Sight difference in 100gr vs lower gr

You're going to have to re-sight the rifle in for the new loads. Every barrel and load seems to be a law unto it'self.

I've yet to have any rifle shoot different weights, sometimes same weight w/different loads, to the same exact POI. And it's not just elevation, there's normally a windage adjustment also. It might be "minute of deer" close enough for short range shots, but not what I'd accept for varmints. Normally the lighter bullets are flatter shooting with less drop, up until the point that the heaver bullets higher BC catches up.

You might have to re-zero, might not. But I'd not go hunting with a different load until I had verified where the bullets impact. At 200 yards some may have a tiny bit more drop, but not enough to change my zero. I shoot 130, 150, 155, 165 178, and 180 gr loads through my 308's. At 100 yards only the 130's are off enough to make a difference. They are not only higher, but for some reason shoot a bit to the right. At 200 yards some loads may impact an inch or 2 lower than others but none of the others change horizontally. But as long as I know which loads I am using it ain't a problem. Once you get out to 500+ yards then there will be enough difference to matter.

But once again, every rifle and load is different. Don't ASSUME anything, Go out and shoot it, or use the same loads you used to zero the rifle.

The other two posters have it right; you can't assume anything until you go out and shoot it.

With that being said, I don't anticipate a 100gr bullet at 3000 ft/s and a 65gr bullet screaming at around 3400 ft/s or 3500 ft/s to shoot close enough to share a zero, so chances are you'll have to rezero your rifle every time you want to switch between deer and coyotes.

Yes, shoot it with both loads. And keep an open mind. I do have a older -06 that shoots in the same place with different weight bullets. But my 6mm and .280 move both vertically and horizontally with different wt. bullets. Depends upon the barrel harmonics, I think.

Generally, a lighter bullet will be capable of greater muzzle velocity and shoot higher. But a bullet with more ballistic coefficient (usually a heavier) bullet that gives less muzzle velocity will oftentimes be less likely to drop velocity and register higher at longer ranges.

You can hire some ballistics guys and a super duper computer and figure it all out, or just go shoot both and take notes.

P. S. Just shooting it is cheaper and more fun.

Even a different lot# of the same ammo may have a different zero. If you are zeroed say 1.5" high at 100 yd and the new load strikes 1.0" left and dead on at elevation just add 4 clicks right and 6 clicks up if you have a quality accurate and repeatable scope with 1/4 moa clicks for the same zero and move it back afterwords. The issue is most budget scopes can get a little squirrelly when you do that. I find my lower end Leupolds are relatively rugged but not the best tracking scope. They won't return to the same zero or move exactly 1/4" at a time. This is fine if it's a one ammo and distance gun. Not so much if you use 3 different loads and shoot from 100 - 1,000 yards.

Matthew Critzman said:

So in picked up a .243 and bought a box of 100gr and plan on sighting it for 200. I also plan on hunting coyotes which I'll end up using 65gr to save the pelts

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As everyone else has said, you're probably going to have to re-zero your rifle for different weight bullets. However, not to change the subject, but I'd like to add that you might find that from a .243, the heavier, 100gr bullets result in less pelt damage on coyotes than the lighter 65gr bullets.
It's a good thought though. Lots of folks rely on .243s for big game rifles that double as varmint rifles during the off-season.

My Ruger.243 loves Remington Corlokt 100 grain. So much that I gave up trying to find a perfect reload recipe.
Sighted in at two inches high at 100 yds, is good for whitetail out to 250.