I zeroed in a Ruger No. 1 at 100 yards with 100 gr Federal Powershok .243

Got a nice group of 9/16" almost dead center

But I used 100 gr instead of the 80 gr that I have left at home

Here are the published muzzle velocities

100 gr 2960 FPS

80 gr 3330 FPS

How much higher should I expect the 80 gr to be at 100 yards

My guess is that it should not be more than an inch or so, everything else being equal

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This is an un-answerable question. While POI will generally change with bullet weight it is totally un-predictable what that change will be. Could be up, down, left or right. It may not change significantly or it may change several inches. My .280 Remington will put any weight bullet in the same 1" group at 100 yards, while my Ruger #1 Tropical .458 will change POI sometimes 6" ar rhe same range simply by changing powder type! The only way to know how your rifle responds to ammunition changes is to shoot them in it!

That's right! Not only may it shoot higher or lower to a greater or lesser degree, it may go left or right.

You have the rifle; you have the ammo. Go shoot and see what happens.

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Pisgah

I would just stick with the 100 grain bullets for hunting and use up the 80 grain bullets to practice with.

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This ballistic program will give you a rough idea of what you're looking JBM - Calculations - Trajectory (Simplified)

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Captain

The number one factor on POI change in rifles is the vibration of the rifle barrel, which will change the position of the muzzle when the bullet exits, which effectively changes the initial direction of the bullet. The basic trajectory difference from a ballistics chart is a secondary effect.
My .270 throws a 100gr bullet nearly 5" low and left at 100yds compared to a 130gr bullet. In addition, it is far less accurate. If you have the threaded weight on the rifle muzzle, you can tune the barrel vibrations to a null; otherwise it just goes where it goes.

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Science plus Art

Using some rough back of the envelope approximations and an online free fall calculator I estimate that the change in velocity would cause a 0.5" change in vertical POI

At 3000 FPS it takes a bullet 0.1 seconds to travel 100 yard (300 feet)

In 0.1 seconds of free fall an objects drops about 1.8 inches

If the velocity is decreased by 10% it takes an additional .01 seconds to reach the target.

In that extra .01 seconds the object drops an additional 0.5" (remember, distance of an an accelerating object goes up as a function of time + time squared)

So if zeroed in at 100 yards and everything else being equal the change in vertical POI is only 0.5"

I agree that the vibrations and harmonics will likely far outweigh the change due to bullet drop

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