Walkie talkies for the tactical gain when playing as a team or squad. Do not forget to include a good headset in your purchase.
The Radio is an important part of your tactical gear to be able to communicate with your team members.
Especially at the large locations it is useful to contact each other to regroup after you have been hit or just to tell where the opponent is in order to gain a tactical advantage. There are Radiosets that are complete with PTT and headset (e.g. those of Topcom) but there are also Radio's that you can buy separately with PTT and headset.