I agree with Swathy. If you don't want to solder you have picked the wrong tank manufacturer. Every single Heng long tank I own would be a shelf queen if I never picked up the soldering iron. 5 bulldogs = 1 Tamiya Sherman. You might want to think about that, although it sounds as if the airsoft feature is important to you. The longer bbl Bulldog will not be as good as the short BBl PZIII. The PZIII out of the box is the best firing Heng Long available.

For me the best fixes I have come up with for the airsoft gun are.

1. Install spacer in spring chamber to cause the spring to be in more compression.
2. Lube the gun with a light silicone spray.

These two little fixes will allow you to break 6mm paintballs at 20 ft. Critical to the operation of the airsoft gun is the litte rubber grommet that provides a seal between the bbl and the "traveling ram". Very finicky and is easy to drastically effect the shot velocity with small changes to the compression this grommet is subject to.