Here'a a couple of links you might find interesting. The .223 WSSM has a larger case capacity than either the 243 or the 6mm case.
Of course since your not worried about barrel life and you have a long action, maybe a .22-06 would be worth trying. /ubbthreads/images/graemlins/grin.gif
22-243 Winchester -
http://ammoguide.com/?catid=197
.223 Winchester Super Short Magnum -
http://ammoguide.com/?catid=362
And here's a link to the .224 TTH and what ammoguide has to say about it.
224 Texas Trophy Hunter -
http://ammoguide.com/?catid=538
"The 224 Texas Trophy Hunter (TTH) was designed by gun writer Ralph Lermayer in 1998. It is essentially a 244 Remington (6mm Remington) necked down to accept .224" bullets. The design criteria were to develop a more lethal .224 caliber rifle cartridge for deer hunting with mild recoil and a flat trajectory.
Rifles made for the .224 TTH have fast twist (8" or 9") barrels to stabilize heavier than normal (75, 80 & 85 grain) .224" bullets. These special bullets have heavy jackets or bonded cores to prevent the bullets from self-destruction in flight at the 300,000+ rpm rotational speeds.
The 224 Texas Trophy Hunter is very similar to the 224 Clark. Timothy Clark of Madera, California began work on his cartridge in 1947. Mr. Clark based his cartridge on the 257 Roberts case, which was based on the 7x57 Mauser case. The 244 Remington is basically the 257 Roberts necked down to accept 6mm bullets. The 224 TTH is identical to the 5.6x57 RWS cartridge except for the neck thickness. There are several other .224 caliber cartridges based on the 244 Remington.
At the cost of expensive custom rifles, highly specialized bullets, expensive reloading dies, short barrel life and no commercially loaded ammunition available; the .224 TTH shows very little performance advantage over the standard factory 244 Remington cartridge.
The .224 Texas Trophy Hunter is for all practical purposes ballistically identical to the .223 WSSM.
Another solution to a problem that didn't exist."