Sounds like to me its a mag issue. The more pressure you put on the spring. It fails somewhere to operate normally. Try taking mag apart. Inspect the spring. I am in the process of building a 6mm arc also.
LOL...... I have to agree aswell. Support is need for gunaholics. I need another safe. My girlfriend & best friend don't understand my gun logic. Was only going to buy 1 more rifle. Now debating on 3 rifles & another hand gun.