I am building a LH AR in 556. I checked on other forums and found a 50/50 result to my ?. I want to use a YHM barrel with M4 ramps and a LH BCG, but... the extension is for a RH. I did read many people used standard barrels in LH builds with no issues. Stag is the only company that has an ambi extension from what I can find. RedXarms makes a LH upper and I was told it is a standard barrel. Anyone have experience in this?