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Education / Training ANOTHER problem with scopes: Not mounting them correctly

ANOTHER problem with scopes: Not mounting them correctly

by B.B. Pelletier

This one comes up a lot, so I thought I’d mention it in case any of you are experiencing it: Your gun shoots to one side at close range and to the opposite side at distance. I’m going to explain the problems and how to deal with them, then I’ll give you all the references to past postings where I’ve explained other pertinent things.

The scope is not aligned right
There are two things that will cause this problem, as far as I know. One is if the vertical reticle is not truly vertical but actually on a slight slant. At close distances, defined as those distances before the pellet first intersects the crosshairs, the scope shoots off to one side. For me, it’s always off to the right – the closer I shoot, the more it’s off. So, at 10 yards it will be off half an inch to the right; at 17 yards it’s off a pellet diameter and still to the right.

At far distances, which are those beyond the second point of pellet/crosshair intersection, the pellet strays to the other side of the vertical reticle and keeps diverging as the distance increases.

My sight-in distance is always 20 to 30 or 35 yards, depending on the velocity of the gun I’m shooting. If it’s above 850 f.p.s., the far distance is 35 yards. Between those distances (20 and 35 yards), the pellet always lands very close to where the crosshairs are placed. Within that range of distances, the pellet seems to be centered vertically.

You align the crosshairs when you mount the scope
Everyone sees things a little bit differently, and this is one time when that IS NOT okay! The scope/gun relationship depends on the precision of alignment, and the sideways straying of the pellet is an indicator of poor alignment. I get it wrong all the time, so I have a lot of experience with it.

When you mount a scope, you turn it until the vertical crosshair appears to bisect the receiver of your gun. If this alignment is not true, you’ll get this pellet shift problem. There is no mechanical tool to help with this situation, but with patience and understanding you’ll overcome it. The scope collimator that some shooters use to assist in scope-mounting is a help, but you can’t rely on it 100 percent of the time. When I notice one of my guns has this problem, I loosen the rings and turn the scope slightly to the left until it starts to look off-center to me. That turns out to be the spot at where it’s correctly aligned. You’re going to have to gain this same experience for yourself.

That’s why I prefer to mount my own scope. If someone else mounts a scope for me, I may end up suffering with their problems instead of my own. I take scope mounting very seriously.

The other alignment problem
If your scope isn’t optically centered when you sight in, you will be using some internal correction to move the pellet strike. I optically center the reticle, then I use B-Square AA adjustable scope mounts and rings to mount the scope on the rifle. I’ve tested most other adjustables, and they just don’t cut the mustard. AA mounts are great! I sight-in using their adjustments and as little scope knob movement as I can get away with. There may be a better way to do it, but this is what works for me!

Thanks to bigvic’s request, we now have an index of all past postings, which lets us see the scope-related stuff at a glance. Read What causes scope shift to get more on the scope alignment operation. Also read Another cause of scope shift: over-adjusted scope knobs to learn about another technical problem. Another great posting was At what range should you zero your scope? Although this subject is full of controversy, at least read the posting to see what is involved when you zero. Take a look at More about sighting-in: How to determine the two intersection points. It’s a companion to the first one. You may be interested in How to optically center a scope.

In August, I did two posts on mounts. See Scope mount basics – part one and Scope mount basics – part two.

Wow! It looks like I’ve written a small book on scopes and scope mounting.

Besides my postings, Tom Gaylord has written a number of articles about scopes. You might find them helpful, too:

All about scopes, Part 1
All about scopes, Part 2
All about scopes, Part 3
What does AO mean?
What is a mil-dot scope?

I hope this summary helps you with your scopes!

author avatar
Tom Gaylord (B.B. Pelletier)
Tom Gaylord, also known as B.B. Pelletier, provides expert insights to airgunners all over the world on behalf of Pyramyd AIR. He has earned the title The Godfather of Airguns™ for his contributions to the industry, spending many years with AirForce Airguns and starting magazines dedicated to the sport such as Airgun Illustrated.

21 thoughts on “ANOTHER problem with scopes: Not mounting them correctly”

  1. There are a couple of tools for aligning the reticle with the gun’s vertical. One is sold by Leupold and fits into the barrel (will it fit .177?) and has an optical colimator so you can see the true vertical.

    The other tool is purely mechanical and is made by the same company that makes a electronic level for scopes — http://www.microlevel.biz/

    Personally, I shoot about 5 shots at my zero, then shoot 5 shots with the elevation turned one or two turns and check to see if the two patterns are vertical. This also assumes the gun is held level — I use a BKL level with BKL mounts.



  2. Joe,

    As always, good advice. I was not aware of either of these two tools, but I’ll have to look into them. Anything that can solve this problem is an essential help for an airgunner.

    I was at a range last week and listened to two firearms shooters discussing the fact that some scopes (including one on their rifles) shoot from side to side at different ranges. They figured it was just something you have to put up with. I would have said something, but I could tell by their general discussion that they already knew everything and couldn’t be told anything new.


  3. If you have a friend in the carpentry biz (or tile setting as I) we use various lazer leveling tools including one called the “GIZMO”. it in particular. projects a vertical lazer line that can be used to clamp the gun then moved to project to the wall to rotate the scope.


  4. turtle,

    I have tested a Microlevel side-by-side against a B-Square bubble level. The Microlevel is easier to see but no more accurate. It is also no less accurate, so if you get one you will have the best that money can buy.

    I didn’t know you didn’t have a level on your gun. All AirForce rifles and most PCPs in general need levels to shoot their best.


  5. i have been tryin to mount a scope on my Gamo Shadow 1000 but when i put the scope on and hold the gun in front of me the whole scope, even the rings are tilted to the left quite a bit and im not sure whats happening. Any sugestions?

  6. Gamo scope problem,

    I don’t think the dovetail claw has engaged the dovetail on both sides of your scope base. It sounds like one side is barely hanging on.

    Another reason for this is that the scope mounts are not correct for the rifle. They would be too narrow or too wide. I’m betting too wide. The mounts would clamp too far in and tend to flop to one side.

    At any rate, I think the problem is down at the dovetail, so look there.


  7. This is not rocket science. First take the scope out of the rings and mount the base of the rings on the rifle. The pointed ends of the scope base clamps have to go into the dovetail cuts on the rifle. Either you have cuts or you have a raised base. The pointed base clamps have to go into the grooves on whichever one you have. When they are tightened down, they need to still be in the dovetails.

    The bases should sit straight, but if they are too big or little for the dovetails, talk to your dealer who sold you the mounts and ask for the correct set. They are too big if the clamps look bowlegged. They are too small if the clamps are splayed out at the bottom.

    I do hope we are talking about good scope mounts, because the cheapies that come from Wal-Mart will all have faults.


  8. i mean yea i know about dovetails and stuff and the scope rings DO fit right and DO bite into the dovetail but it might just be the crappy rings the whole scope assembly came from another rifle i had, but this friday im visiting the local outdoor shop for a one peice mount. maybe this will help and ill let you know how it goes.

  9. well i went to the shop, got new rings,got it mounted and boresighted, at first it was shootin about 3 feet right but got it to shoot pretty accurate but every once and a while there will be a wild shot that will go like 6-7 inces low, i got seated and took a real nice shot about an 8th of an inch so it was good, but them my brother shot it and was about a foot low, might have been user error but hes a pretty good shot, any thoughts of why its shooting wild every once and a while??

  10. Gamo 1000,

    YES! Are you weighing and hand-sorting your pellets? That’s probably where the problem lies.

    Weigh and sort them into lots of the same weight to one-tenth of a grain, then visibly inspect each pellet for defects.

    Also, what pellet are you using? That will make a difference.


  11. As of right now i am using Crosman Premier Hollow Point Hunting Pellets, but i read another blog that you made regarding accuracy and you stated that you need to find a pellet that your gun likes and i am going to try different brands and types untill i find the sweet one. What type of scale would i need to find the weights and how much are they?

  12. The reason i asked the price is because i am a backyard plinker, not a die-hard shooter, i love the sport and all the parts of it, and im a little short on money right now but the pellets im using, have u heard good or bad thigs about them, maybe i should switch to rounded or pointed nose?

  13. does the shape of the pellet effect accuracy? i read somewhere that match and hollow points, because of their flat and cuped nose they grip wind and are effected badly in the accuracy department…..true?

  14. i have just bought the acc shot one piece mount and im not familiar with stop pins so (using the winchester 1000sb)what hole do i mount the pin in th e first hole or the hole with the scope stop in it (scew holding down a piece of metal)it is similar to the shadow 1000 along the top

  15. im winchester deluxe my password wont work tried a lot of time still cant get it to work was woundering if i mounted it in the exaust port if it would ruin the gun

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