Did a scope test. Results are bugging me.

F

frey

Member
Testing 3 scopes on my Turbo sporter.
Two 6x42s with different size dots and a VXII 6-18. All tested at 6X.
New Kelbly bases to replace the Turbo bases and match the Kelbly rings.

Each scope was optically centered before installing on the rifle.
I rotate the scope in a V-block setup, adjusting the windage/elevation
until the dot stays perfectly centered during rotation.

Each scope/rings were set-up individually using a Sinclair alignment tool.
It’s their alignment tool with the 2 bushings and the push through steel rods.
The Kelbly rings aligned absolutely perfect on the Kelbly bases.
I was able to push the steel rod through one bushing perfectly into the other.

The pic above shows the first 3 shots with each scope.
My POA was the upper right hand corner of the green square.
After zeroing to the group, I went to a fresh target to finish the adjustment.
Cleaned the rifle, changed the scope, and repeated.

The results were not what I was expecting.
Although I was impressed with the consistency from scope to scope,
I fully expected the unzeroed shots to land in the 4x4 square.
I definitely did not expect to see that much drop.

Were my expectations wrong?
Should I have expected that much drop?
 
Last edited:
Jeff,

You would expect approximately 4” of drop due to gravity and that would put your POI on the bottom of the aiming square you used. That plus the other obvious sources of error and barrel sag due to gravity, might give you as much as another inch or two of vertical.

The biggest source of error could be the orientation of the bore curve. Bore curves probably averages in the 2 ½” range of dispersion at 50 yds, but they can and do often exhibit much more than that. If it is above average and the barrel’s bore curve happens to be indexed close to the 6 O’clock position…..what you’re seeing wouldn’t be unusual and it could be even worse.

I did all this quickly and off the top of my head, so use your salt shaker. LOL

Landy
 
FredB said:
What was the range and were these shots from a clean, cold barrel?
Click to expand...
50 yards. I cleaned normally between each scope.
Cold? Maybe. Cool might be a better description.
I tried to replicate things as much as possible without getting too wrapped up in it.

HuskerP7M8 said:
Jeff,

You would expect approximately 4” of drop due to gravity and that would put your POI on the bottom of the aiming square you used. That plus the other obvious sources of error and barrel sag due to gravity, might give you as much as another inch or two of vertical.

The biggest source of error could be the orientation of the bore curve. Bore curves probably averages in the 2 ½” range of dispersion at 50 yds, but they can and do often exhibit much more than that. If it is above average and the barrel’s bore curve happens to be indexed close to the 6 O’clock position…..what you’re seeing wouldn’t be unusual and it could be even worse.

I did all this quickly and off the top of my head, so use your salt shaker. LOL

Landy
Click to expand...

I've been trying to do the math too.
I figured I was already starting 1.5"ish above the bore anyway.
I figured some droop due to the reverse profile with the knot on the end, but had no idea how much.
Also figured some ballistic/gravity drop, but couldn't see it being that much at 50.
4" is more than I would have guessed, but I have no clue.
And I also ended up kind of thinking maybe I have this 6:30-7:00 orientation going that's also contributing.
With your numbers, I can see it getting to the 7.5 - 8" pretty easy.
 
If I understand this correctly your swaps were pulled out of the same rings? If so this is easily within the margin of error of dis/remounting an optic on a gun.

Last on a centerfire BR gun I thought I might have had a loose rear ring and durring a winter league match just moderately tightened one ring without touching anything else and had POI shift 1 1/2". I woul'nt lose any sleep over it.
 
3 scopes, 3 sets of rings, all setup individually on the workbench at home before going to the range.
The scopes and rings are mated sets now.

This test was a few weeks ago.
I went out yesterday and remounted all 3 scopes to see if they held zero.
Other than what I would consider normal zeroing for that day,
all 3 were right where I had left them a couple weeks earlier.

Kind of interesting how I started out just wanting to be able to test different
reticle configurations side by side during the same shooting session.

Never figured I'd see something curious at the same time.
 
I wonder if it has something to do with your "optical" centering? When I put a new scope I mechanical center and test fire, if the poi is off too much i know i got mounting issues. thanks, douglas
 
Douglas said:
I wonder if it has something to do with your "optical" centering? When I put a new scope I mechanical center and test fire, if the poi is off too much i know i got mounting issues. thanks, douglas
Click to expand...

Maybe. I aint no gunsmith.
But, I would expect the mechanical center and optical center to be darn close in a BR quality scope.

Before I started spinning the scopes, I used to count clicks to get to the exact center one.
After counting and spinning the first couple, I quit counting and started ballparking the
mechanical center as a starting point.

These 3 aren't the first time I've mounted a scope this way.
Just the first time doing 3 at once and paying attention to the results.
 
I can say this, Husker and Frey are close to what I believe to be the issue. If you have a gun with a removable tuner, no matter the scope adjustment, remove the tuner and watch the groups be much higher that POA. Or for that matter, add more weight to the tuner without removing it and see how much lower the groups are to POA. I would say that optically centering the dot and installiing it on the rifle is a pretty cool idea...but you might actually check the bases for variation to bore it might surprise you. I know of a gunsmith that surface grinds the bases so the the scope can actually be optically centered (in a different way, I suppose).

Carp
 
frey said:
3 scopes, 3 sets of rings, all setup individually on the workbench at home before going to the range.
The scopes and rings are mated sets now.

This test was a few weeks ago.
I went out yesterday and remounted all 3 scopes to see if they held zero.
Other than what I would consider normal zeroing for that day,
all 3 were right where I had left them a couple weeks earlier.

Kind of interesting how I started out just wanting to be able to test different
reticle configurations side by side during the same shooting session.

Never figured I'd see something curious at the same time.
Click to expand...

OK, got it. That does seem a lot, the next thing I'd check is that something is under stress with bases.
 
I have indicated some scopes for forward and rear tube alignment. Its rare to find them real true. When the rings are tightened
on the bases, it will tend to straighten the tubes. So, what you had as an optical center gets lost. Had these rings been bedded
they migh not have shown as much error.
 
Jeff this is a fascinating post.What I would love to see is someone come up with a laser device that can be inserted into the chamber and would print on the target exactly what a straight line trajectory would be then shoot a round and see exactly where it falls in relation to the straight line.Is it a 4 or 5 inch drop does it have any corkscrew effect i.e. does it move sideways at all?If someone had such a device they could log the relation then use it to get to paper without wasting so much of our expensive ammo after changing the scope.I may have to work on something like that.I knew the round dropped from straight line some but I didnt have any idea it was in the 8 inch range at all.Great post.
 
Bob Kingsbury said:
I have indicated some scopes for forward and rear tube alignment. Its rare to find them real true. When the rings are tightened
on the bases, it will tend to straighten the tubes. So, what you had as an optical center gets lost. Had these rings been bedded
they migh not have shown as much error.
Click to expand...

Good point, at the very least I'd lapp them.
 
I'm glad someone mentioned bedding.
Good reason to start a new thread.

I also changed bases on my other Turbo.
When I get a chance, I'm gonna try this experiment again with a couple 36x scopes.
 
Mike: Any of the big sporting goods outfits have a wide range of laser devices that fit into the barrel. They work quite well, but I find it just as easy to bore sight as the laser beam can be hard to see at 50 yards in sunlight. If one puts the scope crosshairs on the laser dot you will be several inches off vertical and how much horizontal depends on how well the laser fits into the muzzle. They also make breech lasers, built into a cartridge, but only for center fire that I have seen. Sorry I did not take notes last time I used it as to how much drop. The one I have is a real el cheapo. I use it mostly to set up my chrony. I put it in the muzzle, turn on the laser, put my hand in the light path and set the chrony for it to be in the middle of the screen. bob
 
I’m not surprised at your results. I used physics, the JBM Ballistics Web site, and geometry to calculate a theoretical displacement of the point of impact.

Theoretical Results: 7.56 inches below the point of aim
Your Results: 7.28 inches below the point of aim

Theoretical Calculations:

Assumptions:

1. Time of bullet flight 0.149 seconds
Source: JBM Ballistics Web site, used their ballistic coefficient values for Eley Tenex Ultimate EPS; inserted muzzle velocity of 1,050 feet/second
Link to JBM Ballistics Web site: http://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi


2. Vertical displacement for gravity, bullet leaves muzzle dead level: 4.26 inches downward
Source: physics formula s = 0.5 X g X t X t, where g is the acceleration from gravity at 32 feet/second squared, and t = time of flight @ .0149 seconds. s = vertical distance of a falling object.
JBM calculates this drop at 4.1 inches.

3. Your barrel is dead level, your scope is dead level, the cross hairs are exactly centered, and the scope is 1.50 inches above the bore line, assuming no barrel sag due to gravity.
Result: downward vertical displacement of 1.50 inches.

4. Your barrel is dead level, but sags 0.001 inch at the muzzle
Geometry: 50 yards = 1,800 inches. If the barrel sags 0.001 inches at the muzzle
Result: downward vertical displacement of 1.80 inches

Adding together the assumptions above (4.26 + 1.50 + 1.80 = 7.56 inches)


Your Results:

I used your ruler and averaged the vertical displacement for the nine bullet holes. I note that your ruler zero point is approximately ¼ inch above your point of aim, so I deducted 0.250 inches from that average. That gave me 7.28 inches of downward vertical displacement.
 
Thanks astronomer!!
It's good to see the numbers work out like that.

I did the test again last weekend with two 36x scopes and not surprisingly saw very similar results.

I'm still on a trek to get the scope aligned to it's optical center. At least from a vertical perspective.
This page from Dan Lilja's site is interesting.
http://www.riflebarrels.com/articles/50calibre/scope_bases.htm

According to the formula on his page, I believe I need (15/3600) X 4.1 = .017" elevation from the
front of the front base to the back of the back base to pull the scope center down 7.5" at 50 yards.

I have access to a milling machine and may give this a shot this winter some time.
 
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