Sighting in a scope at 75 yds

[Lex]

I'm planning on sighting in a scope on my M-4 for about 75 yds (on indoor range). The most I can shoot locally is 100 yds (outdoors). Where should I be on paper at 75 yds so that I'll be almost dead-on at 100 yds? Slightly high by 1/4"??

Any help appreciated.


Lex in NC

[Zak Smith]

Answer these questions first--

1. Altitude ?
2. the sight-center over bore height ?
3. what load, and if not a standard load like XM193, what velocity
4. if no velocit in #3, barrel length for the standard load?

[Lex]

1. Alt. is whatever Raleigh, NC is.....close to sea level??
2. Not sure..scope is shipped today.
3. Ammo is 55gr from GA Arms...not sure of #'s
4. 16" barrel

[KarlG]

This Table (http://www.radix.net/~jchang/ar15DropTable.htm) Might get you close to what you are looking for.

[Lex]

I'll try to digest that info over the weekend.

Thanks!

[Henry455]

Download the Point Blank ballastics software for free here : http://www.huntingnut.com/index.php?name=News&file=article&sid=3 , To use the program you will need to know the following, bullet weight, bullet coef. , (available from the manufacturers website or check out the Ammo Oracle), muzzle velocity (subtract 30 fps for each inch of barrel length less than 24"), altitude of shooting range, temperature, height of scope center above barrel center. Plug this info into this program, set you target distance to 100 yards, run the program and it will tell you the bullet trajectory.
For example, using M193 with a BC of .243,bullet weight of 55 grains, muzzle velocity of 3150 (from Ammo Oracle, m193 from 16 " barrel), scope height above bore 2", temperature of 85 degrees, altitude of 200 feet, target zero of 100 yards, bullet trajectory would be +.02 in. at 75 yards.

[WayneConrad]

This is my go-to site for questions like that:

JBM Small Arms Ballistics (http://www.eskimo.com/~jbm/calculations/calculations.html)

No software to install.

[45crittergitter]

Within about 1/10" below dead on. Your group will likely be much larger than the difference in POI between 75 and 100 yards. To be dead on at 100, your bullet will still be rising at 75.

[LHB1]

45CritterGitter +1. Somewhere around .1-.2 LOW at 75 yds should be close to dead on at 100. Can you/your gun/your scope/your ammo group well enough to discern this small difference? If so, great!

Good shooting and be safe.
LB

[Lex]

Just a hair LOW? OK. I was thinking the bullet would drop a little at 100 yds.

Good info.

Thanks!


Lex

[LHB1]

Lex,
The bullet exits the barrel BELOW the scope line of sight, rises to the line-of-sight and at some distance begins falling below the line of sight. To put things in proper perspective, when the gun is properly sighted-in at 100 yds, the bullet will still be climbing at 75 yds. IT WILL NOT HAVE CLIMBED TO PEAK HEIGHT AND BEGAN FALLING AGAIN IN LESS THAN 100 YDS. Thus, to pre-sight at 75 yds so that the bullet will be at point of aim at 100 yds, you will need to sight it about .1-.2 LOW at 75 yds. Actually the difference between 75 yds and 100 yds impact may be so small that the two distances may appear to use the same sight adjustment.

Good shooting and be safe.
LB

[Lex]

Great! Thanks for the explanation.




Lex

[45crittergitter]

Well, actually it could rise to its peak and start falling again inside 100 yards (assuming improper sighting), but the peak will be below the line of sight, as will the entire bullet path.... :neener: