If i sight my 243 in at 100 yards were will it be at 300 yards?

Lower than it hit at 100 yards.

Honestly, there is no way to accurately answer that question. The one and ONLY way to know for sure is to take the gun out with whatever load you're shooting and actually shoot it at 300 yards.

To give you a ballpark estimate based on ballistics tables we'd need much more information than you've provided.

What is the weight of the bullet, its Ballistic Coefficient (BC), the actual muzzle velocity of the rifle, and height of the sight or scope above the bore's centerline. To get even more accurate, we'd need the ambient conditions like temperature and altitude.

Mike

With a 100 grain bullet sighted in to hit 1.5 inches high at 100 yards you will be back at zero somewhere close to 200 yards and at 300 yards about 7 inches below your aiming point.

Sighted 2 inches high at 100 yards will put you aproximately 4 inches low at 300 yards.

This is just a estemate. To know for sure do like driftrider said and go shoot at different distances.

http://www.remington.com/ammo/ballistics/ballistics.htm

This'll give you a basic idea of the drop for different bullet weights... find a ballistics table for your specific load, and it should tell you exactly.

I like mine to be 2 to 3 inches high @100 for hunting coyotes. Out to 200 just aim low on the body and aim high on the body @300. At 400, I hold on the top of the body. I really like the Federal Premium 70 gr noslers the best. They drop a little more than the lighter rounds, but buck the wind better. Mine is a short, standard barrel on a HS Precision stock. The longer heavy barrels will produce better results.