Standard clays are 110mm = 4.3" or 108mm = 4.25", Midi's are 90mm = 3.5", and Mini's are 60mm = 2.4". So you're talking about just under 1.1MOA for standard clays at 400. Given a calm day and a sub-MOA rifle, it's not really even asking much.
My wife and I use clays regularly as rifle targets, since they represent about the same size of what we consider an appropriate target size for deer and coyote vitals. Our day to day range is 300yrds, but we regularly shoot at 600. We also shoot 20 and 32oz Gatorade bottles laying on their side (circular bottoms) and half gallon milk jugs (square bottoms) which are all in that 3-5" ballpark. Filled with water, the bottles are reactive targets as are the clay pigeons.
When an AR shoots 1/2 - 3/4 MOA at 100-200yrds, keeping it inside 1MOA at 400 either requires a calm day, or a moderate wind doping skill. Ain't magic. If my wife and I can do it, anybody can.