My friend has an Echo 1 M249 (airsoft). If I make a custom battery pack at 10.8volts and 1500mah will I hurt i

a 9.6volt 1100mah battery does not hurt it at all. also current amperage affects rof and power too right?

If you put a 10.8v battery in there, you will wreck every internal gear that there is to wreck, that is way too strong for a stock gun, especially a clone brand. Stick with the 8.4v, or whatever battery came with it, otherwise your friend has just lost a large investment