The military ammo I've measured has all been 1.260"-1.270". 1.275" is just a not-to-exceed maximum that is set to insure fit in all magazines and chambers.
The main issue is nose form. The military bullets use a 234 grain bullet that is 0.680" long and have an elliptical nose profile that is longer and skinnier than the hemispherical profiles you see on some makes of bullets. The Hornady bullet is 230 grains and 0.640" long, so it's nose is likely somewhere inbetween. What Hornady is doing with 1.230" COL is copying the military bullet seating depth.
Seating Depth = Bullet Length + Case Length - COL
Hornady's load data will have been developed with the 1.230" length, so if you are using their data you will possibly get lower velocities with a longer seating depth. It will tend to lower start pressure until it gets so long it jams into the lands. If the Hornady nose form is a less elongated ellipse than the military nose, them seating them out may jam them into the lands of some barrel's throats, causing possible pressure of lock-up issues.
To test for this, remove the barrel from your gun. Pick a round seated to 1.270" and drop it into the chamber and measure how deeply the bottom of the case has gone into the chamber. Seat the bullet deeper to 1.230" and drop the cartridge back in. If the case head is no further into the chamber, then these bullets fit your chamber anywhere in the COL range.
I think the bottom line is you test which COL gives you best accuracy in your gun.
Gunsite Orange Hat Family Member
CMP Certified GSM Master Instructor
NRA Certified Rifle Instructor
NRA Benefactor Member