I've sighted my .270 to be 2.5" high at 100 yds. for a 250 yard zero. When I shoot at 175 yds. (the longest available at my range) the rifle shoots right about point of aim. It should still be over 2.5" high at that range. It does this consistently. Mounts are tight. Changed scopes with no differences noted. Does anyone know what's going on with it?

Thanks,
Telly

Not sure, but a lot of trajectory tables assume the scope is mounted a certain distance above the center of the bore. If the scope is mounted higher/lower (1.5" is typically assumed, I believe), it would make a difference on trajectory.

-Lou

I calculated the line of sight at about 1.6" above the bore and re-checked the traj. table with one of the external ballistics program. I am inclined to think something is not straight, but what?

telly, are your loads chrony'd or factory loads. they lie.

Loads are 130 gr. Hornadys and 60.0 H-4831 chronographed at 3,021 in my rifle.

That is the very reason that you should test your loads at all ranges that you intend to shoot.Trajectory tables are often wrong even with correct ballistic co-efficients and muzzle velocities; and muzzle velocities can vary somewhat with varying conditions.Believe what you see over what you read.People that are foolish enough to blindly believe trajectory tables without actually testing their loads miss a lot of shots at longer ranges.

[ 10-01-2003, 06:56: Message edited by: stubblejumper ]

What's the bullet?
Are you using factory calculated BC? Because that's pretty much academic without knowing the altitude, relative humidity, temperature, etc., in a given shooting situation.
Tell us your bullet/speed and maybe someone can proof your calculations.

very likely that something is out of kilter. is your chronograph reading high? notlikely, since the 60 grain H4831 load in my sons rifle with 22 inch barrel runs just below 3K.

Is your bullet a bluntfaced fatnosed antiballistic bullet? they slow down quick.

Finally, what makes you think that 2.5 inches high at 100 yards makes your POI be 2.5 high at 175? I would think it be lower. but not much, go to the NORMA page and replay your numbers.

but anyway, if its tight, who cares? it is spot on for hunting accuracy.\
Jameister

Telly,

I ran the numbers you posted with Sierra's Infinity ballistics program. With that bullet and velocity I too get the trajectory you are looking for....250 yard zero with 2.5 high at 100yards. With the scope witting at 1.25 and 1.75, 1/2 inch diference it only makes approx 5/16 difference at 100 yards so at 250 yards even if you are off by half inch in scope height it will only be around 3/4 off. Certainly within human shooting error and a hunting rifle, and still a clean kill.

IF you can duplicate this again, ie moving out to 175 and in to 100, then back out to 175. Be sure it is grouping and not moving due to heated up barrel.

I ran the numbers, and got the same thing as well.
In fact, I could find no way to make the thing 2.5" high at 100yd and anywhere near zero at 175yd.
With a 1.6" scope height, and a 130g .277" bullet going anywhere near 3000fps, in order for it to be zero at 175 and +2.5" at 100yd, you'd need a BC of about .078!!!
That would be something like a magnetic wadcutter over an iron field.
Have you checked for that?

I always try to de-magnetize my wadcutters just in case, but may have missed this bunch!! If the scope is not parallel to the bore line would that cause a problem? My scope base is one piece and the rings are Leupold old style with the windage screws on the rear one. Just wondering if that could be a cause. I was going to use the rifle on a late antelope hunt, but now I don't think so.

Adjust your scope to hit point of aim at your desired zero of 250 , then try again at 100 and see what happens .

By the way your scope is never parrallel to the bore , if you sight in high at 100 . The bore is tilted up slightly in relation to the line of sight .

[ 10-01-2003, 17:09: Message edited by: sdgunslinger ]

SD,

While that's true, I just wonder if it could be off enough to cause a big change in point of impact over distance. I can't remember how much I had to move the cross hairs of the scope to get it to hit 2.5" high at 100.

SD gave good advice.
Sight your rifle in at 250 on the target.
Then drop back at 100 yards and check to see where you are at.
It has to be about 2.5 high, no way it can be anything else. It can't be a problem with the gun.
2.5" high at 100 is 2.5" high at 100.
For you to have a sight in 2.5" high at 100 and then dead on at 175 you would have to be shooting extremely velocity challenged ammunition and you are not.
Either your sight in at 100 which you thought was set at 2.5 or your dead on at 175 would have to be wrong if your velocity on your chronograph is right.
Did you make any scope adjustments at all after you set your zero at 2.5" high before you shot your 175 yard group?

Telly,
I just re-read your post. The last line said you tried it with another scope and got the same thing.
Are you saying that you put a different scope on and zeroed it 2.5 high at 100 and then it was at 0 at 75 again.

rukidnme,

Yes I put another scope on and got the same results, sighted in at 100 and shot again at 175 w/o making any changes. Rechecked for tight screws etc. and nothing out of ordinary. I would shoot at 250 if only I had a place to do that. Thousands of acres of farm ground about and not a place to shoot. I am going to put new bases/rings on this weekend and try the drill again.

Telly

Telly.......if you are getting consistent accurate groups at both ranges I really doubt there is anything wrong with your scopes or mounts , and changing out rings and bases is not going to do you a bit of good.......there is one possiblity .......parallex could be present at the longer range.......many scopes are set to be parallex free at 100 yards.

Try setting your scope to hit 2.5 high at 175 , since that is the longest range you can shoot at , then check again at 100 .

Set your final zero holding on to the forened as you would normally shoot . Letting the forend jump free off the bags can throw the shot off several inches at long range ( compared to a normal grip in the field) with a light sporting rifle.....if you are going to be using a bipod , set your final zero shooting off the bipod......

If you end up hitting dead on at around 175 to 200 don't fret too much . Way more 'lopes are propbably killed under 250 than over , and you can still a good hit out to 300 with a high hold on the hair.......

Not sure what the poblem is or if there is a problem at all.
But it does prove the rule that you should be familiar with your load and rifle at ALL ranges even those you do not "plan" on shooting game at.

I used to put weight in those trajectory tables, but more times than not they were off.
Nothing counts like experience.

Good luck

David

The only problem is that you are expecting the trajectory tables to be correct which is often not the case.Believe what you see for yourself and don't worry about the tables.All that really matters is that you know where your bullets will strike.

Try a different 130 gr. bullet. Perhaps you have a soft batch of hornadys that are deforming oddly just by air resitance - I've heard stories of reputed bullets starting to break up at otherwise reasonable velocities.

Just a thought...

9.3

[ 10-02-2003, 07:48: Message edited by: 9.3x62 ]

Stubblejumper, while I agree in principle that you must verify your trajectory, the difference here is a big red flag. Sounds like something is going on with that bullet after it leaves the case and before it hits the target.

If the bullets were deforming in the air the groups would be very large and inconsistant.If your groups are consistantly small the bullets are not coming apart in the air.

In general any bullet like that at that velocity will shoot flatter than what this rig does.

What are the group sizes like? What Hornady 130 gr bullet are you using? Do you know the bullet run out? Are the ranges measrured carefully? What is you load?

High bullet runout will affect a bullets air resistance. So will a twist that is too fast or slow.

Check your data with "Pointblank" at www.huntingnut.com The bullet coefficients are at the Hornady web site.

Most likely this is a measuring error.

Telly,
I still believe it is physically impossible for a pointed .270 bullet at 3000fps to be 2.5" high at 100 and 0 at 175.

check your action screws.
Rings and bases are probably not the problem.
try the same thing with factory ammo, just to rule out your reloads as the problem.
make sure you are using the same exact dead solid rest for all shots.
do not put pressure on the rifle anywhere when shooting especially the foreend.
if your rifle barrel is not freefloated that can be a real problem.
Do Not shoot off your truck hood or tailgate.
let your rifle cool between groups.
shoot all your groups with your scope on its highest power.
shoot all your groups from a fouled barrel and do not clean it till you are done shooting.

If you follow the actions listed above and it still does it then call the Guinness people because your gun is shooting sliders.
Good luck, I know how frustrating problems like this can be.

rukidnme,

Thanks for the advice. I pretty well do most all those things now, but may have a fly in the ointment someplace. I will review my procedures.