These particular recyclers are primarily mods for the laser RCW, but are also compatible with the recharger pistol. It replenishes one shot per four rounds that the player character uses. No electron charge pack is expended on every fourth shot, meaning that the weapon uses 25% less ammunition to fire the same amount of shots. Having the Laser RCW Recycler installed makes one magazine last for 33% more shots than normal (e.g. the Laser RCW's 60-round magazine will now be able to fire 80 shots) - effectively giving you a 33% larger magazine capacity and 33% more ammunition overall. Visually, a light gray disk-shaped object is affixed to the rear of the electron charge pack socket assembly on the laser RCW. No comparable visual changes are distinguishable for the recharger pistol.
This upgrade can be very expensive (depending on your barter skill and discounts), so only purchase it if you plan on using the RCW or recharger pistol enough to justify the money/ammunition saved by having this upgrade. It's useful for hardcore players, since it also means you can get more out of the ammunition you already have or have a larger amount of unlimited and weightless ammunition.