So here's my guess/suggestion....
Disclaimer: I've never heard of this happening before.
I'm thinking there's a drag/snag somewhere in the loading/chambering. This is preventing your rifle from going into full battery.
1. Try a different magazine; steel vs polymer, 10/20/25 rounds. Some rifles just prefer specific mags. One of our members has a CMMG that will not use Gen2 PMAG's, but runs fine with Gen3????
2. Check your extractor. Just make sure there's a full range of motion there. Probably not an issue since your single load is going through no prob.
3. I should have said this first.... WET!!! How wet is your rifle? The .308 rifles like to be very wet at first. I don't mess around with expensive gun oils for break in. I soak all my BCG's in Mobile1 full synthetic 0w50 to start. After they are broke in, I swap to a CLP or whatever else is on hand. I cannot express WET enough. Keep the chamber clear, but the rest of the upper should splash when it cycles.... seriously.
4. Buffer and buffer spring: make sure you're using an Armalite AR-10 buffer spring. You may also look into a heavier buffer (H2 or H3 minimally). JP also makes an adjustable weight "Silent Captured Spring" (SCS) system that might work...$$$$
5. Chuck a chamber brush into a rotary tool, squirt some glitz on it, and spin that little bastard to death inside the chamber. It's a cheap $3 sacrifice to smooth out the inside of that chamber. Clean it really good afterwards. Windex will neutralize and clean the acidic polish, then use any spay cleaner after. Make sure to relubricate. The porous metal will be stripped of all protection and will rust like a MFer if you neglect it at this point.