I had that same exact thing happen to me with the gen 1 version of the R51. I sent it back to Remington and got my money back. Now, I will say that Winchester white box ammo is not the highest quality, I recently had one round in which the case was just slightly longer than it should have been and locked up the PPS M2 that I was shooting at the time (the gun would not go fully into battery and would not fire). With the generation 2 version of the gun I was surprised that Remington did not change the chamber so that it would fully support the case of the 9 mm. I still love this gun and would love to have the gen 2 version but just not ready to buy one,  still waiting to see if Remington gets all the problems fixed.