Pick any typical "deer/elk" cartridge and scope it with a duplex reticle that subtends ~4.5-6MOA to the duplex intersection @ highest power. Sighted in dead-on @ 200yds with anything from a 6.5 CreedMoor up through the 300Win and you'll have an aiming point @ the duplex intersection that's reasonably close to dead-on @ 400yds. He get's a "400yd rifle" with nothing more than he's already used to using.
Leupold 3.5-10x40 w/standard duplex subtends 5.4MOA and works really well for the above scenario. There's probably lots of others that do as well, but that's the only sub tension I know off the top of my head.
