The power down shells claim to reduce your velocity by over 50%. They would be an easy solution if you aren't comfortable opening the gun yourself.
There's no reason you couldn't do the mod yourself.
Dan Wesson Co2 Revolver POWER MOD and TUNE UP - YouTube
This guy does a good job explaining how to take the gun apart in detail. He's using a pellet gun as an example but the airsoft version is he same thing it just fires different projectiles. The idea behind the mod is that the weaker the spring the slower the gun will shoot and vice versa. He's adding to make it shoot faster you need to take away to make it shoot slower.
There are also a couple of guides on how to preform the mod here on ASC. Off the top of my head Kokanee does one and you should be able to find the others using the search function.