[Ricochet]

It's something in the path of the BB. As stated above, it could be your barrel is bent, but that would probably cause them just to go left, not to curve left. Barrel is an obvious one, debris or damage can cause deviations. The big one is hop-up. If the hop is twisted, tilted, or the bucks nvg isn't sitting straight and even it will make them curve away from center. If all that looks good then it could be a relationship between your gun and how the barrel grouping fits in it. If say pressure is applied to one side of the Hop, it could be doing it. Perhaps it's not all sitting together right? Anyways, rebuild the hop and barrel grouping, ensuring a good airseal and that everything is clean and seated correctly, make sure your nozzle and receiver (everything that touched or comes near the barrel grouping) is seated and put together properly. Use good quality BBs and magazines as well.