All tag guns have some things in common. They all have some form of grip. All have some form of trigger, it could be a 'proper' trigger or a simple push button but it makes the gun fire. They all have a lens setup and somewhere for the batteries to go.
They also usually have a speaker too. Some of the more advanced circuits will also have displays and require more in the way of switches too to take advantage of the extra capabilities.
All of these things need to be taken into account when designing your gun.
From personal experience the most difficult physical bit to get 'right' is the grip.
There are several ways to make them, from carving them out of wood, making a master and molding them out of resin etc but probably the easiest method is to steal one from a toy that feels comfortable :)
Your doner grip is simply trimmed down until you are only left with the grip bit you want and possibly with some slight re-inforcement from some filler is used on your new gun design.
Unless you have some experience of carving /casting etc then its probably the method you will end up using.
The simplest thing to use for a trigger is a simple push button switch. Especially if youve carved the grip out of a solid material.
If youve used a doner grip with a trigger already there then its usually quite a simple matter to fit a microswitch such that the trigger will activate it. Hotglue and 2 part epoxy putty are handy in this regard.
If your gun is big enough to fit the lens setup inside it then great. If not then it needs to be mounted externally somewhere.
If your going for a more realistic look then making it look like a grenade launcher is a common way of blending it into the gun design.
Wherever you mount it make sure its SECURELY fastened in place. Its what actually projects your infrared beam so if you have any hope of accuracy it needs to be solid.
Wherever you put your speaker it will need vent holes for proper sound output. If you can make these look like part of the design it looks better, if not do what everyone else does and just drill some holes in front of the speaker ;)
Tip - if somehow you can mount your speaker in a tube facing towards the enemy you can get some really awesome sound, especially as its directed AT the target. You can actually cause panic when you fire from sheer intimidation factor
Think about the placement very carefully. They can make or break how user friendly a gun is.
You want the most commonly used switches in easily accessible places, but not so that you will accidentally press them. This can be a difficult balancing act to master.
You dont want to activate a reload timeout just as you step out from cover to hose a target down, now do you ? :)
If you have a display fitted on your gun then obviously you want it to be visible. Try and make it visible to just you though as its a dead givaway at night.
Either that or fit a brightness control or a display on/off switch so you can just flick it on to check it then off again for sneaking about.
When it comes to internals, fitting the batteries and speaker in can be problematic on smaller designs. Of course on BIG guns thats not going to be a problem :)
Some points to remember
Make sure the case will have enough room in it to fit all the internals in.
Think about access to the internals after its finished. You might need to access them for maintenence or updates.
Battery access is also important. One common technique we use with rechargeable packs is to fit a recharge point on the outside of the gun case. This allows in situ charging of batteries without accessing the inside of the gun at all.
Look at real guns for design tips and check out all the pictures you can of any gun design for inspiration. Movies, comics, other peoples work, all can give valuble ideas for your own masterpeice.
If Ive missed anything out let me know and I'll try to add it in :)