I have read multiple times that people suggested to remove the what they call "air-restrictor" from the plunger and to fill the small hole that is on the top (as shown in the third picture).
DON'T DO IT!!!!
Even though they might tell you that this improves the strength of you blaster, it doesn't! In fact, this actually damages your blaster, because it is not an air-restrictor, but an ingenious method to prevent dry firing. Since after building this 18 shot blaster I had some plungers left over I tried to do the mod and to compare the results by measuring the dart speed with a chronometer.
I also removed the barrel posts from one barrel, to measure the effect. Unless you are planning on using Stefans (If you don't know what I am talking about, here
is an instructable) I wouldn't suggest you to do so either, because it also decreases the strength of the elite darts. The only effective way to improve the dart speed that I found was to change the spring!
The second picture shows the improvement after adding the Orange Mod Works (OMW) kit (from here