Mono bullets can be picky.
In addition to multiple OAL, it's also prudent to load multiple powders behind them, especially when the range is a long ways from home.
But you did learn something. Your gun doesn't like GMX's with RL-19.
Got a dumb question I just thought of. Can grass majorly affect a bullets flight path. I was laying down while shooting. And with it being summer there was longer grass. I avoided as well as I could but there was deffinetly some grass 5-10 feet infront of the barrel.