Ken, The IWI Galil is actually barrel marked 5.56NATO and will fir both 5.56 and .223 ammunition no issue. If you are asking which caliber is best in the IWI Galil Platform, this will depend on your shooting requirements. 5.56/.223 is sufficient but the 30Cal models have more knockdown power.

caliber is .223/5.56 nato you can also get the IWI US, GALIL ACE SAR RIFLE in 7.62x39 ( ak rounds) and in 7.62x 51 of the three for everyday use I would take it in .223/5.56 nato