If your going to buy then yes you have to provide a defence unless you want a two tone.Bucking is the rubber cylinder inside your hop unit that protrudes through the barrel hop up window to provide back spin.
Either use a Tm one costs £5 with standard nub or get a firefly with a flat nub £25.
If your unsure on fitting Firesupport can supply and fit them in your gun,they should do it while you wait its a 10 minute job.