Trouble Shooting Question -- Need Help With the Math

[HumbleEinstein]

Can anyone tell me what .1" of horizontal muzzle alignment will translate to in MILS 100 yards down range? Thank you.

 

[otisray]

If my math is correct, looks to be about 16" or roughly 4.5 mils

 

[HumbleEinstein]

Basically, I took a new Rem 700 SPS AAC-SD .308 20" in an XLR chassis out. It has a 20 moa base, seekins rings, and a swfa 5-20. I zeroed it and when I got home I noticed that I only have about 3.5 MILs of windage remaining to the left and over 10 to the right. I think I should have at least 5 in each direction for long range, right? (I have since swapped out for a lower versions of the seekins rings.)

In addition, the 20" barrel looks centered to the eye in the handguard. However, my caliper measured that it is .1" to the left at the end of the 14" handguard. That could be the aluminum of the handguard or the threading of the barrel in the action. I'm not sure if .1" is within XLR tolerances. If it is the barrel itself then it will be slightly more off at 20" than 14". The numbers above are not encouraging if this is the problem.

I suppose it could also be the tapping of the screws in the top of the receiver, the XLR/talley rail, the rings, or the scope itself.

I'm hoping the problem is just as simple as how I had mounted the scope. I know it was level and torqued to spec. Could the order I torqued the screws possibly have been applying more pressure to one side than the other and pushed it slightly off center?

Looking at the scope mounted in the new rings now, it appears to be centered, but I couldn't find any objective way to measure because of the angles.

I'm new to this and just trying to figure it out. The frustrating thing is I don't get to the range very often and trouble shooting it could take many months at my rate.

 

[otisray]

Here's my early morning math. Figure a 22" barrel, 6" action, and your eye is 2" back from the end of the action. In radians, your looking for sin(.1/(2+6+22)) which is .0033333. X1000 to convert to milliradians and you 3.33 mils.

Wouldn't that be tangent (opposite over adjacent) instead of sine(opposite over hypotenuse)?

 

[KYpatriot]

Basically, I took a new Rem 700 SPS AAC-SD .308 20" in an XLR chassis out. It has a 20 moa base, seekins rings, and a swfa 5-20. I zeroed it and when I got home I noticed that I only have about 3.5 MILs of windage remaining to the left and over 10 to the right. I think I should have at least 5 in each direction for long range, right? (I have since swapped out for a lower versions of the seekins rings.)

In addition, the 20" barrel looks centered to the eye in the handguard. However, my caliper measured that it is .1" to the left at the end of the 14" handguard. That could be the aluminum of the handguard or the threading of the barrel in the action. I'm not sure if .1" is within XLR tolerances. If it is the barrel itself then it will be slightly more off at 20" than 14". The numbers above are not encouraging if this is the problem.

I suppose it could also be the tapping of the screws in the top of the receiver, the XLR/talley rail, the rings, or the scope itself.

I'm hoping the problem is just as simple as how I had mounted the scope. I know it was level and torqued to spec. Could the order I torqued the screws possibly have been applying more pressure to one side than the other and pushed it slightly off center?

Looking at the scope mounted in the new rings now, it appears to be centered, but I couldn't find any objective way to measure because of the angles.

I'm new to this and just trying to figure it out. The frustrating thing is I don't get to the range very often and trouble shooting it could take many months at my rate.

First, the handguard unless it is touching the barrel/not free floating is a non issue. You aren't aiming with the handguard so the .1 offset is of no consequence at all. What matters is aligning the bore centerline with the optical center of the scope. That alignment is controlled by the location of the base mounting screws in the action, and the straightness of the base itself. It is unlikely but is possible to bend a scope with the rings but I doubt you did this. However, try tightening the ring screws in a different order, you want to do this like you do lug nuts on a wheel.

If you really want to fix it right, have the base mount holes enlarged to 8-40 screws, but the smith needs to be careful, because a tap will just enlarge the hole without moving the center...they really need to move the center of one of the holes to truly fix it. You are not off very much so it is up to you whether this is worth it.

Other options....you can buy windage adjustable rings, they are fairly common. You can also shim the inside of a ring using a piece of aluminum from a coke can, or sacrifice a feeler gauge.

Frankly, considering you can still zero with some good adjustment left, you may just want to shoot it and not worry about it. I dont know anyone who dials windage for wind, they hold it using the reticle.

 

[HumbleEinstein]

First, the handguard unless it is touching the barrel/not free floating is a non issue. You aren't aiming with the handguard so the .1 offset is of no consequence at all. What matters is aligning the bore centerline with the optical center of the scope. That alignment is controlled by the location of the base mounting screws in the action, and the straightness of the base itself. It is unlikely but is possible to bend a scope with the rings but I doubt you did this. However, try tightening the ring screws in a different order, you want to do this like you do lug nuts on a wheel.

If you really want to fix it right, have the base mount holes enlarged to 8-40 screws, but the smith needs to be careful, because a tap will just enlarge the hole without moving the center...they really need to move the center of one of the holes to truly fix it. You are not off very much so it is up to you whether this is worth it.

Other options....you can buy windage adjustable rings, they are fairly common. You can also shim the inside of a ring using a piece of aluminum from a coke can, or sacrifice a feeler gauge.

Frankly, considering you can still zero with some good adjustment left, you may just want to shoot it and not worry about it. I dont know anyone who dials windage for wind, they hold it using the reticle.

I'm with you on the handguard. I was just using it as a reference point to determine if the bore is not properly aligned with the receiver. I did change the rings since I did the zero that is in question. Hopefully, that will solve the problem and I can blame it on a poor job with my original mounting. Thanks for the advice.

 

[HumbleEinstein]

I found out what the problem is. The scope base is twisting. I'm going to have to get it bedded.

 

[HumbleEinstein]

Problem solved! I bedded the scope rail. I just finished mounting the scope and bore sighted it in. The windage ended up dead center. Thanks for the help.