Changing POI on Pumpguns by Shimming,etc......


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Dave McCracken
January 18, 2003, 08:03 AM
Had a request for this, and I see that some folks here might need a little guidance, so....

I bet 80% of the folks on this BB own at least one pumpgun. They're the goto for a lot of shotgun activities, from busting clays to hunting to life insurance.

And unfortunately, they sometimes do not shoot where we're looking. Correcting this can often be done in an hour or two over two days,at minimal cost and small effort. Here's how...

And we're assuming(Yeah, I know) that we know something about shotgunning and LOP is about right.

Tools and materials needed will include:

A screwdriver long enough to reach the stock throughbolt, and another for the screws on your recoil pad. Tha last will probably be a Phillips head 1 or 2.

Scissors.

A tape measure or yardstick.

Shim material. I see toothpicks and business cards endorsed for this, but I prefer stuff that's non-biodegradeable and not easily compressed. Old hulls can be scissored into pieces that will work, but I use folded Aluminum foil. This can be easily made into the right thickness, and it's darn near free.

Polyurethane finish.

Rustproofing liquids like oil and CLP.

And now the method.

First, dismount the stock and while you have it apart, seal the grain at both ends with the Urethane to stop water migration if there's raw wood showing. Next, protect the rear of the receiver metal by applying CLP, SLIP, or oil. Wipe on,wipe off. Use a bit on the throughbolt except for the threads, there a bit of wax from a candle, or the wax bowhunters use on their strings will protect from rust while not letting the thing loosen under recoil.This does nothing for POI, but you might as well protect your weapon while your in there...

Once the Urethane's dry, add a shim where it'll do the job and reassemble,then head for the range.

And how do we know where the shim should be?

It's a reverse process. If we want the shot to go high,the shim goes at the bottom of the receiver.

Measure the thickness of the shim, and write it down.I'd start with 1/16".

Of course, we've already patterned with the load of choice, and know how much off it is.

Head for the range and set up at the pattern board at 16 yards. If your shotgun has tubes or Polychoke, set it for Full or tighter. Shoot from a rest and note where the shot goes in relation to the aiming point. Repeat a couple times, using new paper each shot.

A few repetitions should give you a fairly good idea of where it's hitting for you. Now the fun part.

A 1/16" change in height(NOT the same thing as adding a 1/16" shim)should give about an inch or two change in POI. Fiddle with this until you've gotten the pattern where you want it. For GP use, I like about 60/40, but YMMV.

Now move back to whatever distance you choose and check again.Fiddle if needed.

Now shoot a few rounds offhand to see if fit has been compromised. A small shim at the heel of the butt may be need to return the pitch to normal, or the toe of the pad may need sanding down to keep it from digging into your chest.

Measure all the dimensions and write them down.

Go shoot a round of clays and see if things have improved, and you're comfortable. If so, you're done and your shooting has probably improved.

HTH, sing out if something's unclear....

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Hkmp5sd
January 18, 2003, 08:15 AM
And I have one to try it on too. I have access to a wide range of stainless shims at work, so that isn't a problem (and they ARE free:) ). I'll definately give it a try! Thanks.

HSMITH
January 18, 2003, 10:57 AM
Great post Dave.

In my opinion shimming is only applicable for smallish errors in fit, and the real purpose is fine tuning pattern placement. I would not try to move the pattern more than about 6" in any direction on a pump gun. Bigger errors in fit need to be addressed in the wood itself. I only pipe up about this because I have seen 1/8" and larger shims in the wrist break stocks when not fitted well. If you can work wood you can get some pretty drastic changes safely by shimming one "side" and relieving the other, you must maintain good contact around the wrist, if contact gets patchy there will be problems.

I like 70/30 and even 80/20 does not bother me if I know where it is.

T aking care of the details and fine tuning the fit is the difference between good and great shooting for most guys, it should be done.

Dave McCracken
January 18, 2003, 12:45 PM
Thanks, guys...

HK, what's the POI (Point Of Impact) now?

Howard,agreed. Fit is best handled elsewhere, and the shims are for fine tuning the POI. Plenty of us have shotguns that are just a little off. This is one way to address that. I wouldn't try for more than 6" either.

Good shooting occurs when mistakes are eliminated. This can dial in the right picture.

Any decent smith can inlet the head of a stock to match a shim.Or, duplicate the dimensions to make a stock identical to this one.

sm
January 18, 2003, 12:54 PM
Dave, Great post. Most are not aware of shimming as a method of fine tuning a shotgun.

<grins>
Of course NOW that blows another excuse one can use when a miss occurs;)

Hkmp5sd
January 18, 2003, 02:13 PM
HK, what's the POI (Point Of Impact) now?

I have one that is hitting about 8" low and about 2" right of aim point at 20 yards.

HSMITH
January 18, 2003, 06:29 PM
8" is a major amount, but up and down is the easy adjustment. I would NOT fix the left and right until I get the elevation squared away, fix the big dents, then tap out the smaller dings.

Either buy one of those stick on comb kits with the padded comb pads or one that you lay the comb pads on and then roll a neoprene sleeve over the stock covering them. For 8" at 20 yards start at about 1/4" thick pad and go from there. You can also use a folded towel and tape if you want, lots cheaper but not quite as easy. Double thickness of an old mousepad maybe? Tape that on and see what shakes out. At any rate you want to increase the comb height of the stock raising your rear sight (your eye).

Keep adding or taking stuff off until it shoots to POA, at 40 yards. For SD guns I like slugs to hit right where the bead lays, as if the bead could cover the hole the slug will make. That means it shoots 50/50 for shot. When you get it shooting where it belongs take the yardstick or tape you got out in Daves lesson from earlier and record what the changes have been. You can go to your smith with that or even back here for some advise. Holler if you have any questions.

Dave McCracken
January 18, 2003, 09:49 PM
Howard has it right, here's an addendum...

KickEez makes a neoprene pad called CheekEez, available in 1/8 1n3 1/4" thicknesses. Get one for less than $10 and scissor a thin strip say, 1/2" wide. Mount this on the comb and layer it up with other strips until the height's right. Do not use neoprene where it would be in between you and the stock. As the comb's raised, your cheek and jaw may move over enough to take care of the left/right misalignment. Moleskin can be used also, but it takes smore layers.

HTH...

Andrew Wyatt
January 19, 2003, 11:08 PM
I used the mousepad technique, and it works pretty well.


I taped them securely in place with black duct tape, and they show no signs of shifting around.

HSMITH
January 20, 2003, 09:51 AM
Awesome Andrew!!

sm
January 20, 2003, 08:27 PM
OH great!!
So I guess moleskin self adhesive rectangles are now "old fashioned" huh? :D

Umm didn't have mousepads and neoprene per se [read: duh? we were not high tech I guess] We'd buy moleskin from the DR Scholl (tm) foot display and keep in range bags for new shooters.
[does this mean the ones I still have are antique and worth something?] *g*

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