Barrel loose in the middle after machined at breech

[Aldhy]

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

 

[subscriber]

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

 

[ChuckHunter]

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

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!