Link Arduinos over 5 acre laser tag field
You can use nRF24L01+

The chips offer:


Really high range with power amplifier (1km open field radius (776 acres if I'm right) , but fairly high power consumption 100-150mA @ 3.3V) 
High range with PCB antenna (50-80m open field, low power consumption 14mA peak)
Low cost (0.8£-6£ a piece)
Many easy to use libraries available, possible to create a mesh network
High usable voltage range (direct Li-Po for example without step-up)