Modifications, DIY, FAQs and other info for Discovery and Crosman 22xx airguns.

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NEW!!! TKO Productions! Crosman 2240 Teardown Videos on Youtube.com

Part 1 (Teardown) 10:00: http://www.youtube.com/watch?v=pTHChcW9ktI

Part 2 (Assembly) 10:00: http://www.youtube.com/watch?v=QuGSaO_B_gU

Part 3 (Assembly) 4:00: http://www.youtube.com/watch?v=Wp245fjsCus

(Not sure why the volume is so loud....)


This page will always be under construction!

Because I actually shoot each gun for accuracy (18 yards) and velocity (Chrony) when one comes in for a tune, I can plot to what effect each modification has on the overall performance of your airgun.

Here is what I have found:

For accuracy

#1. Fix the barrel band. Probably the easiest way to maintain accuracy from session to session.
The barrel band has 2 set screws that are, in effect, squeezing the barrel and airtube together. Since the band is "split" at the juncture, the band spreads apart as the set-screws are tightened. The tighter they are, the more the barrel is pinched towards the airtube. You can see this effect if you sight down the barrel and look at the space above the barrel and below the airtube. (You probaby won't be able to see the gap under the airtube unless the action is out of the stock. You will also see this on all current Crosman guns that use the barrel band.)

This squeezing causes two problems:

#1: The barrel is being tensioned downward. If you slip the barrel band off the action, you will notice that the barrel is actually angling upward, away from the airtube. That is because the design of the Crosman breech port seal and the tiny forward breech screw act as a pivot point while the rear action screw is of a much more stout construction and acts as the lever.
#2: The Discovery is a great gun, but a little loud. The addition of a muzzle brake makes the gun more pleasant to shoot at home, etc. The smaller .75" brakes I and others make allow the filling of the gun while installed, but that downward angling barrel can cause it to be a very tight fit for the fill adapter. Hence setting up the gun with a parallel barrel helps while filling the gun also.
We can see that clamping the barrel parallel to the airtube is the desired effect, but the design of the band doesn't allow that if the screws are tightened securely.

Shimming the barrel band…
The quick and dirty solution (and one that works quite well), it to cut two (or more depending on the amount of distortion our band has) 1/2" X 1/2" pieces from a soda can. Form one piece to the curvature of the diameter of the barrel, and the other to the curvature of the diameter of the airtube. With the barrel band set screws loose, slip the 1st piece UNDER the barrel, between the barrel and the barrel band, and the other piece OVER the airtube between the airtube and the barrel band. Then tighten the set screws. Since the barrel band is "split", overtightening the set-screws will cause the same tensioning problems.

 

 

Drilling for additional barrel set-screws…
A bit tougher but much better method to adjust the barrel is to drill the barrel band and install additional set-screws.
This makes a "3-point" cradle for both the barrel and airtube and offers a much better and sturdier attachment. This is the method I use when I build a gun based on the Discovery and am looking for ultimate accuracy with the factory barrel. Here's a picture of one that has been drilled with set-screws added. Two holes angle upward and two holes angle downward. I used 6-32 set screws and sightly grind the points off the screws to avoid scarring up the airtube. I don't want to cause any gouges where a stress crack may develop. (Cranking down on the factory set-screws will already have scarred up the airtube.)
It's tricky to drill the flat at an angle. I use an end mill and do a litte side cut flat then drill with a #36 bit. You can do it in a drill press. Make a small dimple at the desired points, then angle the band and drill your angle.

We also sell the bands ready-to-go! (sorry --- discontinued.)

 

#2: Smooth the leade-in ("chamber")to the barrel. We are trying to achieve a nice, smooth transition of the pellet into the rifling of the barrel.

I have not found any difference in accuracy from a barrel that has a very long angle to the rifling compared to one that has a sharper transition, but fine accuracy from a crosman barrel always requires that the pellet head be seated in the rifling to eliminate any yaw being intorduced to the centerline of the pellet compared to the bore of the barrel.

I use a tool I made that has a collar depth gauge to prevent cutting the leade too deep. While the barrel is in the lathe, I take off about 001 from the face of the breech end of the barrel for a tighter lockup of the bolt handle.

#3: Recrown the barrel. I have factory crowning on barrels from Crosan that were far far off center. Not all crowning is bad from the factory, but I would say 85% of the barrels can be improved.

I cut the barrel flat, then cut a 11 degree inset, and lap with a brass lap and 1500 grit wet-dry sandpaper soaked in thread cutting oil. I run the lap both clockwise and counterclockwise 7-8 times, always ending with the barrel spinning clockwise (reverse from usual lathe turning direction.) A cotton swap in the muzzle pulled out slowly tests for burrs.

#4 Lap the barrel. I concentrate on lapping the first 2/3rds of the barrel, with a quick final lap of the entire length. I'm trying to keep the barrel from scouring any lead from the fine pellet skirt.

More to come!!!

Contact Info:  

E-MAIL ADDRESS:
sales@tko22.com

TKO Airguns
Mike Tiehen
323 South 50th St.
Omaha, NE 68132
USA
 
Phone: 402-889-9222