I never left that idea of covered relief port. But I do not want to make existing closed (sealed) muzzle more complicated that it is now. I usually just lift the gun in horizontal position above water surface, after inserting the shaft in muzzle, and after some water is drained out push in the O-ring using the slider. But with use the gun in scuba, when under water, this idea having the shaft end filed as on image, might be a soultion.
Well it is really what your customers do that counts and I expect many load their guns with the muzzle completely underwater, as in deeper water you have to do that even when free-diving as you cannot brace your feet on the bottom to reload your pneumatic gun. That is how I load my gun and I certainly did that with the " Taimen", i.e. kept the gun entirely under the water all the time. I would never have time to tip my gun's muzzle above the surface to let water out as I am looking to keep an eye on the fish movements around me.