Re: help please 7mm rem mag
If you consistently shoot .5" at 100yds with those bullets, you should be able to get close to 1.5" at 300yds.
There are several reasons why this doesn't always occur.
Your barrel might've heated up and/or is getting fouled which would affect accuracy.
If you changed shooting positions, rests, bipod, bench/prone, etc... then something could creep into your technique that would degrade accuracy.
Next, you may need to verify that your parallax is set correctly. If you bag up your rifle good and solid on the target and it remains steady yet the crosshairs appear to move when you move your eye from side to side and up/down, then the parallax is not correct and any variation in cheek weld will cause your groups to open up.
The greater the magnification, the bigger the problem with parallax becomes. There are lots of threads here about adjusting your scope.
Any wind at any distance will move your bullet around. This is increasingly true at longer distances and is very tricky for shooting groups if the wind is shifting.
Additionally, variation in muzzle velocity may not have a noticeable affect at short range. However, it will have an increasing significance at longer ranges.
Hope this helps.