Smaller on the inside
The diameter is very similar so it's not going to mess up the look of the gun and should mean is going to be an easy fit.
No chance of fitting the muzzle flash LED inside the emitter as I've bought it complete. It'll have to be somewhere on the front face of the gun, but this shouldn't be hard to pull off.
I need to buy some NiMH batteries to turn into a 7.2V brick. The 3.6V Li-ion ones you get are appealing but more complicated to charge so ideally need to be removable.
I also need to buy an assortment of switches as I've nothing but little PCB tactile stuff at the moment. This will need a power switch, reload switch and trigger. I've a half formed plan to use reed switches but it might not work out.
The Lasertag gun I bought made up by Phil was constructed out of a Playstation Time Crisis style light gun and I can see why. You get something already designed to emit IR with a switch at the trigger and a smattering of other buttons.