If this is the first time it's happened in 1k rounds then I'd assign it to an ammo issue. I had this happen the first time I ran Remington thunderbolts. Rifle had at least 3k round of non-Remington ammo at that point. Hasn't happened in additional 2k rounds since then (not using that ammo).

How much of this ammo have you used? If it happens repeatedly after switching ammo brands, it's a rifle issue.