Re: Min barrel length for .223 AR?
You should be able to go from a 24" to an 18" barrel with a rifle-length gas system without any problems, but as with everything...your mileage may vary. You may need to open up the gas port slightly depending on the ammo that you are using, etc. to obtain 100% reliability in terms of cycling, etc. I personally wouldn't go any shorter than 18" with a rifle length gas system on a .223/5.56, but you'll find others who disagree and say you can go shorter with other modifications like again...opening up the gas port...as well as lightening your BCG, etc.