Magnum primers are generally hotter and loads need to be reduced not added to.

As for 3gr that is a whole lot of powder. In most of my manuals it is nearly the whole load window for the 06' family,w/some powders.

.3gr might be enough to use as a guideline.

Here's a place where I've caught hell before you may have a very touchy gun and it may settle down w/100 plus rds through it or not. I've 2 rifles same cal same brand similar modles. 1 has shot everything its fed well enough for the field and holds the vast majority of loads under 6" at 300yd. The other 1 will throw out groups for a case that is a gr heavy or light, the case mind you not the charge. For it I have to match cases to less than .5 o/u.

Another thought is that the load may be fast enough to be over stablized not a common problem but it can happen. It usually shows up way out w/bullets staying flat rather than following the flight arc. It may also show up as blown groups like you discribe if the bullet were to wobble and restablize between 100-200yd. Used to be very unlikely but is becoming more common.

Don't get in too big a hurry the gun will settle down. And you may find that it is an 1 1/2"gun.

Another thought check your action screws.