You've left a lot of variables out of the equation. Your 280, may just be you: IE, trigger control and breathing. Is it a real light rifle? The 270 has definite problems. Is it bedded? Probably has uneven pressure on one side of the barrel as well...Properly bed that rifle and freefloat it and let us know how it shoots. Start shooting some 5 shot groups too, that will give a better representation of the ability of both shooter and rifle...
What bsa said. For the 280, I'd add grip pressure on the pistol grip. When I do my part, my rifles shoot nice little groups. When I start hanging onto the gun a bit (r.e. gripping the pistol grip in an inconsistent manner), my groups look exactly like your 280 - opens them up to 1.5" or so. For the 280 it looks like a consistency issue with some facet of your form. For the 270, I'd bed and free float it if it was mine. Then follow the aforementioned form/consistency suggestions.
