. Imp. dies work just fine for .222 Rem. Mag. Imp., and I now stock them.
While on the subject of large diameter cases and .22 cal. bores, Howard also had a .22 Bower, which is nothing more than the .22-250 Ackley Imp. This also was on a Contender, and he could get little better than .223 Rem. velocities from it, no where near what the .222 Rem. Mag. Imps. deliver. And I said Mags., plural, since he has I believe at least 3 of them I have done for him, plus the one that I rechambered to .224 Weatherby Mag. So we are not speaking of "me and Joe" anecdotal reports involving just a barrel or two.
Advertising hype & aura are hard to overcome when facts are less sensational, but the fact remains that large diameter cases and small bores are counter productive in Contenders. The small diameter .222 Rem. Mag. Imp. case permits hot loads in the 55,000 psi range safely and holds about 33 gr. of powder, ample capacity to propel .22 cal. bullets at velocities pretty close to on par with the standard .22-250, which of course is too hot for the Contender.
The .222 Rem. Mag. Imp. is one of those gems that most people will trip over in favor of hype.