The .223-size 6.x rounds strike me as even worse than .223 because I assume they have a worse trajectory than .223, yet not a lot more downrange energy.
Your understanding does not match what I have read.
Matter of fact, what you wrote is the exact opposite of the usual 5.56NATO vs 6.5Gremlin comparison. The 5.56mm starts with greater velocity(1) (though less energy) and proceeds to shed velocity and energy faster than a prom dress, while the 6.5G holds what its got. The advantage, trajectory-wise, at the 300+m ranges goes to the 6.5G.
There is a great energy graph I saw a while back on wikipedia, that holds the 6.5G 142gr pill as 100% and places all the other pills relative to it, percentage-wise
<looks for graph>
Here it is:
http://en.wikipedia.org/wiki/File:AssaultRifleCartridgeComparisonChart.PNGHere's a velocity chart comparison a fellow on the Beartooth Bullet forum did:
http://shootersforum.com/showthread.htm?t=8865My understanding of your problem is accuracy, the longer-ranged targets (300-400m), and energy to dispatch deer. If the shooting is shorter ranges, the 5.56mm will be adequate, even for deer given a decent broad side presentation and a quality pill (Nos Partition, etc,)
Any of the beefier 6.5mm rounds (.260Rem, 6.5x55) will be even better, if you want to go that route.
(1) In the 62gr & less pills. Use anything heavier and stating velocity will be close enough to be determined by handloading and individual gun characteristics.
230RN:
Very good point on ricochet avoidance.
Thing is, anything that will hold together enough to dispatch deer at moderate range will not disintegrate at long range. The farmer in question will likely have to settle on either having to do a mag change to use different bullet types or settle on a compromise load.
Frankly, the requirements listed are going to be very tough to meet:
* 400m varmint-accurate
* Powerful enough for deer
* Rapid firing (semi-auto)
Tossing other requirements into the mix will result in no rifle/cartridge combination being up to the job. This actually points toward a semi-auto with different mags loaded with different cartridges for different applications.