Ok, so I just finished putting together my long range precision rig. I bought a CMMG MK-3 and rebarreled it to .260 Remington with a Lilja barrel, and mounted it with a JP Enterprises 15.5" rail. Everything seemed to click together no problem. Then, I went to dry cycle some rounds through it, and immediately found a major problem. With both PMAGs and LaRue mags, the round does not feed at a high enough angle, and smashes into the bottom of the chamber face. Most of the time it will chamber the round all the way, but it sets back the bullet into the case a considerable amount, and damages the meplat. Ammo is factory fresh Southwest Ammunition 142gr SMK on new brass. Nothing on the rifle has been changed other than barrel, trigger, and rail. What did I do wrong?

Thanks in advance for any help - I need to get this thing running for the GAP Grind!

Since you have tried 2 different mags, I'm afraid your mag catch in the lower might be indexed wrong with regard to magazine height, or you have tolerance stack between the upper, lower, and mag catch slot.

You can see that the cartridge needs to feed at a higher angle in order to transition into the chamber, rather than having the meplat hit the edge of the breech.

It looks like the magazine needs to be seated just a little lower.  Not what you wanted to hear, but that's what it appears to be right now.