Barrel Life

[Brillo]

Here’s a question for the gunsmiths on here.

If a barrel is set back completely, that is the whole tenon cut off and the barrel re-chambered, will the barrel life still be the same or is it likely to be longer? Assume bullets not pushed at light speed.

My previous experience of barrels going south is through throat erosion or fire cracking at tge start of the rifling, so weird logic says re-chamber should extend the barrel life.

[meles meles]

Might we offer a few observations, ooman ?

We'll begin with some caveats.

• First of all, we aren't a gunsmith;
• We may get shot down by those on here who don't have a pawful of metallurgy degrees;
• Our opinion is worth exactly what you paid for it.

Technically the barrel life will be shorter as you've already used some of it up. Now the linguistic pedantry is out of the way, let's think about reality.

Throat erosion occurs at the throat because that's the part of the barrel that sees the worst of the service conditions: basically high temperatures and pressures. The metal is, in a typical rifle, subject to pressures of 50,000 psi or more and temperatures in excess of 2,500 K. The weakest points of the iron from which the barrel is primarily made are the grain boundaries where there will be segregation of tramp elements as well as the intended, desirable, alloying elements. These, particularly phosphorus and sulphur compounds, will have lower melting points than iron and will try and melt. Probably succeed too. That's why the throat develops the 'crazy paving' look as it wears. This wear accelerates as the high temperature and pressure exploit these weaknesses, eventually causing metal grains to scab off. 

Once the throat is eroded, chopping it out and setting back the barrel brings fresher, less affected, steel into play again and the erosion has to re-start. Barrel life is thus prolonged in that sense. You may be able to set back the barrel several times depending on how long it was initially and what final velocity you require from it.

Now consider this too.

Maybe throat erosion doesn't matter as  much as you think it does.

Yes, throat erosion will increase the jump to the lands, and perhaps the initial pressure in the cartridge case and so forth. Somebody will be along in a moment to tell us all about that. They will probably be mostly correct too. However, if you aren't trying to eke out the highest possible muzzle velocity and aren't an Olympic marks'ooman, maybe the gun is still accurate enough for you. Wear and erosion at the muzzle, should it occur,  is far worse than at the throat. Even with a worn throat, the bullet will still be gripped by the good rifling further up the bore, centred and rotated. It will still exit the muzzle and spin on its way to the target at a goodly speed. Paradox guns only had a few inches of rifling at the muzzle and were fit for purpose. 

*Bolts down into the sett, prepares for incoming...*

 

[Popsbengo]

Dear Dr. Badger,

Why is "wear and erosion at the muzzle" far worse?  Assuming one is careful in cleaning practice and not plonking the muzzle in the mud how does the muzzle wear faster?

I assume you will be eating Xmas worms while we bald apes will be enjoying turkey/duck/goose/ham etc.  Tough life for you critters what with the cough and all.

[The Gun Pimp]

2 hours ago, Brillo said:

Here’s a question for the gunsmiths on here.

If a barrel is set back completely, that is the whole tenon cut off and the barrel re-chambered, will the barrel life still be the same or is it likely to be longer? Assume bullets not pushed at light speed.

My previous experience of barrels going south is through throat erosion or fire cracking at tge start of the rifling, so weird logic says re-chamber should extend the barrel life.

If you do your own barrel work, then chopping off the chamber plus maybe a bit more - depending on what the borescope reveals - is something I frequently do.  Example - my 6mm Dasher had done two seasons of benchrest - about 600 rounds with testing.  I could maybe have squeezed another season out of it but it would likely 'go south' half way through the season so, top n tail and squeeze another year out of it.

However don't expect the kind of accuracy you'll get with a new barrel.  If you do it yourself, it costs nothing and gives me another season rather than scrapping the barrel.  If I had to pay a gunsmith to do the work then - no.  It will cost just as much to chamber and thread your old barrel as with a new one so you may as well start with a new tube.

[lapua]

1 hour ago, Popsbengo said:

Dear Dr. Badger,

Why is "wear and erosion at the muzzle" far worse?  Assuming one is careful in cleaning practice and not plonking the muzzle in the mud how does the muzzle wear faster?

I assume you will be eating Xmas worms while we bald apes will be enjoying turkey/duck/goose/ham etc.  Tough life for you critters what with the cough and all.

He means it has a bigger impact on accuracy

[baldie]

To "set back" successfully, you have to whip an entire chamber length off, before the fire cracking and burning have extended much past that distance, otherwise , if left too long, the first 9-12" of bore could be torched. Not much point in setting back into that , is there ?

In some hot calibers, it might only be 300 rounds, or less.

You need to use a barrel with a ridiculously long tenon too.

[meles meles]

Dear Popsbengo,

Pertaining to muzzle wear,  we wrote 'should it occur', not that it would occur. Muzzle wear is far less likely to occur than throat wear,  accidental damage and corrosion being far more common but, as lapua says, it usually has a far greater effect. Most of the damage we have seen at the muzzle of a firearm tends to arise from botched attempts to fit or remove a moderator or muzzle brake (or bayonet), or poor cleaning  - often due to a moderator being left in place and an accumulation of hygroscopic combustion products then giving rise to corrosion*. 

Yours, 

Professor Badger

 

* We aren't a corrosion scientist or a tribologist either, but we do hang out with them occasionally... 

[Popsbengo]

24 minutes ago, meles meles said:

Dear Popsbengo,

Pertaining to muzzle wear,  we wrote 'should it occur', not that it would occur. Muzzle wear is far less likely to occur than throat wear,  accidental damage and corrosion being far more common but, as lapua says, it usually has a far greater effect. Most of the damage we have seen at the muzzle of a firearm tends to arise from botched attempts to fit or remove a moderator or muzzle brake (or bayonet), or poor cleaning  - often due to a moderator being left in place and an accumulation of hygroscopic combustion products then giving rise to corrosion*. 

Yours, 

Professor Badger

 

* We aren't a corrosion scientist or a tribologist either, but we do hang out with them occasionally... 

a-ha,  must pay more attention in future

[meles meles]

We're  currently listing 'Erosive/ corrosive wear of steel under extreme conditions of temperature and pressure' as a Master's Thesis at the Badgerkademy. If a student takes it up we'll be happy to share the results. Bald apes are a big brained species so we'd expect some good findings...