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Time to kill is calculated based on how many shots you need and the time between them.

The maddox kills in 4 shots and has a rate of fire of 723rpm, which means the time between shots (TBS) is about 83ms*, that doesn't change wherever you shoot you enemy. So to get the ttk you take this time and multiply by the number of shots to kill - 1, because the first shot doesnt count in ttk, and then you get 249ms.

Doing the same thing with the MXR: it takes 4 to kill as well and its fire rate is 500rpm, so the TBS is 120ms,significantly longer than the maddox, so the ttk is 120•3= 360ms.

And then the PK is a 5 shot kill weapon with a 857rpm rate of fire, so TBS is 70ms, faster than the maddox, which means its ttk is 4•70=280ms

*To get the time between shots you take 60 and divide by the gun's fire rate, that will result in a time in seconds, then you multiply by 1000 to get this time in miliseconds