Authority to the Airgunning World

Where instigating argumentative debates and stepping on egos are the rule.

Sunday, December 24, 2006

Here it is by popular demand: Bending an airgun barrel:

The methods described so far in this post and the post a short time back are very crude and not practical as far as controlling the amount of bend imparted to the barrel. It also subjects the gun to possible damage. Cut three blocks, 2 inches square by 1 inch thick of delrin or similar hard material that will not deform when pressure is exerted on them. Use tape, masking or other to attach them to the jaws of the vise so the 1 inch side is against the barrel. You will position two on one side and the third one on the other side. You can now position the barrel in the blocks so that the applied pressure will push the barrel in the desired direction. Pressure can be applied in a gradual manner and without the requirement to remove the scope, the point of impact can be easily checked until the desired result is attained. There is not the brute force requirement as described in the other methods.

you look down the bore and check for concentric rings of light and reflection. A bright light in the bore is not how it's done. A well lit black and white pattern on a wall is what real barrel makers look at thru the bore when straightening. They look for ripples and curves in the reflected pattern inside the barrel, then bend and adjust as needed to make the reflected pattern consistent.

I ended up mounting the barrel/breech on a 2X4 and squeezing it between 2 blocks of wood with a 5 inch C-clamp. I inserted some wood shims under the length of the barrel (about 4 inches from the breech 'cause that was where the bend was evident) then compressed the breech part of the barrel with the clamp. Since the barrel was bent upwards I mounted the barrel upside down on the 2X4 and compressed the breech until it met flush with the mounting board. Did the trick. This allows more precise pressure and you can eyeball the progress you're making. I did such a good job I had to shim my scope to get the sighting in correct.

Assuming it's not in the mount or scope, the barrel and receiver could simply be out of line with each other, and I've also seen scope grooves that were clearly machined non-parallel to the receiver itself.

On many springers the barrel can be bent for re-alignment instead of using adjustables. Fixed drooper mounts are avavailable from Mac1 to take care of it without adjustable mounts, or you can carefully take your chances with the shim method.

Its not unusual to get an airgun to run out of windage/and/or elevation when using higher magnification scopes and suggest you use the B Square AA Adjustable 2 piece mounts and the risers too if you need to increase the height. (Pole risers).
If the B Square won't cure the problem, then I don't believe there is anything else that can short of replacing the receiver or bending the barrel.

For a long time 85% of guns needed 30 minutes of droop. The manufacturers AA & HW have made the barrels point another inch higher at 10 yards in the last few years.

30 minutes=30"@ 100yds
=15"@ 50yds
=10"@ 33yds
=7.5"@ 25yds
=3"@ 10yds
=1.5" @5yds
=.030" @ 4"

It seems that the 85% these days would be covered by a 20 minute drooper but the need to droop accurately is more critical when scopes with very little total adjustment are used. Elites come to mind as having about two turns up and down. Not much.
We provide beefy mounts so POI shift problems are eliminated.
If you can't sight the gun in with straight mounts or the mounts delivered, put the scope in the middle of its range of adjustment and see where the gun shoots @
10 yards. With this info we can figure out what it will take for your application if you aren't in the 85%.
Only so much droop correction can be made in the mounts (usually 50 minutes)and roughly half the droop correction can be made to the windage if needed.
On barrel break and underlever rifles it is easy enough to bend your barrel into alignment but with rigs with barrel supports and fixed barrels the best solution is to machine rather than shim. Shimming is very hard to do without stressing or denting the scopes thin tube. Whatever thickness in thousanths (.001") the material your adding adds up to, that is how many minutes of adjust you need. Most ring spreads are 4" and at that distance minutes=shim thickness or droop needed in thousanths of an inch.

More than likely you need 20 minutes of droop like all HW's. If you center the scope you'll find it hits about 2.5" low at 10 yards. It should shoot 1/2 to 3/4 low at that distance to be zeroed at 25 yards( highly advisable to zero at your apogee) so you need to put a mount on there to let the Elite use the middle of its adjusment range like the designer intended.
You can shim but you should remachine. By putting shims in the back you bow the scope tube and can dent it if you get carried away.
I make a mount that has a tie rod connecting front and back rings and a solid stop in the rear ring that sets in the guns stop hole. The mount is based on the 5040 Beeman which is a $35 set of rings. We charge $60 for the HTRD. It is available in 20, 30, 40 and 50 minutes. Thatb number of minutes roughly equals the number of thousanths of shim thickness you need. So in a 4" ring spread 20 thou shim or angle adjust(1/3rd degree)= 20 minutes = 2" at 10 yards = 20" at 100 yards.
20-30 minutes is what most springers mounting a 40 MM objective need.
When you buy quality optics the makers expect you to make the coarse adjustments and the scopes ajustments do the fine adjust. They often supplky a shim quide but the proper way to do it is to cut the droop in the rings. You can send us your 5040 rings and trade up to an HTRD for $30.

Remove the barrel complete with cocking arm and breech block. Remove the rear sight from the breech block if so equipped. Place barrel/breech unit on your workshop table with the breech block resting on its bottom side (the side the cocking arm attaches to). Swing the cocking arm forward out of the way. Place a small wooden block under the end of the barrel (beneath the front sight). Place a small wooden block on top of the breech block and one under it. I'm not sure if the HW55 has a breech opening latch or similar device that would prevent the breech block from sitting flush on the block/table, if so you may have to remove it. Take a 4 inch C-Clamp and place top (the part with the tightening screw part) of the clamp on the wooden block that sits on the top of the breech block and wedge the "bottom end" of the clamp under the edge of your workshop table. This works best if you have a protruding table ledge to slip the end of the C-Clamp under. By pressing down the wooden block that sits on the breech carefully with the C-Clamp, the end (or middle depending where you put your block under the barrel) barrel is "levered" or bent up according to the height of the block beneath the barrel and by how much pressure you apply to the C-Clamp. This is more precise than the wack with a 2X4 across a sawhorse -----just compress slowly the first time, take the unit out and measure for straightness and re-do the procedure as necessary. I had a HW30s that someone fired with breech block open and the barrel was bent upwards-this procecure (I just flipped the breech on its top edge though) worked to align the barrel. Note the procedure above is for barrel DROOP but can be used for any bend by the location of the breech -on its side or edge.

Open the action and point the muzzle towards a brightly lit area, not at a light. Hold the barrel 6 to 8" away from your eye, you should see perfect concentric circles as you look down the barrel. Turn the barrel around and look down the muzzle end, it should be the same.

If you see ovals and not circles, the barrel is bent. Rotate the barrel slightly while observing this and the ovals will retain their shape in the same spot while you rotate the barrel.

A bent barrel can often be repaired by simply bending it back to a level trajectory. Often this can be done without adversely affecting the accuracy. Several people have posted simple techniques on this forum and others.

If you want to replace the barrel, the technique is the same for the R series spring guns (the barrels and breech blocks are interchangeable from R1, r9, R10, RX, RX1, RX2 guns, while the RX guns have a different cocking linkage).

- Make sure the gun is uncocked
- Find a pair of LARGE flat head screw drivers to fit the screw and cap of the pivot bolt snugly.
- While placing a driver into the bolt on the right to hold it still, use the other driver to remove the cap from the left side. Then remove the bolt itself by counterclockwise turning of the bolt (Right side). You may find less tension on the barrel if you slightly break the barrel.
- Once this bolt is out, the barrel and breech block will freely move out of the receiver arms. Be careful to collect the thin washers that goe between the receiver arms and the breech block.
- Carefull rotate the barrel and cocking linkage downward and slide it out of the cocking shoe - if you rotate too far, you can inadvertantly damage the cocking shoe.

Reinstallation is the opposite procedure, first insert the cocking arm into the shoe, then slide into the receiver arms with the thin washers aligned with the hole (kinda tricky, a nice punch with a smoothly increasing diameter can make this a lot easier). The shims should have moly lube on them and the smooth portion of the cross bolt should also have moly, while the threads should be clean.

Then insert the bolt from the right - once again slightly breaking the barrel can decrease the pressure on the parts or you can try placing the muzzle on a piece of wood on the floor and then pressing to compress the detent so that the bolt will line up.
Place the cap end on the Left side.
Now cock the gun and adjust the tension of the cross pin. The barrel should slowly move with response to gravity. Once it is this tight, you have to hald the bolt with a screwdriver while tightening the cap on the left. Test the barrel movement again. Often takes a little tinkering, but only a few minutes.

As I mentioned earlier, the R series barrels and breach blocks will work on the RX guns, but will need a RX specific cocking arm. The cocking arm is attached by a pin and can be swapped. However, I would try to get the barrel bent back....

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1.Remove scope, take action out of stock.
2. take front sight off.
3. grab action near the middle of the cylinder tube with both hands like a golf club.
4. Hold barrel 10" over thick wooden table, fencerail, or sawhorse. Make sure the gun is aligned so the direction the barrel needs to be corrected is straight up and down (trigger down if gun shoots high)! A cloth towel or piece of old carpet should cover the wood to prevent marring the barrel.

5. with arms fully and stiffly outstretched and gun parallel with wooden surface, bring barrel firmly into contact with wood in a chopping motion (do NOT bend wrists, or too much force will be developed). The contact point should be between center and where the barrel enters the breechblock.
6. Determine results of straightening by sighting down the action tube and seeing of the barrel is parallel to it.

7. Repeat steps as needed until barrel is overcorrected a VERY slight amount, then tap it the other way very lightly.
8. reassemble and test. Repeat as needed. If you are reluctant to attempt
this, send gun to dealer so he can do the same thing, but more carelessly.

If the bend is as severe as you say, and a distance of at least three or four inches from the breechblock i would suggest making a jig from a section of 2"X4" wood around three feet long by drilling a hole about 6" from one end about 5/8" in diameter thru the board.

Remove the barrel from the action and take the sights off. Push the barrel thru the hole in the board so the short end is supported by the wood and the bend is right up against (and partly in the hole) with the the long end able to be used for leverage.

Place the board and barrel end so they are close to the ground and the long end of the board is up. The bent end of the barrel should be facing up as well. Using your padded palm on the end of the barrel, press slowly but firmly down while noting if the barrel is bending the way yo want (support the board of course). If it doesnt bend, press harder, even "bouncing some", and if it STILL doesnt bend, use foot pressure. Don't overdo it, just bend it til it begins to look a lot straighter, then lay it against a flat surface, even roll it, and gage your progress. Once you have it so it looks pretty straight, you can reinstall and testfire ... if it needs a bit more,

BTW, it might help is someone holds the breech end in proper position when you push down on the other end, just to make sure it doesnt rotate ... again, easy does it, those barrels on small guns arent usually very stiff.

On real stubborn bbls i have resorted to padding the barrel and slipping a piece of 1" water pipe about 3 feet long for leverage. The trick is to hold the other end of the barrel firm .... a 5 foot 2"X6" board can be used with you actually sitting on the board with barrel protruding up and breech end pinned to the ground under the board so long as you know its aligned right. You can bend any (almost) airgun barrel with this kind of leverage.

I have found the rubber mallet to be near useless for this work. I once had a BSA Mercury with a short custom Harper .25 barrel epoxied inside the std barrel as a sleeve ... it already had been "triggered off" and straightened a couple times before, making it way stiffer than original. I could NOT straighten it no matter how hard i hit, pulled, pushed, or whatever, it refused to go! Even Tim had give up on it, and its bent upward around a degree to this day! thje bend was right at the breech and i had no covenient way to hand onto that breech at the time ... even holding it in my mill vice and shoving it just moved the mill, as it wasnt bolted to the floor at the time an only weighs around 800lbs or so.

For FT: The mount is a Beeman Sportsmatch set of high split rings that have been fitted with a tie rod and drooped so the scope can be used in the middle of its adjustment range. HTRD's are currently $65. They include a threaded solid scope stop to replace the factory roll pin.
We call them High Tie Rod Droopers. They are a great way to mount high heavy optics on springers solidly, with no shims.
Most HW and AA guns need 20 minutes of droop.

http://www.mac1airgun.com/droopers.html

http://www.bushnell.com/support/manuals/riflescopes/Shim%20Guide_2.pdf

I had an R9 with a badly bent barrel, I took it to a friend of mine who is a welder/fabricator and he wrapped a towel aroud it and clamped in a vise and straighted it. I never would have believed it, but it didn't affect accuracy at all. I doubt that I could have achieved the same results, but I had seen him work wonders with metal before, and was pretty sure if it could be fixed he could do it.

Early american gunmakers typically bent barrels (straighting them) They used an arbor press. The barrel is bridged across the top of two wooden blocks spaced about 18" apart. The barrel is bridged across the blocks with its "high side" up and the arbor press is brought down (bounced) onto the barrel. A series of a dozen or so stacked thin shim spacers "under" the center of the barrel directly below the arbor press ram will limit the movement (bend) of the barrel to keep it from deflecting too far. I've done this many times. The barrel will "flex" (deflect) quite a long way before any real change (bend) takes place. Keep deflecting the high point of the barrel against the shim spacers, and remove the shims one at a time to increase the deflection until a "set" (bend) takes place. Looking down the barrel against a bright light will show when it is straight. The "eye" can determine straightness with about 2 thousands of an inch in a 24" length of barrel.

Slip a fresh cotton swab type Q'tip in and out of the crown carefully while watching with any eye loupe or magnifier of some sort. If there are BURRS, they will pull cotton fibers on the way out. The burrs are hardest to detect.
I used to try to detect it by feeling the pellet "hang" right at the muzzle as i slowly pushed it out, and this works, but Russ's trick works too, and is appreciated when its hard to push the pellets due to breech access probs.

As to the crown itself being damaged, if it looks burnished on one side or the other, or there is an obvious dent or ding, or if some rifling lands or grooves appear differant than others i would just go ahead and touch up the crown via the old brass screw and JB borepaste method i use on virtually every gun i've owned.

See, recrowning done right never hurts accuracy, and i've seen it improve accuracy when it didnt LOOK like there was anything wrong. A bad crown can act like a barrel with a bend right at the muzzle, or it can simply lead to a few extra "fliers" in your group that widen it. I know many have claimed the crown is over-emphasized, and some even claim that an uneven one does NOT cause problems, but i've crowned perhaps five hundred barrels now, and shot em after, so i've formed an opinion that it IS important, and its been reinforced many times.