The Better Bolts will add approximately 100ft in range in the gun they are designed for. They are direct replacements. The undershot puts a sever back spin on the ball so it flies like a golf ball. You get a flat-line shot to the extreme range. You get more shots per tank because the velocity is lower (260-270). You will not believe what this bolt does.
The Cooper-T Products Spyder Rear Charging Better Bolt is older, so while it may be available used and in a few cases new, it is not commonly available anymore.