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Impacts lower than expected for a 20 MOA base

B

bstaz0831

Private
Banned !
Minuteman
May 9, 2017
17
5
Hi, I've searched many places for this and haven't found anything. If the Simpsons did it, feel free to blast away. Secondly, I apologize, this is gonna be a long one.

Note: I've moved certain information to the bottom of the post to keep the main issue as clear and accessible as possible.

Citations to external content are marked [cite: n]
Calculations are marked [calc: n]
Processes are mark [proc: n]

Note: ~ (tilde) = approximately (not minus, or negative)

----------
SETUP
----------

Rifle: Ruger Precision Rifle (18008)
Caliber: 6.5 CM
Optic: Vortex Razor HD Gen II FFP (mrad) 4.5-27x56mm (EBR-2C)
Rings: Vortex precision matched rings 34mm (1.45" height)
Sight height above bore axis: 2.6" [cite: 1]

EDIT: (thank you @supercorndogs): 20 MOA base only mounts in one direction. 20 MOA engraving is at the rear of the action.
EDIT: (thank you bogeybrown n): Both rings are the same height


Settings:

LTEC Vertical Zero Stop: 0 mrad
Turret lock position: 0 mrad
Installation process [proc: 1]

--------------------
AMMUNITION
--------------------

Make: American Eagle
Projectile: 140gr OTM
MV: (nominal) 2700 FPS
BC (G1): 0.58
BC (G7 *converted using link [cite: 2]): 0.291
Drop @ 100yd: ~2.5" (~0.7 mrad)

Note: Using 3.6 in/mrad @ 100yd [cite: 3]

---------------
PROBLEM
---------------

Impacts are lower than line of sight at 100yd, despite having a 20 MOA base.

Explanation:

This is a problem for the following reasons
[a] Considering the rifle comes equipped with a 20 MOA base, I should expect to see a bore axis above the line of sight.
I want to make sure the products I purchased are in the proper condition while they are still new and I can contact a manufacturer for any assistance.

--------------
RESULTS
--------------

Expected:

Bore axis ~5.1 mrad above line of sight [calc: 1]
Impact ~4.4 mrad above line of sight [calc: 2]

Actual:

Bore axis ~7 mrad below line of sight (LTEC zero stops set to 0 (as if from factory) and locked in place at 0 on the turret, when bore sighting)
Impact I confirmed this with a shot 3 group (~0.3 in) that came in ~7.6 mrad low.

Note: After adjusting the zero to account for the 7.6 mrad difference, the vertical adjustment is cut to 19.5 mrad (29.5 if you count the holdover in the reticle). Sure, 19.5 still gets me out to ~1500-1550 yds (depending on zero), and 29.5 gets me out to ~1850 yds. So, I'm not too worried, but I want to figure out why this is happening.

----------------------
ASSUMPTIONS
----------------------

1) The line of sight runs parallel to the surface of the base when at 0 zero stop, and 0 turret elevation settings.
2) The bore axis is straight (i.e. - no barrel droop)

Note: ^ this is where the majority of my suspicion lies

---------------------
CONCLUSION
---------------------

Feel free to share your wisdom on why reality isn't matching up with math.

------------------------
CALCULATIONS
------------------------

[1] Expected vertex of bore axis with reticle considering assumption #1

a. 20 MOA = 0.333° (canting the line of sight forward toward the bore axis)

NOTE: Steps b-d account for the path of bore axis while below the line of sight. I used a triangle calculator to help [cite: 4]

b. leg A = 2.6 (sight height over bore)
c. angle a = 0.333°
c. tan(angle a) = leg A / leg B
d. leg B = leg A / angle a ~= 447.35 in

NOTE: ^ approximate distance from chamber where bore axis should cross line of sight
NOTE: Steps e-i account for the path of bore axis while above the line of sight. I used a triangle calculator to help [cite: 4]

e. 100 yd = 3600 in
f. leg B = 3600 in - 447.35 in = 3152.65 in
g. angle a = 0.333° (vertically opposite angles are equal [cite: 5])
h. tan(angle a) = leg A / leg B
i. leg A = 3152.65 in * tan(angle a) ~= 18.3 in (bore axis above line of sight @ 100 yds)

[2] Overall impact relative to the line of sight, dependent on bullet drop

a. 18.3 in - 2.5 in ~= 15.8 in
b. 15.8 in / 3.6 in ~= 4.4 mrad above line of sight

-------------------
PROCESSES
-------------------

[1] Optic installation
a. Clean/Inspect 20 MOA base and action mating surfaces
b. Install base to torque spec
c. Clean/Inspect rings lower halves base and action mating surfaces
d. Install ring bases to torque spec while pushing down and forward
e. Adjust for eye relief, then focus, then final eye relief, mark tube with pencil
f. Index Optic level to base level using 2 bubble level system.
g. Index Optic bubble level ring to base, and optic.
h. Confirm all bubbles index to same position as best as possible
i. install rings top halves with even gap, tighten to torque spec

-----------------
CITATIONS
----------------

[1] http://imgur.com/gallery/o3FmX
[2] http://www.jbmballistics.com/cgi-bin/jbmgf-5.1.cgi
[3] http://vortexoptics.com/uploads/web_...6-13a_mrad.pdf
[4] http://www.mathportal.org/calculator...calculator.php
[5] https://www.mathsisfun.com/geometry/...te-angles.html
 
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Boy thats the longest why don't I have as much adjustment as I think I should I have seen. Could be its a 900 dollar production factory rifle, not everything is straight. Other things to check, if its possible, is your 20 moa base backwards? Remove it and check for debris that may not be letting it sit flush.
 
supercorndogs

Thanks for trying to assist. You're right this was a big'n, hence the warning in the opening statement.

"is your 20 moa base backwards?"

Answer: No, its not possible, the mounting holes only line up in one orientation.

"Remove it and check for debris that may not be letting it sit flush"

Answer: See Processes 1

"[...] not everything is straight [...]"

Answer: See Assumptions 1,2 & Results Note

Thanks for raising those questions again, it helps validate my thought process. That being said, I had provided a technical question, and I'm not exactly looking to settle for just, "cuz, yer rifle is a cheap POS'. If that does happen to be the case, I'm willing to accept it. I care more about the 'why', than the actual symptom, as stated in my post.

If there is an issue with alignment in one or more components, a method to test and confirm such suspicions would be appreciated.
 
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Welcome to the Hide. I'm gonna guess you may have spent some time in school, and that it probably wasn't studying Art History.

I tried working through your data but still had too much blood in my coffee stream. Corndogs already hit on the possible reversed scope rail. Lemme see if I can translate your data into English and re-approach it.

As he said, that may be the most quantitative post of its kind that I've seen. We're not typically used to that level of info on a "Why is my shit not working" post.
 
bogeybrown

Thank you. I can understand I provided a lot of information. Thanks in advance, for taking your time to help. If you have any questions, please let me know.

And I'm a film school drop-out (ASU), by the way.
 
 
supercorndogs & bogeybrown posts got me thinking. What if the base was mounted incorrectly by the manufacturer?

20 MOA @ 100yds is approximately 20.92 in. Add LOS height (2.6 in), and bullet drop (2.5 in), and you get approximately 26.02 in. Divide by 3.6 to convert to mrad, and you get......

approximately 7.3 mrad

Digging around for my calipers. I'm going to yank the base and measure both ends. Also called Ruger and asked about the proper orientation of the base to the action, waiting on a response.
 
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bogeybrown

It's ok, I'm using a format we use in a software company for documenting problems. I had to learn the thought process somewhere, and it did take some time. So, you were justified in your assumption.

Correct I'm seeing a 11.4 mrad discrepancy between mathematical and bore axis, and an 11.9 mrad discrepancy between mathematical and impact

I did check to make sure I was in the correct revolution, twice. My initial thought was there, too.
 
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Working on the possibility that your starting assumptions may have been off, I went back and worked through your findings and assumptions from the very beginning and even checked them against an outside reference just to keep myself honest:

http://warnescopemounts.com/20moa-explained/

Thus far, I'm not finding a disconnect in your starting assumed values, at least not to any degree that would account for 12mrad. I keep coming back to a reversed rail or similar issue. However, you say that the RPR rail can only be mounted in one direction. To be sure we're on the same page, your rail should be sloped downward towards the muzzle, correct?
Barring a manufacturing/mounting issue from the factory, another highly unlikely possibility comes to mind: Vortex typically has great quality, but is there any chance you may have mismatched scope rings? If by some ridiculous chance you ended up with rings of different heights you could induce a cant. It would have to be one hell of a cant to overcome a properly sloped rail however.
 
The top one in the link is what i was talking about, if the scope and the scope base are good to go, then its the muzzle that is pointing down.

Montrose
 
bogeybrown

I heard back from Ruger, the end of the base with the 20 MOA engraving is supposed to be at the rear of the firearm, as it is on my rifle.

I also was told that the base should be ~1.215 at the front of the base, and ~1.238 in at the center of the base (no tolerances were available). What point on the rifle those measurements are in reference to, the tech on the phone had no clue. I am guessing the above are measurements from the bore centerline, as determined in main post citation 1.

Note: I forgot I had lent my calipers to my father, for pistol reloading over the weekend. I am waiting until I get those back, later today, to pull the base and measure front, center, rear heights.

As far as I can tell, the base slopes down in front, in the correct direction, but given the above measurements,

~1.238 in - ~1.215 in = 0.023 in slope over 1/2 the length of the base. So,

0.023 in * 2 = 0.046" in slope over the entire distance of the base.

I am not sure I can accurately judge that with eyes alone

As for rings, I mic'd the rings previously, and they are the same. I forgot to mention that.
 
Following MJB13SRT8 's idea, are you able to measure the barrel at its protrusion from the handguard to determine if there's an issue there?
 
tnichols said:
Trying to follow along, but this is making my head hurt...
Click to expand...

Thanks man, I almost bypassed it for the sheer effort of re-converting the analysis to plain speak. However, how many times has someone posted a similar issue with no usable information. The OP certainly did his part, I figured the least we could do is help, even if that's just scratching our heads along with him (which as you can see is mostly what we've been doing).

I consider you to be WAY smarter than me, so jump on in. I'll hold it if you'll hump it for a while.
 
You guys are awesome thus far. I'm not going to have access to calipers and time until this evening (MST). I will try to follow up before I hit the rack, but it's possible I won't post until tomorrow.

Thank you all for helping so far.
 
Do you have enough clicks left in the scope to bring POI to POA?
If so, your problem is solved. Beware the zero stop if there is one on your scope.
All this math stuff is wonderful but it doesn't shoot your rifle.
20 MOA worth of tilt should let any scope zero at 100 yards.
 
SubOptimal said:
Do you have enough clicks left in the scope to bring POI to POA?
If so, your problem is solved. Beware the zero stop if there is one on your scope.
All this math stuff is wonderful but it doesn't shoot your rifle.
20 MOA worth of tilt should let any scope zero at 100 yards.
Click to expand...

I know where you're coming from, but unless we're all missing something I think he has a reasonable concern. A 20 moa base should have him adjusting down to zero, and instead he's adjusting up. In this case he's adjusting by almost the perfect amount but in the wrong direction.

Im hoping a smart guy will pop in and say "you guys are all screwed up, HERE is your problem" but it hasn't happened yet.
 
I finished some stuff early and was able to grab a new set of calipers (just figured I'd let my pop keep the others).

I measured the bottom edge to the bottom of the picatinny at the left and right of the base at the front and rear. I repeated 3 times each, zero-ing the calipers between measurements

Right Front: avg 0.395 in
Right Rear: avg 0.441 in

0.441 in - 0.395 in = 0.046 in

Length of the base = avg 7.802 in

tan(angle a) = leg A / leg B = 0.046 / 7.802 ~= 0.3378°

It has to be the barrel, not sure how to go about testing that. The tech at Ruger forwarded the information we've worked on so far to Engineering. Now, the waiting game.
 
tnichols said:
Nah, you're doing fine ;).
Click to expand...

Loose barrel nut, I would remove the handgaurd and look at the barrel and nut. I am still here scratching my head. Always start with the easiest things to check. Do you have the tools to remove the barrel nut?

I don't know how I ended up quoting there.
 
There is a bolted on piece that stops the barrel nut from moving, so that shouldn't be the problem also unless you have headspace guages it's not a good idea to remove the barrel nut or unscrew the barrel.

Montrose
 
Maybe your muzzle velocity isn't what you think it is. What kind of scope is it? Are you at mechanical zero or just where the knob says zero?

Check your total travel and make sure you're starting in the middle, then coming down approximately 6 MILs for the base.

Are there strikes inside the brake?
 
bstaz0831 said:
Bore axis ~7 mrad below line of sight (LTEC zero stops set to 0 (as if from factory) and locked in place at 0 on the turret, when bore sighting)
Impact I confirmed this with a shot 3 group (~0.3 in) that came in ~7.6 mrad low.

Note: After adjusting the zero to account for the 7.6 mrad difference, the vertical adjustment is cut to 19.5 mrad (29.5 if you count the holdover in the reticle). Sure, 19.5 still gets me out to ~1500-1550 yds (depending on zero), and 29.5 gets me out to ~1850 yds. So, I'm not too worried, but I want to figure out why this is happening.
Click to expand...

A wise man cuts through the noise and imparts wisdom with a wave of the hand....

There is no problem here. You have 19 mils of travel above your zero, that's normal for a 20 MOA base. Go shoot.
 
tnichols said:
Trying to follow along, but this is making my head hurt...
Click to expand...

Holy math vs reality Batman!

Bottom line, the bullet never lies and the target will give you the answer. I mount my scope, bore sight, adjust my scope the necessary elevation/windage for my zero, shoot a few rounds to confirm my zero and I'm done. I've never looked at what the necessary corrections are in relation to the scope's true zero and the bore line, but that's me. Having a good zero is important and doing a tall target test to verify your scope's tracking is critical.
 
bogeybrown - thanks for defending what I (now, a few of us) are trying to accomplish.

supercorndogs - Thank you for considering an option "outside the box". MJB13SRT8 is correct in reference to the barrel nut.

MJB13SRT8 - I do not have go/no-go guages, nor the proper tools, nor desire to remove the barrel nut, or barrel, at this point. Yet. (not snark, honestly think this is a maintenance task beyond my skill level at the current time)

LawnMM - Thank you for sharing.

Muzzle velocity - see original post, drop properly correlates to velocity used, within a neglibile margin, for this matter.
Scope - see original post
Zero - see orignal post (yes, full mechanical zero)
Break strikes - None. The rifle is grouping 0.2 to 0.35 MOA @ 100 yds.

Sheldon N - This problem is like the bully that picks on other kids, but not you. Sure, you're fine ignoring it. But why the f*** does he feel the need to do that? And, what can I do f*** up his day, just so I know I don't have to worry about him in the future.

Sig Marine - The tracking of the optic is not an issue. At mechanical zero, the bore axis intersects with the reticle 7 mrad below the line of sight, and approximately 12 mrad below the mathematically calculated bore axis intersection given the current configuration and known measurements and variables. After adjusting mechanical zero to meet actual zero, the rifle performs fine. See my above response to Sheldon N why I care.
 
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HOUSEKEEPING UPDATE

I appreciate the village knowledge being shared, for my, and other's benefit, because of this thread. You guys are great, and I appreciate it immensely.

I fully expect something simple to get suggested, and for all of us to go "shit, how didn't that come up?". Because of this, every suggestion is worth considering.

If you want to help, thank you. But, please read my original post, and the thread, in full, before doing so.

My post was highly detailed to help answer common questions and issues that may be raised in an effort to keep this thread on topic, and to the point.
 
Let's start by addressing your stated problems:

This is a problem for the following reasons
[a] Considering the rifle comes equipped with a 20 MOA base, I should expect to see a bore axis above the line of sight.
I want to make sure the products I purchased are in the proper condition while they are still new and I can contact a manufacturer for any assistance.
Click to expand...


You have a RPR that shoots 0.3" groups. That's excellent, you have a very good example of the rifle. Congratulations, there is nothing wrong with the gun.
You have a Vortex scope. Lifetime warranty. Run it over with your truck, they will give you a free one.

I would call this problems solved, and the rest is just mental masturbation. So what you seem to be searching for is your missing mils.

The optical axis is the center of the scope, not the bottom zero stop. If you really want to find the optical center, put it in a set of v-blocks, spin the scope and dial the reticle until it stays centered no matter which way it spins. That's your optical center and line of sight.

So with that done on a zero MOA base you should have half your mil adjustment above and half below. Your scope has 28.5 mils of total adjustment. Half that would be 14.25 of vertical adjustment from center. Adding a 20 MOA base should buy you another 5.5 mils of adjustment. 14.25 Mils of adjustment from midline + 5.5 mils from the base = 19.75 mils of travel. You said you're getting 19.5 so you're not missing any mils. My Kahles on a 20 MOA base gives me roughly the same, as does my Bushnell on a different gun.

If you want to do lots of math and chase measurements, take up reloading with the goal of getting your standard deviation of velocity down to a perfect zero for a 10 shot string. It'll be fun, I promise. :)
 
Can I interject something here?
I'm going to anyway so here goes.
First a couple of definitions.
Line of sight - What you see through the scope, discounting mirage for a moment, it will be a straight line to the target. This pre-supposes that the optical center of the scope coincides with the crosshairs centered within the erectors.
Line of bore - What you see through the bore when looking through the barrel with the bolt removed.
Height of target vs height of barrel - At the rest, if both are equal, line of sight will be parallel to the line of bore and offset by the sight height.
Path of bullet - A parabola defined by change in velocity over the distance and drag on the bullet at that varying velocity.

So, if you fire a shot and the scope is parallel to the axis of the bore, the bullet should hit low by the sight height plus the drop of the bullet over the distance as defined by the path of the bullet above. Now you move your cross hairs in the scope to compensate for the bullet drop and the sight offset and you are at zero. What happened? Line of sight hasn't changed but the axis of the bore relative to the line of sight has. The bore is now pointing up such that the bullet now crosses the the line of sight path at the distance specified. In fact, it may have crossed LOS twice, once on the way up and again on the way down.

Now let us change nothing but the angle of the scope relative to the angle of the bore. The OP's situation. The scope is now pointing down towards the muzzle and introducing a shift upwards in POI relative to LOS because angle of bore to LOS is now different. By 20MOA in this case. This changes the points at which the path of the bullet will cross LOS. It might even be still traveling upward at 100 yards and this could be the cause of the discrepancy in his calculated vs actual results.

Then again, I could be full of shit.
 
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SubOptimal said:
Line of sight - What you see through the scope, discounting mirage for a moment, it will be a straight line to the target.
.
Click to expand...

Keep in mind that line of sight through the scope changes when you dial. Lock the gun in a vice and dial the turret up and the entire sight picture moves and your line of sight angle also changes. Line of sight is only parallel to the scope tube axis when you have it dialed for the optical center of the scope.
 
bstaz0831 said:
...
Sheldon N - This problem is like the bully that picks on other kids, but not you. Sure, you're fine ignoring it. But why the f*** does he feel the need to do that? And, what can I do f*** up his day, just so I know I don't have to worry about him in the future.

Sig Marine - The tracking of the optic is not an issue. At mechanical zero, the bore axis intersects with the reticle 7 mrad below the line of sight, and approximately 12 mrad below the mathematically calculated bore axis intersection given the current configuration and known measurements and variables. After adjusting mechanical zero to meet actual zero, the rifle performs fine. See my above response to Sheldon N why I care.
Click to expand...

This is a classic case of asking a question, but only wanting to hear answers that line up with what you already are predisposed to believe. Both Sheldon and SigMarine are correct.

I take it by your presentation and information following, that you've never shot a scoped rifle before and haven't anyone around that does know how that can point things out in real time on a range. This truly does not sound like you have a real issue. Go back and read what Sheldon, SigMarine and now SubOptimal have written. Then trust them for a bit and go out and shoot. Unless of course, you would prefer to write a thesis for us and we'll grade it and provide you with a glowing diploma at the end.

 
Sheldon N said:
Keep in mind that line of sight through the scope changes when you dial. Lock the gun in a vice and dial the turret up and the entire sight picture moves and your line of sight angle also changes. Line of sight is only parallel to the scope tube axis when you have it dialed for the optical center of the scope.
Click to expand...

Note: In deference to Sheldon's very accurate observation, I have edited my definition of LOS.
Line of sight - What you see through the scope, discounting mirage for a moment, it will be a straight line to the target. This pre-supposes that the optical center of the scope coincides with the crosshairs centered within the erectors.
 
Sheldon N Thanks. your post helped out a lot. Found the wrinkle because of your explanation.

TL;DR: You are correct, I have the proper adjustment.

I had originally called Vortex at the start of this and asked if the optic came from them at mechanical zero, they said yes.

After Sheldon's post, I checked the scope out further. The zero stop dial didn't match up (most likely my fault.) It is free floating and if lifted/bumped up while the set screws are undone, can rise above the zero stop dial and lose it's index.

I called Vortex again.

The scope has 28.5 (29, if you count the -0.5 mrad) mrad of upward adjustment in the turret from stop-to-stop. The actual adjustment in the zero stop is a nominal 33 mrad (actually 34.8 on my scope).

Optical Zero (as I understood it, could be completely wrong) would be, with the turret locked at 0, dialing the zero stop all the way up, then down 14.5. This would allow for 14.5 mrad of adjustment in the turret. Add the 20 MOA rail 5 mrad and theres the 19.5

Mechanical Zero would be dialing the zero stop all the way up, then down by half of the total adjustment. Nominally, they said 33 mrad and suggested dialing down 16.5 mrad. Actually it would be 17.4. The tech confirmed.

I followed the steps and am good to go. Everything matches up and Sheldon was right. Thanks for helping me understand this.
 
Glad that you got it worked out. I'm sure that it eases your mind and you can now be comfortable behind the scope.
 
I posted this same issue a bit before you but you posted it much more clear and concise. I attached a link for reference. I went over the entire setup with bubble levels and a set of starrett calipers and couldn't readily find the issue. For the time being I purchased a 20 MOA base as well for a total of 40 moa to at least get some elevation adjustment back (5 mils)

https://www.snipershide.com/shootin...fle-scopes/6396922-rpr-scope-mounting-trouble
 
I've never seen a RPR except in pictures. Is the Pictinny rail screwed onto the receiver? If so take it off and make sure that there is not crap between the reciever and the rail. Measure. Inspect the rail for being straight---put it on a grannit countertop and press on the corners and see if it wobbles. Take your calipers and measure thickness front and back.
 
bstaz0831 said:
Sheldon N - This problem is like the bully that picks on other kids, but not you. Sure, you're fine ignoring it. But why the f*** does he feel the need to do that? And, what can I do f*** up his day, just so I know I don't have to worry about him in the future.

Sig Marine - The tracking of the optic is not an issue. At mechanical zero, the bore axis intersects with the reticle 7 mrad below the line of sight, and approximately 12 mrad below the mathematically calculated bore axis intersection given the current configuration and known measurements and variables. After adjusting mechanical zero to meet actual zero, the rifle performs fine. See my above response to Sheldon N why I care.
Click to expand...

Bstaz0831,
First off I don't see anyone trying to "bully" you or "f*** up" your day. There are many shooters on this forum who, when asked, will offer their advice and opinions based on experience in the field. If YOU decide that their comments are "bullying" or trying to "fuck you up", so be it, we can't change how you perceive things. Glad you found the answers you were looking for and can move forward.

As as for my post where I mentioned tracking, you don't know if you have a tracking issue unless you've tested for it. Getting your zero and tracking are a separate process. My point was that your zero is important because all of your elevations will be based on that point of reference and your tracking is important because it relates to how the reticle moves/tracks incrementally and vertically/horizontally in relation to your zero and any error will influence you more as your shooting distance increases.

Best of luck with your shooting and reaching your goals...
 
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