PT1 ASSEM. UPPER, .223WYLD, 18" HEAVY FLUTED MID LENGTH BARREL, 12" TACTICAL EXT HANDGUARD
 
The .223 Wylde is the answer if you like to use both .223 rounds and 5.56 NATO rounds. The .223 Wylde was engineered to be able to safely handle the 5.56 NATO pressure.