by B.B. Pelletier
This is a very tough question, so please stick with me as I try to make sense of it for you. I’m not trying to confuse you, but there are so many things you have to know to understand this problem.
1. The bore of your barrel doesn’t go through the center of the barrel!
Shocked? I hope so. Man has yet to discover how to put a hole exactly in the center of a long tube. The easiest way is to drill the hole first, find out where it is then cut the barrel in a lathe until the outside is concentric with the inside! That’s been done for custom barrels but not for production guns. SO, that barrel that you THOUGHT was mounted straight in the receiver is probably looking somewhere else. Read The Bullet’s Flight From Powder to Target by F.W. Mann to learn more.
P.S. Turning the barrel between centers on a lathe is a popular fix for this problem BUT IT DOESN’T WORK! What it does is ensure that the hole at the MUZZLE is centered, with no assurances of where it is anywhere else. I have cut barrels at varying spots along their length and found their bores up to one-quarter inch off center! That’s extreme, I admit, but it illustrates my point.
2. Your barrel probably isn’t centered in your action!
In fact, it’s a safe bet it isn’t. Unless you’ve spent a lot of money having a machinist center your barrel in your action, it probably isn’t centered. The design of most firearm actions makes it almost impossible to center or align the barrel this precisely. Most airgun actions do, too. Hold on, ’cause it gets worse!
In The Bullet’s Flight From Powder to Target, the author built a special cylindrical action and attached it to a concrete pillar sunk deep in the ground. It stood 26 inches above ground and 40 inches below. He called it his “Shooting Gibraltar,” and with it he proved that no barrel is ever bored perfectly straight. He shot all of his test shots from this incredibly solid bench. He used barrels made only by Harry Pope, the most famous barrelmaker that ever lived. The best of Pope’s barrels were shooting ten-shot groups under one-half inch at 200 yards with black powder.
3. Your scope rail is not aligned with your action or your barrel!
Even if your barrel were perfectly aligned with your action, there is absolutely no guarantee that your scope rail is aligned with either your action or your barrel! That’s why it’s pretty futile to try to align the barrel with the action.
Is there any hope?
Yes, of course. All I’ve said thus far is true, but that doesn’t mean you can’t mount a scope on a gun. It just means that the Hollywood version of a rifle coming out of a box and the scope being snapped into place with perfect zero is pure fiction. Remember how long it took NASA to get the Hubble Space Telescope up and running? Well, your job is easier, but there is still work to do.
This posting was not about how to mount or zero a scope. It’s purpose was just to inform you of what you have to overcome when you do the job.
The problems I have mentioned here can be overcome with good scope mounts and a little patient work.
If someone can compete and win with a scope mounted like this, your job is easy!
I’m interested to know more about the pic you posted. Was this a left handed right eyed shooter?
Holdover calculations with this gun must have been a headache!
This explains why a scope zeroed at a given distance is “off” at another range,ie.,not vertically, but horizontally.
The rifle pictured is for a right-handed, left-eye dominant shooter. It actually worked very well AT ONE DISTANCE, but when you tried to shoot at a different distance, it had to be resighted because of the converging angle of the scope.
Lefty,
You got it! This is one possiblereason for not lining up with the vertical retical at different distances. Two others are parallax (the lack of a repeatable spot-weld) and the scope not being correctly aligned up-and-down when installed.
B.B.
I shoot right handed but am blind in right eye. Where can I find scope mounts like ones shown on 10/20/05? Will they fit on tech force 99, 22cal., which is my first purchase, so far. Had similar device on 308 caliber. Bill D
Bill,
These mounts were made by a custom maker who said he would never make another pair. They cost $350 about eight years ago.
A good machinist should be able to make something like this for you if you show him the photo.
B.B.
I wrote the first comment this morning. I got mixed up….I meant to say left-eyed right handed like you said. My initial thought was that the rifle would only be accurate at one range as you mentioned….but I figured anyone who would go to all that trouble to do that sight system and have a range like the one in the picture would be able to put together a holdover chart for different yardages. Anyway….I have the same situation being left-eyed and right handed. What advantages did this gentleman find from this shooting method? I eventually gave up and shoot all my rifles left handed. It has worked pretty well…..the only problem I find is that friends and family have a difficult time shooting my rifles because their sight picture is completely different from mine.
Holy cow, you’d need a holdover/hold???..”next to” chart. maybe a mildot mounted at 45 degrees…its making my brain hurt.
I’m left eyed too but this is pretty extreme!
The “next to” chart is correct. The convergence of this sight is from the side, so the pellet will drift sideways against the bull.
The shooter who owned this rifle used it for BRV, a sport of bullseye shooting in which all shooting is done at one range. There was never a need to adjust the sights once it was on target.
For normal use a mount like this is pretty impractical, though I suppose a convergence chart could be made up.
B.B.
Hi B.B
I’m new to airgun hobby and currently having accuracy problem with my hw 35. The pellet always shot at about 5 cm below my target, I’ve already wind the up level on my scope to the maximum ( it is a bushnell sharpshooter, is this ok to use on a hw 35?)
but with no effect. Do you have any suggestion to fix this
thanks
Robert
Robert,
I’m not familiar with a Bushnell Sharpshooter scope, but if it is made for airguns it should be okay.
Having the elevation wound all the way up is no way to operate. With the erector tube spring relaxed all the way you will get shots all over the place.
What you need is a set of adjustable scope rings. B-Square adjustable rings are the best in the world. They adjust in both directions and will never stress your scope tube. They are a little harder to adjust to begin with, but once they’re dialed in, they work fine.
What caliber is your HW 35? And what pellet do you shoot? That can make a big difference.
You said you had an accuracy problem, too. Could you tell me about that?
B.B.
Hi B.B
Thanks for the response. Problem solved,I’ve made adjustment to the rear scope mount so it is higher than the front. Now it shoots ok. My HW 35 is 4.5mm cal/.177
is an old air gun, I got it from my dad who bought it in 1981.
Robert?
That’s great! Isn’t it wonderful when things work like they should?
B.B.
I have a Gamo Big Cat 1200. My wife and I were in the Army and have shot rifles.
When we first bought the rifle, we were easily killing squirrels and rabbits with one shot.
Over night, we were off by inches from the aiming point and point of impact.
I had heard the Gamo scope could have problems, so I bought a BSA Essential AR 2-7X32 and attempted to sight the rifle at 30'.
I fired four pellets with with the rifle supported on a tripod and had a 1" shot group
(yes, I was in MI, thus the large shot group) and adjusted the windage and elevation after each shot. The shot group was 1.75" left and 3.5"down from my aiming point.
I adjusted the scope after each shot, but since each click adjusts 1/4", wouldn't expect to be able to move it inches.
This was similar performance with the Gamo scope, so is this the result of barrel droop, the scope mount, or something else?
Curt,
First, you cannot shoot a spring-piston airgin from a bipod and expect accuracy. You must learn the artillery hold. That will shrink your group size. Here is a short video on how it works:
/article/The_artillery_hold_June_2009/63
Next, how high is the elevation of your scope? If it is around the three-quarter mark or higher, then the problem may be caused by a drooping barrel. A scope will not hold its zero when adjusted three-quarters of the way up or higher, due to the vibration of the spring-piston mechanism.
If your rifle has open sights, the proof of this is to remove the scope and shoot some with the open sights. The groups will shrink right up, if you use the artillery hold.
A spring-piston breakbarrel air rifle is the most difficult rifle to shoot accurately. But once you learn how, all other rifle shooting will improve.
BTW, you can go on the current blog page and ask your questions. We don't worry about staying on topic on this blog.
/blog//
B.B.
Curt,
one last thing to check – while BB's advise is spot on, big power air rifles like this tend to loosen screws frequently. Go over the stock screws and the scope screws and make sure they're all tight. Then come on over to the current blog which BB writes. Your comments and questions will be viewed by thousands of enthusiasts who are anxious to help a fellow airgunner.
Fred DPRoNJ
I am left eyed and right handed and yet another way on low recoil rifles is to use a high rise rail with a rail on three side and mount a scope on the left side of the rail.
Then roll the rifle over to the right until the scope lines up with my left eye. Then modify the stock so the I can get a cheek lock and the butt fits my shoulder. The scope is over the bore more or less and there is still a clear rail above the bore for right handed shooters. On a Kel Tec SU-16 I can still use the open sights thought the tunnel of the high rise rail.
If the there were more beef in the stock or if I were shooting .308 it wouldn't work for more than a shot or two until I got tried of getting slapped in the face.