As well as a circuit and a lens assembly there are a few other 'bits' that make up a tag gun.
BATTERIES - all that electronics needs batteries to power it. The best for tag use are nickel cadmium rechargeable batteries. While they may cost a little more and require a charger they soon pay for themselves, in both monetary savings and performance.
Rechargeables are capable of giving more power at any given time than even alkaline making them ideal for a tag gun where we need lots of current to power the IR, sound, muzzle flash etc all at the same time.
A 7.2 volt 'racing' pack is ideal or one of the 8.4 volt packs found in airsoft guns. Try to get a battery pack with at least 1.5 aH capacity.
rechargeable battery pack
MUZZLE FLASH - There are 2 common forms of muzzle flash, a bulb or a hyperbright LED. The hyperbright LED is the prefered method as it doesn't require anywhere near as much current as a bulb, lasts longer and is just as bright. All the circuitry on the web site is designed for use with a hyperbright LED as the muzzle flash.
A hyperbright LED is classed as any visible led with a light intensity of 1000mcd (or more). I use 5mm 3000 mcd red LEDs for my muzzle flashes as they are cheap and readily available.
WHY do I want a muzzle flash ? - well one reason is that its only fair that if you shoot someone that they can see that you are firing at them. It also looks really cool at night, especially when 20 people all open up from ambush :-)
SPEAKER - You want to go for a speaker as large as you can confortably fit in your gun case. I generally use a 3 inch 7 watt speaker on rifles as you can get some really decent volume with one. If I can't fit a large speaker in the gun (on pistols and smg's etc) then a 2 inch is ideal.
A mylar cone type speaker is great as they are splash proof from the front and much more water resistant.