Drilling out gun barrels is easy to do as long as you take your time and are careful. If you mess up, it's a big pain to try and fix, so it's worth going nice and slow.
You only need a couple things to pull this little trick off.
Pin drill (I'm a fanboy and use the GW version).
X-Acto blade (#11 blade with super fine tip)
Fine point marker or felt tip pen.
It's four simple steps.
1. Clean the mold lines from the face of your barrel.
2. Place a dot where you want to drill each barrel.
3. Use the X-Acto blade to make a small pilot hole in the plastic. It doesn't have to be a deep hole, just enough that the pin drill won't skip around when you start drilling.
4. Use the pin drill and drill out the barrel.
Additional things to keep in mind
From Alex: You can't emphasize step 3 enough, especially if you have an electric Dremmel tool.
From GDMNW: I'd add that when you start drilling you should stop as soon as the drill has bitten to see if your mark really was in the centre or not. If it's to one side then you can rescue the gun by drilling a small pilot hole in the real centre and starting over.
From SC John: Use a dental pick or push pin type thumb tack to make the guide hole, as they are round.
Once you're done, if you've got any excess in the barrel opening, you can always go back with your X-Acto blade and clean it out. Just be careful not alter the shape or size of the barrel opening.
This works the same for larger guns like multimelts and heavy bolters.
I just end up using a larger drill bit after starting out with the normal GW one. It in effect becomes a second pilot hole so the the bigger drill bit doesn't move around on me.
When drilling, make sure you're drilling on the correct axis. Meaning your in line with the gun horizontally and vertically. You don't want to end up drilling a hole out the side of your gun in your zealous attempt to make it look cool.
And then you can go all out and drill the side openings in the barrels too. That makes it super real.
If you happen to make a mistake and damage your barrel in the process of drilling it out, don't worry, here's how you can fix it.
