I mean absolutely no disrespect by what I'm about to say.Its my honest best guess.
The rifle /shooter/scope/ammo has demonstrated it will shoot 1.5 MOA at 100 yds.
If something occurred like loosening scope mount screws,a change in the rifle or scope,then your problem is explained.
If no such change occurred,lets think about what is possible,or not possible.
If the bullets flying to the 300 yd target would have made a 1.5 MOA target at 100 yds,those bullets left the rifle and scope a long time ago.A group will open up some over distance.If you shot 1.5 MOA at 100 yds,it might be expected you would shoot (guessing)10% or so larger at 300yds.1.5 MOA at 300 yds would be a little over a 4 .5 in group.Allowing for the group to grow a little with range,it does not make sense your group would go over 6 in at 300.(Unless wind or extreme mirage came into play)
From my own experience with a 7mm Rem mag,in hunting weight rifles it can deliver enough recoil to begin penetrating..punishing,the shooter.
We being humans,the mind and body will begin to anticipate the next recoil.
IMO,the source of your groups opening might be you had better concentration on the first group.It may very well be that you shot the 300 yd group with your eyes closed...a fraction of a second before the trigger broke.
For myself,the most powerful technique I know to keep myself focused is to totally concentrate on seeing the crosshairs on the target at the moment of recoil.Calling the shot.I want a mental picture of exactly where the crosshairs were at the shot.If I have a target on the bench next to me,I want to be able to put my finger on the target and show you where the crosshairs were when the the scope jumped with recoil.
If I can do that,my eyes were open.If I can't,my eyes were closed.
The classic ,revealing test is to let your buddy load your rifle for you and surprise you with a dummy round or an empty chamber.Close your eyes,let him close the bolt for you.
When you squeeze off on an empty chamber,expecting recoil,if you still see the crosshairs resting center bull,good for you!!
But if you cringe a bit,you know why your groups open.
The good news would be you then can make your rifle shoot better by acknowledging the flinch and overcoming it.
You may even find your 100 yd groups improve.
IMO,if the scope screws stayed tight and the guard screws are snug,If this were happening to me,I'd look real hard at myself,rather than the rifle,as the place to make improvement.