New barrel/cartridge recommendations

Hello All,

Been shooting PRS with my son for a couple of years in CO and would like to upgrade his setup a little. We both shoot gas guns - I'm either shooting 6.5 creed or 6.5 grendel.

Current setup:
Platform: Aero AR-15
Barrel: Ballistic Advantage 16"
Cartridge: Hornady Frontier Match 5.56, 75GR BTHP
Mostly standard hardware

Distances up to 1400

Thoughts:

1. Could just go with a longer barrel in .223 wylde to keep cartridge costs low
2. Change up to a 6mm
   1. Arc/Creed/??

Please advise on cartridge and barrel length