Will shooting 70-grain bullets affect my rifle’s aim?
To determine this you need to know the exact velocity of the Federal load you use.
The ballistics listed on the box of cartridges or in the Federal catalogue are for a test rifle at their factory, and may not be the same as those produced by your rifle.
However, assuming the Federal 100-grain load you use produces 2,960fps as listed and you are zeroed at 100 yards, then at 200 yards you will be 3.1in low, and at 300 yards 11.7in low.
If you switch to a lighter bullet, namely 70 grains, then it is more than likely that your zero will shift not only vertically but also horizontally, and you will need to re-zero your rifle.
Sometimes, however, ballistics do not always match paper statistics and you may get lucky with a zero close enough for your purposes.
If a 70-grain .243 bullet travelling at 3,450fps is zeroed at 100 yards, then at 200 yards it will be 2.1in low and 8.6in low at 300 yards.
This is flatter than the 100-grain deer load, but you need to shoot the actual load to determine any zero shift before using the rifle in the field.