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The first thing you need to address is that the SAAMI length of a 22-250 cartridge is 2.350" while the 223 Rem cartridge SAAMI spec is 2.260". AR-15 magazines are typically 2.28". The 22-250 also has a 308 size bolt face. Olympic Arms briefly offered a 22-250 upper in their "UMAR" extended magazine version. It's no longer in their 2004 catalog.
Building one would not simply be assemby of available parts.
If you want to shoot .224 caliber bullets really fast from an AR-15 I'd suggest buying an Olympic Arms upper in 223 WSSM. http://www.midwayusa.com/find?userSe...ympic+223+wssm. The Olympic bolt and barrel extension are not standard AR-15 designs and capable of handling the WSSM cartridges.
cart ________case L capacity pressure
Length in inches
Capacity in grains H2O
pressure in PSI SAAMI
That's not to say I recommend the 223 WSSM. The downside is limited barrel life. I like the 223 WSSM cartridge and shoot it in a Winchester 70 bolt action. Velocities are well above 22-250 and 220 Swift.