How does shimming a scope affect bullet trajectory?

[pacrat]

I have a scope that sits 3 inches above the bore of the rifle. There is not enough downward elevation on the scope to zero the scope - it is shooting several inches too high. I have never done this but in theory if I shim or use adjustable elevation scope mounts so that the front of the scope is higher than the rear, I will be able to lower the point of aim/impact for the bullet and zero the scope, correct? So, how would that affect the trajectory of the bullet? How do I determine the bullet drop at various ranges for target shooting? Can anyone link me to a web based trajectory calculator that allows me to input the amount of shim and then calculates the trajectory? Thanks for any help!

What range are you trying to zero at? Important question, and answer is very relevant to a proper answer.

"TRAJECTORY", is a parabolic curve. LINE OF SIGHT, [LOS] is STRAIGHT.

A bullets path aka, "trajectory" crosses the LOS twice in flight. Because the rifle barrel is pointed slightly upward in relation to the LOS. The higher the scope CL is over the bore. The farther out the first crossing of LOS is.

With scope CL, located 3" over the bore. "IF" you are trying to ZERO up close. Like 25 yds. You have to reverse the turret adjustments. UP becomes DOWN.

Have you bore sighted the rifle? In the thread that kcheung2 linked to. Sounds like the scope is faulty. If so, I doubt you got it fixed in 4 days.

ETA ..........Youtube vid. https://www.youtube.com/watch?v=rvE0aJVVVXU

[ar15barrels]

Let's assume that there really is no more downward elevation adjustment since I've already had two gunsmiths take a look at my setup and they said there was no more downward elevation adjustment.

But does anyone know how angling the scope might affect the bullet trajectory?
I use the Ballistic Calculator by Gundata.org but I don't know it this calculator can factor in an angled scope, or if that needs to be done.
Thanks again for all replies and any help!

Angling the scope has no effect on trajectory because you will adjust the erector to the necessary zero angle so that your bullets land where you want them to.

You only need to angle the scope when you don't have enough erector travel to get a proper zero.
Once you can get the erector to zero, any additional angle is just taking away potential elevation travel.
It makes no difference to the bullet flight what angle the scope is at because you are simply aiming the bullet before you send it on it's way to the target.

A good ballistic program will tell you the departure angle.
The departure angle is the difference between the bore centerline and the erector centerline.

[subscriber]

But does anyone know how angling the scope might affect the bullet trajectory?

Angling the scope is the same as angling the internals, if they had the travel to zero the scope.

Obviously something in your setup is pointing the barrel up, relative to the line of sight on the scope. Undoing some of that will help.

You question is a bit like asking if you turn a car's steering wheel upside down on its splines, will the car corner differently. The answer is no.

You need to be careful with shimming a scope in the sense that you might be applying bending loads to the scope tube. If the scope is mounted such that there is no tube distortion, then pointing the tube in a direction that causes the bullets to land at your point of aim, at your chosen distance, can never be bad. It is actually good if you can zero your scope while it is close to "mechanical zero". Good for field of view; compared to cranking in max elevation. This is why 20 and 30 MOA mounts exist. So you can shoot long range without cranking in a lot of elevation.

The same reason why cranking in max elevation is undesirable, makes cranking in a lot of drop undesirable.

[ar15barrels]

I think that discussing the exact type of rifle would be a distraction from my original question.

Having the whole picture of what's going on is never a distraction.
A distraction is when someone asks a technical question and leaves out or flat out refuses to give relevant details.
Important details like the type of gun, scope and mount are absolutely relevant.

[edgerly779]

Randall correct. Shimming front of scope up lowers point of impact. OP is being obtuse or paranoid. Gives us info and someone can help with issues. I have shimmed scopes before with aluminum foil under scope rings on top of mount.

[Jimi Jah]

Had a similar issue with my Henry Golden Boy and Leupold scope. I could not get elevation up enough to hit 100 yards. The scope was maxed out. Henry sold a rather wanky scope mount.

It was replaced with a newer version with rails on top. That mount corrected the issue.

[Starslinger]

Yes, the rifle is an AK. Yes, I know that a red dot would be more appropriate than a scope. Yes, I have adjusted the controls on the scope back and forth as best I can and with professional help. I think I will try an adjustable mount and experiment with it. Thanks everyone!

[edgerly779]

Just take scope off mounts on receiver & put couple layers of aluminum foil under front ring see how it affects point of impact. Does your scope have weaver type rails same as picatinny?

[audiophil2]

That setup should not require any special mounts.
It is either defective scope/parts or user error.

Before buying a mount I suggest buying a laser bore zero.
Take everything apart.
Find dead center of scope by spinning elevation turret all the way in one direction and then count each click as you spin the opposite direction of total travel. Divide by 2 and spin back those clicks. (ie, 80 total clicks, spin back to 40 clicks) repeat with other turret so both elevation and windage are at dead center.
Install mount.
Lock AK into a lead sled or vice and get everything plum. Gun plumb to earth, pic rail plum to gun.
Install bottom rings to mount a little loose and pushed against the backs of the slots.
Set scope on rings and get your proper eye relief.
Make sure rings are located in correct area of that particular scope.
Adjust ring positions and scope until everything is per manual and eye relief.
Torque rings to rail.
Use laser bore zero and aim somewhere at least 25 yards. If you can't get that then try as far out as possible. I use my back yard brick wall as the bricks are already level so I can get my windage reticle to match the bricks.
Plumb scope to mount.
Torque upper rings to scope once plumb is verified with everything.
With scope locked in place and plumbed properly adjust reticle until center is on laser dot.
That will get you very close to a 25yd zero in your house.
Go to range and put zero grid target at 10 yard line. Use laser to check point of aim. If it is within 3 inches of bullseye move target to 25 yd line.
Use laser to check point of aim. If it is within 10 inches of bullseyes you can now adjust turrets. While looking through scope and not moving rifle (should be locked in sled) spin each turret until the reticle moves over to the laser dot. Now you are very close to any zero from 10-100 yards.
Remove laser
Fire 1 shot at 25 yds
Spin turrets until reticle is over hole in target. Fire 1 shot to confirm. If it is close move aim to other area of target. Fire 3 shot group. If they are within 8 moa (2inch circle at 25 yards) you are pretty much zeroed for an AK.
You can zero at 25, 50, 100 yds/m or whatever you like.
Remove turret caps and set both to zero. Screw back down.
You can go further now and count the clicks again. This way you know exactly where to go if you start spinning durinbg shooting and forget your count. You will always know zero is at 44 click from bottom and 38 clicks from left.

I can usually zero my guns with less than 5 shots doing it this way. Typically I can zero with 2-3 shots from a bolt gun or 3-5 with a semi auto and red dot. It takes me about 5 minutes to zero at the range but I spend about an hour at home setting up the scope and gun.

[audiophil2]

You will always have a hard time with an AK because they are not accurate, they are semi auto, and x39 ammo varies a lot in charge. A typical AK is about 3-4moa once you factor in ammo. Good ammo can get down to 1moa with a good gun but that is asking a lot. The scope is not your friend because it just magnifies the lack of accuracy. The gun is made to hit a torso at 25 yards so 40moa is acceptable. That is why a red dot is so much better. You can be sloppy with a red dot and it will work. I shoot Aks out to 500 yards with dots and 200 with the irons. But I only try to shoot 10" plates. I'm not trying for tight groups.

[hermosabeach]

OP

Snap some pics

Go to imgbb and upload the pics
Click edit and set the later side to 1,000
(Most camera pics could be 3,000-6,000 and too big)

This will make the image a good size to see

Post the links here


Www.imgbb.com. Free and no account needed.

[hermosabeach]

Some AK's are build better than others.

If the side rail is crooked, it can make any side mount way off

Rings should be parallel
So shimming inside the bottom of one ring and the top of another is bad.

So post some pics and maybe we can offer accurate or relevant info...

[subscriber]

Starslinger,

Nobody has spelled this out, so here goes:

A high sight mount helps with a wide "point blank" aiming ability at 200 to 300+ yards, but messes with short range point blank aiming (25 to 100 yards).

If you want to aim fast at targets between 10 and 100 yards, then open sights or a low mounted dot are actually better because the line of sight and bullet trajectory are closer to parallel. Yes, the trajectory is curved, but not as much, short of 100 yards as it is beyond 300 yards.

As the AK is not really a long range rifle, optimizing for fast shooting between 25 and 100 yards might have more value.

A 3" tall sigh mount, zeroed at 25 yards will shoot very high at 50, 100 and 150 yards. It will only come back near zero at 300 or 400 yards. If you type in known or estimated info this ballistic calculator and change sight height from one inch to three inches, it will show you how that affects near and far zero (and drop compensation at other ranges). Thus, "point blank" aiming ability over a range of ranges (just aiming center mass without range compensation): https://www.hornady.com/team-hornady...alculators/#!/

[pohorsky]

You could also look at the ultimak products for the AK. I have one of their mounts for the mini 14.

[kenl]

Did you check to make sure your scope and mount are ok, like several people mentioned on your thread in optics? Before throwing good money after after bad, you need to make sure everything is working correctly. A laser bore sight would help you a lot.

If you're in the Sacramento area, maybe we can meet at the Sac Valley range near Ione to see what's going on.