Zeroing at 25 yards for longer ranges?


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DontBurnMyFlag
September 28, 2007, 10:48 AM
Hello all,

I was reading up on how to do an expedient or hasty zero for a .308 M24. Well I recently purchased a Rem 700 in .308 and only have access to a 25 yard range as of now...

I was reading something about doing a 25 yards/900 inch zero, or something to that effect. It basically involved having the point of impact be 5/8 inches above point of aim at 25 yards. Would this mean that at 100 yards my POI would be my POA?

Im still very new to anything that isnt close range pistol craft or open sighted rifles. I'll admit my knowledge is lacking.

Is there a way to achieve an accurate 100 yard zero at 25 yards?

After that is completed, do I calibrate my dials to "1" for 100 yards and "0" for windage?

Its pretty confusing, but Im having so much fun learning about all this. :D

Any help is appreciated.

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twodawgs
September 28, 2007, 10:51 AM
I always use it for starters. Bore sight at 25 feet, first shot(s) at 25 yards and then move on out.

Just did it last week with a new scope on my .308.

DontBurnMyFlag
September 28, 2007, 11:05 AM
my weapon is boresighted as of now.

If I was too take it out to 25 yards, where should the POI be if I want it to be zeroed at 25 yards before I have the ability to actually take it out to 25 yards.

I will soon, its just a matter of getting the guy in charge of membership for the outdoor range to get better hours than 0600-1300hrs!!! Its impossible to catch that man.

Then I will take it outdoors and have fun.

Canuck-IL
September 28, 2007, 11:19 AM
Use this ballistic calculator to try out different zero points ... it'll show the second crossover point. You should zero out the wind speed and try ballistic coefficients for a few different bullets...very educational.
http://www.handloads.com/calc/

/Bryan

Pax Jordana
September 28, 2007, 11:46 AM
I believe M1's were sighted at one click above 25 yards' zero in the armories, and because of ballistics they were back on at 200yds, up to two inches high before that and two inches low at 400.

That's 30-'06 though, with open sights. Use the ballistic calculator.

308win
September 28, 2007, 12:07 PM
You need to have the external ballistics for the load you are zeroing. Any of the online ballistics calculators will provide you with zeroing information as do many of the load manuals. Ignore windage as was already mentioned. Keep in mind that if you are 1/4" at 25yds this will be the same as 1" high at 100yds, etc. Also, the click values of your scope will be 1/4 of the 100 yd value, e.g., if one click moves POI 1/4 inch at 100 yds it will move it 1/16" at 25 yds.

GunTech
September 28, 2007, 12:09 PM
You also need the height of you scope above the bore. The you can calculate your zero at 25.

DontBurnMyFlag
September 28, 2007, 12:18 PM
Ok, so far what Ive achieved is:

I have a Barska scope with 1/4" adjustments at 100yards. Therefore 1/16" at 25 yards.

I did the basic calculations for a 168gr .308 boat-tail bullet fired at 2600fps with an elevation over the bore of .5". I am almost positive that is the correct elevation of my scope.

The calculation stated that the drop would be .22 at 25 yards.

Im kind of confused.

Do I want my shot to be 1 inch low at 25 yards to it will be dead on at 100 yards? Thats what Im assuming from the fact that if I am 1/4 high at 25 yards I will be 1" high at 100.

If only they taught me this kind of math in high school maybe I would have enjoyed it. This is actually kind of fun. :cool:

308win
September 28, 2007, 12:22 PM
I would check the height of my scope center above bore center again; .5" doesn't seem reasonable.

If your calculated trajectory is .22 low at 25 yds for a 100yd zero then that is where you want your 25yd POI.

DontBurnMyFlag
September 28, 2007, 12:29 PM
So I want my POI to be .22 low on the target at 25 yards?

Or vice versa. I want my POI to be .22 high on the target since the drop is .22?

Sorry for all the questions. And I will double check my elevation of the scope.

308win
September 28, 2007, 01:19 PM
.22 low. Remember, you are applying calculated trajectory that is supposed to replicate actual trajectory.

TallPine
September 28, 2007, 01:45 PM
DBMF: are you getting the fact that the line of sight (through your scope) is a straight line and the bullet trajectory is a parabolic curve? IOW, the bullet starts out below your LOS, rises above your LOS and then falls below you LOS. So the bullet crosses your LOS twice.

Anyway, you're probably making this too complicated. If you zero your rifle (most any modern hunting caliber) at 25 yards, then you will almost certainly be on the paper at 100 or 200 yards.

You should check that at 100 yards before shooting game. Typical "zero" is to be an inch or two high at 100 so right on at 200 and not too low at 250. For hunting big game, that's probably closer than you can hold under field conditions.

I used to bore sight at a store with one of those gadgets that just holds a target card up over the top of the muzzle. People told me that my bore sighting was pretty close. One customer even went out and shot a deer without checking it on the gun range. The deer was so surprised about that, he just fell over dead. :D

Canuck-IL
September 28, 2007, 02:38 PM
Maybe have a look at the diagram and explanation here...
http://www.hornady.com/ballistics/external.php
/B

GunTech
September 28, 2007, 02:57 PM
You need to merasure from the center of the bore to the center of the scope.

Then run the ballistic calculator with a 100 yard zero. See what the elevation is at 25 yards. Adjust your scope so that you are hitting at the predicted 25 yard elevation and you should be zero'd at 100 yards.

For 308, this will get you close for a 165gn bullet:

At 25 yards, aiming dead center, your bullet should stike about 1/2 inch low of the bull's eye. That should get you close to dead on at 100 yards, assuming you scope is about 1.5 inches over the bore line.

Jim Watson
September 28, 2007, 02:59 PM
Short range zeros will get you on the paper at 100 or so, but it is not a substitute for actually completing adjustments at the range you will actually be shooting at.

CNYCacher
September 28, 2007, 03:03 PM
64727
http://thehighroad.org/attachment.php?attachmentid=64727&stc=1&d=1191002538

GRIZ22
September 28, 2007, 03:12 PM
You need to have the external ballistics for the load you are zeroing.

This is true and if you chrono that load in your gun even better. Height os sights, altitude, and other factors come into play.

Try http://www.ipgeneral.com/~renegade/rifle/rifle1.shtml

DontBurnMyFlag
September 28, 2007, 03:20 PM
Muzzle Velocity: 2600 fps Bullet Weight: 168 grains Bullet Ballistic Coef.: .25
Sight Height: 1.5 inches Target Range: 100 yards Altitude: 500 feet
Temperature: 75 Fahrenheit

This is close to what I go using the other table. The other table has a .22 reading.

how does this effect my POA?

RubenZ
September 28, 2007, 03:26 PM
What people seem to forget is YA you can sight for drop and elevation at 25yrds but you better be damn sure your left and right is DEAD ON! Because up and down is not of that much importance when sighting in. It's left and right that can be way off when sighting at 25yrds.

ball3006
September 28, 2007, 04:50 PM
Here is what you do, and it does not require book reading.......set your rifle on some sandbags and remove the bolt. Line the bore up on a target at 25 yards, then look through the scope and see where the cross hairs are. Adjust accordingly, trying not to move the rifle. Recheck as above. Then shoot the target and see where the hole is. Adjust the scope accordingly. Then move the target to 50 yards. Shoot and adjust the scope accordingly. Then move the target to 100 yards and shoot. Adjust the scope accordingly. The reason for the short range target is to at least get a hole on paper so you know where the sight is set at. I have seen guys burning through boxes of ammo trying to get a hole in the target at 100 yards because they don't know where the gun is shooting. The comment is always "my buddy bore sighted it for me, the gun/scope is a POS...........chris3

PS....I forgot to tell you to reinstall the bolt before trying to shoot the 25 yard target........chris3

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