Vasu AreAsked on 18 October, 2018
In achievements, it says to kill 500 enemies with an assault rifle and the reward is a gun so can we use that gun in the match?
The reward for killing 500 opponents with an assault rifle is not a gun, but a gun skin. More specifically you will get a skin for the M416 assault rifle, which you can equip in your inventory. Once done, every time you pick up the M416 in-game, you can see your gun in that special gun skin, instead of the default color.
Amjith AmjAsked on 9 October, 2018
Is all these kills are need to done in a single macth
No, absolutely not, your results in every match are recorded and keep getting accumulated (as long as you are logged in from your account). Once you reach the magic number required to unlock the achievements, you will get your reward.