I have a problem with my M4. I was at the range today and have no pressure on the rear gauge. I was shooting the rifle at 250 bar on the front reg, 150 bar on the rear, front adjuster in front below the barrel was at the 4th line, hammer adjustment at 12 and the micro at 3.5. Rifle was shooting good. Then had a loud pop and burst of air by my cheek against the cheek riser. So where should I start looking? What might be the problem? This is my first PCP rifle.