I built a Model 70 Vietnam Era Sniper rifle that I shoot at 1000 yard matches.
Using the 175 SMK w/a muzzle velocity of 2800 FPS, sighted in at 100 yards, it drops 30.5 inches at 1000.
I was just checking some old score books using a M14 Service Rifle in HP, including 1000 yard matches.
At 200 yards, my zero is 12 clicks up. At 1000 its 48 clicks up (one min. clicks). That with a 175 @ 2500 fps or so MV. Thats 36 MOA difference.
One thing you need to put in the equation is that if one shoots a 6 o'clock hold then @ 200 yards your point of aim is about 3 min below your point of impact where as on the 1000 yard target your point of aim is about 2 min below the point of impact (meaning you aim at the bottom of the black the bullet will hit 2 or 3 minutes high.
So basically I think the 40 MOA sight change from 100 to 1000 should get you on paper. Take into account the temperature difference. Consider that for ever 15 degree change in temp, you impact will be 1 MOA off. You sight your rifle in at 1000 yards when its 40 degrees in the spring, then you go to a late summer match where its 80 degrees you're gonna shoot about 2-3 min lower.
Also, as I've seen some inexperienced shooters do, getting ready for a match, take their ammo out of the stool and lay it on the mat so they can get to it, they also lay it in the sun where it heats up, like most metal, sometimes almost getting too hot to handle with bare hands, think what that does to your impact.
Something else that needs to be taken into account, just about every rifle, even high dollar target rifles have "dead spots' in their sights. Meaning that you normally have one minute per click, as you count up you have a dead spot, where the tiny treads on the sights skip, meaning when you click it doesn't move. You need to know this. Putting a dial indicator on your sight, and counting up will show this. (How many have used a dial indicator to tell how much your rear sight moves.
An example the M1 has a 28 inch sight radius. Meaning for ever 1 MOA movement you move the sight .0077 inches. The 1917 Enfield has a sight radius of 31.5 inches, each 1 MOA of movement on the target means the sight moves .00875.
So if you have a dead spot, you're gonna be off 1 MOA or 10 inches at 1000 yards. Use a dial indicator to find if you have dead spots.
Anyway, 40 minutes should get you on paper. The above is why its so critical to keep a data or score book, recording every shot fired.
As to adding Williams sights on an unmodified 1903, I think there is a special spot in shooter's hell for those people. There are getting less and less "as issued" un-modified Springfield's out there. The sights are calibrated for the military ammo used at the time period the rifle was made. If you load your ammo to match, your sights will work.
Play with your ballistic calculators, its fun, may get you close, but nothing beats taking the rifle out and shooting it. Recording every shot you fire and in ever environment you shoot that round in. That and only that will tell you what your rifle will do, the data or history of past shots will give you an indication of what your rifle/ammo will do the next time you shoot.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071