Aceofpyro

So this is a brand new build. I have a 40 cal ar-15 built on a spikes tactical glock lower. It has a 7 inch barrel with a linear compensater. It's gas blow back only. Im using and Ar-10 buffer spring and a 9 oz buffer. I took it to the range and it would randomly double feed (about every 3 to 5 rounds) the bolt slammed pretty hard at one point pushing the bullet into the casing. Anyone have any ideas -Thanks