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Barrel loose in the middle after machined at breech

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Aldhy

Member
May 7, 2024
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Hi Everyone,

I bought a new LW polygon OD16mm 60cm blank barrel. This blank has even friction / tightness from the breech to the choke when I push a pellet on it.
Then i asked a gunsmith here to machined the breech of the blank to fit my airgun. After finished , it has a loose in the middle of the barrel.
Is there any explanation about it? And how to avoid it in the future? Is there any tips for the gunsmith when machined the blank?
I already asked the gunsmith, and he just said that sometimes its happened.

Thank you
Aldhy
 
Aldhy,

Did the gunsmith reduce the outside diameter of the breech end of the barrel?

Did he do anything to create a chamber, or machine an o-ring groove?

If the barrel had even friction before, then the breech end may have a section of reduced diameter, that sizes the pellet down so when it enters the middle, the pellets are loose.

In other words, the middle section of the barrel is not actually larger, the breech end is smaller.

If the inside of the breech end was machined, it is possible that there is a slight burr where the loading chamber ends and the rifling starts.

If there are barrel retention screw dimples machined, those could dent the barrel slightly and reduce the pellet diameter a little when the pellet passes the dimples. If you over tightened the retention screws when mounting the barrel you can also restrict the barrel bore, so it seems loose after the pellet passes that point.

If a new transfer port hole was machined, that could also have burrs that project into the bore and that can shave of constrict the pellet as it passes by.

Tightness at the chamber / start of the rifling is the easiest to fix. But you should have certainly about the location of any restrictions, because assumptions may have you "fix" a problem that did not exist before; making matters worse potentially.

You should push pellets through the barrel and see where the tightest spots are. Before pushing pellets all the way through, push them back out of the breech and look at the rifling marks. They should be even all the way around, and clean; not as if the pellet is being chewed by burrs.

This guide should help show potential trouble and what can be done about it:

Barrel troubleshooting and accurizing guide

The following article is intended for the do-it-yourselfer that is interested in either finding and fixing a problem with a barrel, or improving the accuracy of an already good performer. You don't have to be a master machinist to get excellent results. The techniques described here can be...
www.airgunnation.com www.airgunnation.com
 
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subscriber said:
Aldhy,

Did the gunsmith reduce the outside diameter of the breech end of the barrel?

Did he do anything to create a chamber, or machine an o-ring groove?

If the barrel had even friction before, then the breech end may have a section of reduced diameter, that sizes the pellet down so when it enters the middle, the pellets are loose.

In other words, the middle section of the barrel is not actually larger, the breech end is smaller.

If the inside of the breech end was machined, it is possible that there is a slight burr where the loading chamber ends and the rifling starts.

If there are barrel retention screw dimples machined, those could dent the barrel slightly and reduce the pellet diameter a little when the pellet passes the dimples. If you over tightened the retention screws when mounting the barrel you can also restrict the barrel bore, so it seems loose after the pellet passes that point.

If a new transfer port hole was machined, that could also have burrs that project into the bore and that can shave of constrict the pellet as it passes by.

Tightness at the chamber / start of the rifling is the easiest to fix. But you should have certainly about the location of any restrictions, because assumptions may have you "fix" a problem that did not exist before; making matters worse potentially.

You should push pellets through the barrel and see where the tightest spots are. Before pushing pellets all the way through, push them back out of the breech and look at the rifling marks. They should be even all the way around, and clean; not as if the pellet is being chewed by burrs.

This guide should help show potential trouble and what can be done about it:

Barrel troubleshooting and accurizing guide

The following article is intended for the do-it-yourselfer that is interested in either finding and fixing a problem with a barrel, or improving the accuracy of an already good performer. You don't have to be a master machinist to get excellent results. The techniques described here can be...
www.airgunnation.com www.airgunnation.com
Click to expand...
Yes sir if this gun smith was a little ham fisted or not used to working with more delicate airgun barrels that's exactly the problem!
 
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Presumably no machining operations were done to the region of the barrel where the perceived loose spot is now. In other words, there is no logical explanation for how the middle of the barrel just spontaneously expanded. Therefore I lean toward subscriber's intuitions that something is happening to the pellet at the breech end...either a constriction putting a squeeze on the pellet or a burr shaving material from the pellet.

A possible cause of a constriction would be from excessive clamping pressure in the lathe chuck/collet. Approximately how much was the 16mm OD turned down to fit the receiver? Note the act of removing material from the OD causes the bore of a button-rifled barrel to expand very slightly, so that would have the opposite effect. But if the machinist at some point secured the breech end in the chuck, particularly after it had been turned down (reduced wall thickness), it may have squeezed a constriction into it.

The other thing I'll mention here is that virtually every LW blank has a slight constriction near the breech. More pronounced with the larger calibers than with .177. In other words one can expect to detect some looseness in the middle of a LW blank in its as-delivered form.

Then lastly we have the potential of a subtle burr shaving material from the pellet as it moves over/through a machined feature of the breech. The barrel troubleshooting & accurizing writeup that subscriber linked above has some guidance on detecting and correcting those kinds of issues.
 
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subscriber said:
Aldhy,

Did the gunsmith reduce the outside diameter of the breech end of the barrel?

Did he do anything to create a chamber, or machine an o-ring groove?

If the barrel had even friction before, then the breech end may have a section of reduced diameter, that sizes the pellet down so when it enters the middle, the pellets are loose.

In other words, the middle section of the barrel is not actually larger, the breech end is smaller.

If the inside of the breech end was machined, it is possible that there is a slight burr where the loading chamber ends and the rifling starts.

If there are barrel retention screw dimples machined, those could dent the barrel slightly and reduce the pellet diameter a little when the pellet passes the dimples. If you over tightened the retention screws when mounting the barrel you can also restrict the barrel bore, so it seems loose after the pellet passes that point.

If a new transfer port hole was machined, that could also have burrs that project into the bore and that can shave of constrict the pellet as it passes by.

Tightness at the chamber / start of the rifling is the easiest to fix. But you should have certainly about the location of any restrictions, because assumptions may have you "fix" a problem that did not exist before; making matters worse potentially.

You should push pellets through the barrel and see where the tightest spots are. Before pushing pellets all the way through, push them back out of the breech and look at the rifling marks. They should be even all the way around, and clean; not as if the pellet is being chewed by burrs.

This guide should help show potential trouble and what can be done about it:

Barrel troubleshooting and accurizing guide

The following article is intended for the do-it-yourselfer that is interested in either finding and fixing a problem with a barrel, or improving the accuracy of an already good performer. You don't have to be a master machinist to get excellent results. The techniques described here can be...
www.airgunnation.com www.airgunnation.com
Click to expand...
Hi Subscriber,

Thank you for your explanation . Really appreciated 😊
My unit is a Weihrauch HW100.
Yes the gunsmith reduce the outside diameter and made a groove for oring seal at the breech end of the barrel.
No transfer port hole.
The inside of the breech end is not machined. But a little touch at the end like crowning 30-deegre at the end of the breech so the pellet able to sit where the skirt flat to the end of the breech without damaged and the head sit in the land.

Yes looks like the breech end is become smaller 😅 And it is quite dissappoint me since I choose the blank carefully by myself at the store after forcing the store clerk to show 20 pieces of blank stock (of course after offering big tips) 😁

Is there any tips when machined the blank to prevent breech shrink ? Maybe sharp knife, slow rotation, cut thin , watery?

Also I already tried to push pellet to the barrel and find loose after 5-6 cm and go further another 5cm. Then push them back and getting tight and stopped at 5-6 cm from the breech end.
Should I lapping 5-7 cm of the breech to remove the tight?
 
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Aldhy,

The easiest to fix would be if steel is raised into the barrel bore at the edge of the o-ring groove because it would be very little material to remove. Your comment about 8 to 7 cm at the breech being tight makes it sound more like a longer section of the bore has been reduced, possibly by tight gripping in a collet or chuck.

There may be another possibility: If the unmachined blank had the both ends of the barrel bore cleaned to form a "crown", that might have been restrictive and sized the pellet down, so that it hid the tighter rear bore diameter, until the gunsmith removed that restriction. In other words, it is possible that the gunsmith made it possible to feel differences in bore diameter, rather than causing them.

A very light lapping of the breech end until 5 to 7 cm deep can be used to diagnose the problem further: If the O-ring groove edges are raised, lapping them down will happend quickly and change the feel of pushing pellets through with little effort. You can always lap more, but you cannot put metal back, so do less, then reassess.

If after a light lapping the feel does not change, then more lapping near the breech end to even out the bore can be used. You do not want to abruptly stop the extent of where you lap, but use overlapping strokes to blend into the rest of the bore. The goal may be to have the bore the same from breech to muzzle (before the choke), but having the breech slightly larger than the middle and muzzle end is also good. Having a tight breech and a loose muzzle is bad for grouping because the pellet will be able to rattle and yaw down the barrel, and not leave the muzzle straight and consistently.

The question about which lapping compound to use is going to get many answers. The rougher the compound, the faster it will cut. Some like a mirror finish, while others swear that a 400 grit finish or even rougher is better. Having even friction will be how you measure the bore diameter, and roughness feeds into friction. My attitude is if you match the LW factory gloss level you can't be hurting anything. No matter if that is considered "too smooth" by some experts. An actual mirror finish is not required.


You do need to be careful of your lapp leaving the muzzle and cutting a bell shape. Perhaps you do not plan on lapping all the way to the muzzle, because the tight section does not extend that far. That should make it easier.

At some point before you think a bit more lapping might be better. Clean the parts, assemble and shoot the airgun to see how it performs. If it shoots well and keeps shooting well for hundreds of pellets without getting so dirty that groups open up, or throw "fliers", then you may be done with lapping.

The video below shows how the HW100 barrel is retained. The deep groove machined for the angular positioned grub screw may actually be the source for the bore reduction at 5 to 7 cm; if the cutter was not as sharp as it should have been:

Watch video from about 9:38 from start:


Either way, lapping concentrated around the area of minimum diameter should help. Check frequently, rather than lapping too much...

Rather than overtightening the barrel retention screw, use wicking Loctite 290 added from the exposed end, after you are happy with the assembly and are concerned with the screw coming loose. The angle of the screw seems to make it less likely that the screw could squash the barrel than many PCPs, but the barrel wall is thinned out there.


next.henkel-adhesives.com

LOCTITE® 290

LOCTITE® 290 is a green, medium-high strength threadlocker designed for the locking and sealing of threaded fasteners. Because of its low viscosity, the product wicks between engaged threads by capillary action and eliminates the need for disassembly prior to application. This makes it ideal for...
next.henkel-adhesives.com next.henkel-adhesives.com
 
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subscriber said:
Aldhy,

The easiest to fix would be if steel is raised into the barrel bore at the edge of the o-ring groove because it would be very little material to remove. Your comment about 8 to 7 cm at the breech being tight makes it sound more like a longer section of the bore has been reduced, possibly by tight gripping in a collet or chuck.

There may be another possibility: If the unmachined blank had the both ends of the barrel bore cleaned to form a "crown", that might have been restrictive and sized the pellet down, so that it hid the tighter rear bore diameter, until the gunsmith removed that restriction. In other words, it is possible that the gunsmith made it possible to feel differences in bore diameter, rather than causing them.

A very light lapping of the breech end until 5 to 7 cm deep can be used to diagnose the problem further: If the O-ring groove edges are raised, lapping them down will happend quickly and change the feel of pushing pellets through with little effort. You can always lap more, but you cannot put metal back, so do less, then reassess.

If after a light lapping the feel does not change, then more lapping near the breech end to even out the bore can be used. You do not want to abruptly stop the extent of where you lap, but use overlapping strokes to blend into the rest of the bore. The goal may be to have the bore the same from breech to muzzle (before the choke), but having the breech slightly larger than the middle and muzzle end is also good. Having a tight breech and a loose muzzle is bad for grouping because the pellet will be able to rattle and yaw down the barrel, and not leave the muzzle straight and consistently.

The question about which lapping compound to use is going to get many answers. The rougher the compound, the faster it will cut. Some like a mirror finish, while others swear that a 400 grit finish or even rougher is better. Having even friction will be how you measure the bore diameter, and roughness feeds into friction. My attitude is if you match the LW factory gloss level you can't be hurting anything. No matter if that is considered "too smooth" by some experts. An actual mirror finish is not required.


You do need to be careful of your lapp leaving the muzzle and cutting a bell shape. Perhaps you do not plan on lapping all the way to the muzzle, because the tight section does not extend that far. That should make it easier.

At some point before you think a bit more lapping might be better. Clean the parts, assemble and shoot the airgun to see how it performs. If it shoots well and keeps shooting well for hundreds of pellets without getting so dirty that groups open up, or throw "fliers", then you may be done with lapping.

The video below shows how the HW100 barrel is retained. The deep groove machined for the angular positioned grub screw may actually be the source for the bore reduction at 5 to 7 cm; if the cutter was not as sharp as it should have been:

Watch video from about 9:38 from start:


Either way, lapping concentrated around the area of minimum diameter should help. Check frequently, rather than lapping too much...

Rather than overtightening the barrel retention screw, use wicking Loctite 290 added from the exposed end, after you are happy with the assembly and are concerned with the screw coming loose. The angle of the screw seems to make it less likely that the screw could squash the barrel than many PCPs, but the barrel wall is thinned out there.


next.henkel-adhesives.com

LOCTITE® 290

LOCTITE® 290 is a green, medium-high strength threadlocker designed for the locking and sealing of threaded fasteners. Because of its low viscosity, the product wicks between engaged threads by capillary action and eliminates the need for disassembly prior to application. This makes it ideal for...
next.henkel-adhesives.com next.henkel-adhesives.com
Click to expand...
Dear Subscriber, thank you veru much for the detailed explanation. Really appreciated 😊👍
I Will try to inspect the barrel again this week based on your instruction and update you once I have update.

Thank you
Aldhy
 
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Aldhy said:
Also I already tried to push pellet to the barrel and find loose after 5-6 cm and go further another 5cm.
Click to expand...
subscriber said:
Your comment about 8 to 7 cm at the breech being tight makes it sound more like a longer section of the bore has been reduced, possibly by tight gripping in a collet or chuck.
Click to expand...
This tight region sounds like the aforementioned issue present in most LW airgun barrel blanks. Mike of Thomas Air has called attention to it at times, for example:

POLISH AN LW POLYGONAL BARREL?

22lr. It's not too dirty at the moment, but I'm following the Vudoo cleaning method, so no solvents will ever go down this barrel, only C4 for the carbon ring.
www.airgunnation.com www.airgunnation.com

Working it with a poured cast lap and abrasive is indeed the best way to deal with it. In my experience, abrasive in the vicinity of 300-400 grit will take an absurd amount of time and effort. 180 or even 120 grit is advised.
 
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A cast "hard" lap is the best way to relieve a tight spot in a barrel, because it preferentially cuts the tight spot. Rougher grit can be used with a hard lap, but a soft lap such as a tight patch will cut everywhere the lap moves, including where not required.

Soft laps are OK for polishing the whole barrel, but not so good for relieving a short section of tight barrel. For that reason, finer grit should be used with a soft lap, or it will reduce land height and round land corners, rather than preserve the basic shape of the barrel bore. Yes, you can use a soft lap for preferentially removing material from the bore, by applying more strokes in tight areas, but you do not have the control to cut to an even diameter that you have with a hard lap.
 
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