Well, as rdtricks mentioned, you have affected the barrel harmonics. In most cases you might not notice any difference, but sometimes you will. Barrel harmonics can do some funny things with little changes. You probably will just need to play around with the speeds of the pellets you were previously using to get the muzzle to settle down again. But you might need to try different pellets too. There is also a small chance that you are clipping pellets with the adapter, but that would mean something was out of alignment. Likely not the case. Just try slowing down or speeding up the pellets first, little bits at a time. I assume you have a chrono? You will need one if you want to own a pcp.