depends on the height of the scope from the barrel but most centerfires like .270, '06, .308 you should shoot around 3/4" low @ 25 yds.
if you use see thru mounts it different, or if you have say ,a 50mm objective and use high mounts to clear the barrel. most of my scopes are 40-44mm and if i want dead zero @ 100 on my ruger '06 i shoot a tad less than 3/4 @ 25.
but that said i shy away from 100 yd zero and opt for max point blank range to avoid holdover out to 300 yds
I set my .243 in at 50 its about 3/4 in high at 100 and 0 at 150 bout 1 1/2in low @ 200 and like 3 1/2in at 300 and 19in @ 400 and 42 in at 500 with 100 gr rem core lokts. I figure out to 3 i got a basic no hold over and regardless of what i set it in at i would have a hold over or big hold under andwhere else and its dead on at two places instead of one here. From what i can observe the bullet actually rises a bit after leaving the barral and most calibers peak at around 100 so i see no reason in setting it in there since it will be low at every other distance.
To get a precise answer to your question just google 'ballistics software' and you can find a virtuall laundry list of programs to which you can plug in all of the variables for various ammo/ firearms combinations to accuratly calculate the external ballistics you are in need of.
As others have said most rounds from scoped firearms should shoot slightly low @ 25yds to be near zero @ 100yds.
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Actually the 25 yard sight in will get you close but will still need to be adjusted at 100. I have had some that were really close and others that got me on the paper when using a 25 yard zero. It will deffinately get you in the ball park and save major ammo. What it won't do is tell you anything about the tracking of your optics or the grouping of your firearm. Good luck and you are on the right track.
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neo- This is purely academic, so accept it as such, sir !! If you hit a target, regardless of size and thickness at 25 yds, the rest of the data is skewed/useless. It changes, even just a tad bit, the coefficient and velocity of flight.
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I have a 3-9x50mm on high rise scope mounts to clear the barrel and scope and I know my gun at 25 yards an inch high is roughly 4 inches off at 100 yards, that is why I say still shoot it at 100 because you most likely will not be dead on. My scope sits a bit higher then some because of the big bell on the end has to be up high to clear. Even with high rise scope rings I am only clearing the barrel by 1/4 of an inch.
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generally, most rifles hit their first "zero" point between 25-30 yards (the bullet rises over line of sight to hit zero again at 100). slower cartridges like the 30-30 or 45/70 tend to be closer to 25 because of their higher arc to 100. (assuming scope centerline is at the standard 1 1/2" over bore centerline).
other rifles like the 308 up to the 300 win mag are going to be 3/4" to 1" low at 25 yards. every gun / load combo I have is sighted at distance (whatever distance that may be), then fired at 25 yards to give me a quick reference for a rough re-sight if I should happen to change out the scope for some reason.
the only rifle I have that is "zero" at 25 and dead on at 100 is my 45/70 shooting a 405 gr. cast bullet at around 1500 fps. on the opposite end, putting my 300 win mag at zero at 25 yards means I'm completely over top of the target at 100. (did that once just to try it years ago).
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At home,when mounting a scope site on a new rifle I first bore site it in at about 40 yards, I put the rifle in a sturdy rest, remove the bolt then shift the rifle around so that the two inch target can be seen in the center of the barrel bore. The windage is first adjusted with the rear base rings, and the elevation with the scope adjustment knob to were the cross hair is on the bottom of the target. I reload four groups of five rounds with different powder weights and go out to a 100 yard range with rifle, ammo, and cleaning kit. If the shot group is way off the target I will suspect there is a action bedding or a stock barrel channel problem. I fire five rounds then adjust the scope, By the time I have shot the last group of five round the rifle is sited where I wont it. At home,If I have had to make a lot of scope adjustment I remove the barreled action from the stock to inspect the wood metal contact points. It is common for me to glass bed the action, freefloat the barrel and adjust the trigger on bolt action rifles. The barrel is cleaned for each five shot group. From siting in I have 20 fireformed cases for neck size reloading, see the group size of the different powder loads, and see if I wont to inspect the stock.