I had the same problem with a reelgun, and did some homework.
On some barrels the plug sits to far to the back in the barrel, so the screw-holes leaks water into the barrel, that fills with water and is then to heavy to float, luckily this was not the case with me.
On my gun the barrel plug was very far forward, this meant that the back 30cm of the barrel filled with water, I filled this with foam, and it helped a lot.
The second thing I did was to replace the dynema on my reel with a different tipe of line, (don't ask me the name, but I got it from Southern Ropes) This new rope, that looks almost like a 2mm thick climbing rope floats, while dynema is neutral. This new line is not as strong/hard wearing as dynema, but I think Dynema could be a bit of a overkill, especially at more than double the price!
My gun now floats when the spear is not inside!
PM me if you need more info!