HELP!!!! SCOPE SHIM QUESTIONS

[454 Big Block Chevy]

ok...

well, my factory gamo scope, decided to go the wayside a few months ago, and i bought a CP 4-16x50 ao for it, mounted it on with the factory scope mount,   (realy solid one piece dealio with a stop pin)

scope mounted fine, very solid, but very close to the barrel.  normally i'd be fine with this...

sighting it in... mount adjustment is al the way up, and at 10 yards, i'm shooting 8 inches low... dead on left/right... what can i do?  where do i shim, front/back and what can i use, the scope nearly touches up front, i'm assuming if i shim the front up, it would give me more scope/barrel distance and move the POI up, am i correct with this logic?  and i am planning on using a piece of aluminum soda can, will that work?  or is that overkill?

Thanks 454

(untuned Gamo Big Cat 1200 that will shoot one hole ragged groups with stock trigger and CP4-16 scope, just really really low.

[TCups]

If the POI is 8 inches low, and if you are out of adjustment on your mount and scope, then you will need to shim the the back ring to raise the back of the scope.  This will tilt the scope downward so that the point of aim can coincide with the point of impact.

That said, at 10 yards, 8 inches low is WAY off.  8 inches low at 100 yards would be about 8 minutes of angle.  8 inches low at 10 yards is 80 minutes of angle.  You will need to tip the scope's line of sight down by 1.3 degrees, so you are talking a pretty thick shim on the underside of the rear ring.  At least 1 mm thick and maybe as much as 2 mm.  The likely cause is barrel droop and you may need special mounts to compensate for that.

[454 Big Block Chevy]

see, i thought barrel droop, but the barrel lockup is very very tight still... it boggles my mind at why it shot dead on with old scope, scope broke, and new scope holds POI, but was way low, and by 8" low at 10 yards, thats with full upward adjustment, i just don't get it.

[TCups]

Not saying for sure that the scope couldn't be the problem.  And a tight lock up at the breech is no guarantee that the barrel isn't drooping.  Was the scope shimmed before?

[Motodad]

Wow great info here, thanks TCups. I am having the same issue with my brother-inlaws gamo silver shadow. I shimed the front mount and the poi went lower and I didn't understand why. Now that I visualize how you explained it, it makes perfect sense to shim the rear mount instead of the front. THANK YOU


LOL- I am such a newb on this stuff!

[454 Big Block Chevy]

no it wasn't shimmed, i will buy a scope mount, i can look through the scope, and watch the crosshairs move, while moving the turret, of course, i did notice something... the last 2 turns of hte turret, at each extreme, do not move the crosshairs... malfunctioning scope possibly?

i tried shimming the back of the scope with electrical tape, seemed to help, but i'm still 4" away... i can't go another strip of tape or the front will hit.   i will see if wally world will take the centerpoint back...  until then... we can only see what will happen

[TCups]

Typically, the last few clicks don't move the cross hairs.  If you can, do this put the scope on the rifle and the rifle on a solid rest or bags.  Take a yardstick and put a tack through the center and tack it to an empty wall about 10 ft or so from your scope.  The distance isn't critical, but it does need to be far enough to measure how far the cross hairs move.  Start with the yardstick vertical and then adjust the elevation first to all the way up then all the way down.  At each extreme, jot down the measurement where the horizontal reticle crosses the yardstick.  If, for example, the travel is from 12.5 inches to 23.25 inches, then, the adjustment will travel 10 .75 inches.  Divide that in half -- 5.4 inches say -- add that to 12.5 -- 17.9 inches, then dial the elevation back to the optical center at the 17.9 inch mark.  Next turn the yardstick horizontal and repeat to find the optical center for windage adjustment, and dial the windage back to center.   Now your scope is optically centered.

Next, take your rifle outdoors, and again off a stable rest or bags, fire 2 or 3 shots at a fixed target at least 10 yards distant.  Take the top of the rings off, and tip the back end of your optically centered scope up and add enough shim until the reticles are about on the point of impact.  

Shim material that seems to work well:  cellulose film from old camera negatives (for very thin shims).  Sections of plastic cut from soda bottles for intermediate thickness.  If you cut your shim from the bottle neck, it will be curved.  Or pieces of an old credit card or hotel room card key.  The thickness of a credit card is about as much shim as you can practically use.  The shim can only cover a 90 degree quadrant of the scope ring.  If you were, for example to use 180 degrees of shim, clamping the ring down would bend the scope tube.  You can position a 90 degree shim in the bottom of the ring, but not on either side, because you can open the vertical gap in most rings, but not the horizontal gap.  Obviously if you were to use rings that fastened top and bottom rather than on either side, you couldn't shim the bottom of the back ring.

So -- optically center the scope.  Find your point of impact with the rifle "fixed" when you fire it.  Take the top half of your rings off.  Figure out about how much shim you need.  If you can't reasonably shim the scope to adjust to the POI, then you will need special "droop compensating" mounts.

[arathol]

Its not good for the scope and in your case it will probably cause the scope to fail if it hasn't already. There are two proper ways to correct for droop, neither one involves shimming. First, use a mount that corrects for droop, second, bend the barrel up a bit. Shimming means you are putting a spacer under the rear of the scope tube inside the ring, effectively changing the height of the rear ring. This causes the rings to be out of alignment. If the rings are not aligned, the scope tube will bend when you tighten the rings down. A bent scope tube  means the ends are not in alignment. If the ends are not aligned, when the gun fires the energy from the recoil will not travel in a linear fashion through the scope.    The recoil energy needs to be transmitted in a straight lne from  the objective end through the tube to the eyepiece. If it isn't, the scope will flex in all the wrong directions and the inner workings can fail.

[fisherdude]

I too am a proponent of bending the barrel.  This may sound scary, but the steel in airgun barrels is soft and easy to bend.  You should be able to lay the rifle sideways and look down the side of the compression tube.  The barrel should visually run parallel with the compression tube.  If it appears to dip, you have droop (not uncommon actually) and you can probably correct this by bending the barrel up.  I think there may be a discussion on how to do this in the library.  I have done it several times without incident.  If there is no droop, check out the mount and grooves to see that the mount is properly seated.  This is also a common problem.

[thebookdoc]

don't shim. See this: http://www.gatewaytoairguns.com/airguns/forums/thread-view.asp?tid=27688&mid=218050#M218050

[TAJ45]

Just a suggestion here.

When zeroing my varmint rigs (powder burners), I pick my yardage that is the closest I'll probably shoot and set up my Burris inserts/rings so that I leave about 85-90% of my "up" elevation available.  No reason to center my elevation and leave 25 clicks "on the table" when I won't be shooting so close to need the adjustment capability.

Using this concept, I'd zero my air rifle probably around 30' with only about 10-12 clicks left to lower my poi - for pellet wgt adjustment.  By doing this, I save the majority of elevation travel for ranges requiring the greater adjustment and being more likely to be encountered.