Let me explain -- years ago I built a gas operated (DI) AR in 5.7X28 and it shoots great but I wanted to build something that was closer to 22 magnum since I can't find that ammo any place around here. This new AR is manually operated (like the fellows in the UK do) so I don't have to worry about operating a gas system. The cartridges are loaded with 6.5gr of H110, CCI BR-4 primer and the 40gr V-max. I tried 30gr Barnes Varmint Grenades with the same powder charge and primer but they did not shot well. When I went to the 40gr V-max the rifle shot unbelievable. I shot 4 five shot groups that you could cover with a dime -- this is at 50 yrds.
The barrel on the rifle is a heavy fluted Green Mountain barrel designed for 22 magnum so the twist rate is 1:14. I had the barrel shortened, chambered in 5.7X28 and a barrel extension added so it would work in an AR. After I got home from the range I decided to check some of the barrel twist rate programs to see why this rifle shot so well and come to find out the programs say the darn thing shouldn't work at all. I can't be driving the bullets at more than maybe 2000fps, the temperature was 90 degrees and I'm at close to sea level. Do I have some type of freak rifle? Anybody got any ideas as to why the rifle shoots that well with the 40gr V-max bullets?
The picture below is an old picture of the rifle -- I've since added an A1 butt stock and a Magpul pistol grip.