I had the same issue with an Apex Carbon 130. Seems both barrel sealing plugs were not doing their job. Ended up having to get 2 new ones and replacing the old ones. Guess there was a problem with a batch of guns/plugs. Installed the new ones with RTV. Another problem is the plugs sit several inches back in the barrel. That leaves another open place to fill with water. After I installed the plugs I sealed both ends of the barrel with common sealing foam found at your local hardware shop.
After all that, I finally got my gun to float again. Its a lazer now
Good luck
Thanks woody...