Sub-moa barrels

[subscriber]

If it is Bartlein, they single point cut rifle their barrels: https://www.bartleinbarrels.com/

The problem with bore springing does not apply to single point cut rifled barrels, because the process leaves almost no residual stress in the steel.

Contour Info | Bartlein Barrels

www.bartleinbarrels.com

"Why is single point cut rifling better than button rifling?
A lighter weight contour when the barrel gets button rifled (Bartlein Barrels only does single point cut rifling) the stress being put into the blank is much greater. As you turn the contour smaller and smaller in size this can have a negative effect on the bore sizes changing and opening up. When this happens you cannot make the bore size smaller. This is not an issue with single point cut rifling. We don’t see this happening at all. Same can be said about threading the muzzle of the barrel for a muzzle brake or suppressor attachment. On button rifled barrels, threading the muzzle this is a common problem with the bore opening up. The last place you want the bore to open up or go sour on you is right at the muzzle's crown edge. Anything going wrong here has a negative impact on accuracy."

 

[caliusoptimus]

If it is Bartlein, they single point cut rifle their barrels: https://www.bartleinbarrels.com/

The problem with bore springing does not apply to single point cut rifled barrels, because the process leaves almost no residual stress in the steel.

Contour Info | Bartlein Barrels

www.bartleinbarrels.com

"Why is single point cut rifling better than button rifling?
A lighter weight contour when the barrel gets button rifled (Bartlein Barrels only does single point cut rifling) the stress being put into the blank is much greater. As you turn the contour smaller and smaller in size this can have a negative effect on the bore sizes changing and opening up. When this happens you cannot make the bore size smaller. This is not an issue with single point cut rifling. We don’t see this happening at all. Same can be said about threading the muzzle of the barrel for a muzzle brake or suppressor attachment. On button rifled barrels, threading the muzzle this is a common problem with the bore opening up. The last place you want the bore to open up or go sour on you is right at the muzzle's crown edge. Anything going wrong here has a negative impact on accuracy."

Hey don't take my word for it... listen to frank green at 13:30

 

[subscriber]

I believe you, caliusoptimus

I was making a conditional statement that applies to button and hammer forged rifling; not cut as Bartlein does.

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[Jaxjax]

Or normalize the barrel in an inert atmosphere so that when you contour the barrel, there is no residual stress to spring it open where steel has been removed from the OD. This applied to hammer forged and button rifled barrels. On the other hand, hammer forged barrels are sold for their work hardened bores, and that will go away if you stress relieve them, before contouring.

Name a barrel maker that contours barrels before rifling them?

How do we avoid above said problems when we do fitment into receiver . I always wonder about this messing up a barrel potential on button barrels. I had Lilja do a SS .177 1/26 3or 4g for me. They did it at .75" then had it fit to .625 just in reciever area of a RAW. I thought I was safe on the .75 contouring as I thought they would measure TIR .?

 

[subscriber]

Your situation should not cause a functional problem: reducing the barrel OD at the breech would tend to expand the bore a small amount. Providing the projectile fit there is not so loose that it drops into the barrel, the projectile simply encounters a slight but very long parallel choke beyond the turned down section.

The risk to be avoided is of the projectile becoming loose towards the muzzle. That is why airgun barrels have muzzle chokes. Your potential choking affected barrel would not go loose towards the muzzle, assuming the 0.75" OD is constant all the way to the muzzle. And that the barrel had a parallel bore to begin with. So, you should be OK.

If you turn the barrel profile down towards the muzzle, then the breech end would be tight, and would size the projectile down, to then travel into a bore that becomes looser towards the muzzle. That is a bad situation for shooting well. Your barrel should not have that, unless you taper or step the OD down towards the muzzle.

You could get away with tapering the barrel down towards the muzzle, if you left and inch or two long section at the original OD, right at the muzzle. Many German guns that were made over a 100 years ago had such a reinforced muzzle look, after tapering to a thin walled OD; even if it were not done for a choking purpose (image below).

The other fix would be to choke the barrel near the muzzle, after tapering it. But, that seems like risking the goodness of an expensive quality barrel.

If a barrel OD is reduced evenly over the whole length, the bore should spring open evenly, and not affect how well the barrel shoots. Not unless the bore was on the large end of the spec (what spec?); and your projectiles are on the small size. Consider that Lothar Walther airgun barrels have bores several 0.001" larger than Weihrauch, for example. Yet LW barrel are known for shooting well. When they don't, people find larger diameter projectiles and usually succeed; and / or choose the chocked version of that barrel.

 

[Jaxjax]

If you turn the barrel profile down towards the muzzle, then the breech end would be tight, and would size the projectile down, to then travel into a bore that becomes looser towards the muzzle. That is a bad situation for shooting well. Your barrel should not have that, unless you taper or step the OD down towards the muzzle.

I didn't want to risk any problems at muzzle so I went with simple crowning prep & NO threads. I suspected alot of what you said so I didn't want any contouring after the fact. Makes me wonder about what's left on the table with all the muzzle threaded CZ barrels & LW for that matter. If your effectively added a makeshift choke by threading muzzle it seems that's exactly what a NO choke barrel user would not want.. That said my muzzle threaded CZ barreled Gen 1 Uragan is lights out with .22 pellets

 

[subscriber]

Jaxjax,

The devil is in the details. Reducing a 0.75" barrel to 0.5" will "unchoke" it. Threading it after is likely to "re-choke" it. So, you may end up with very little to no choke.

Thread rolling (not used for barrels as far as I know) would choke a tube the most, followed by cutting it with a die (amateurs method). Gentle single point threading with a sharp cutter would do less choking. Thread milling on a machining center (combined lathe and mill) would do even less to the steel beneath the thread.

What I am saying is that many people seem to be getting away with threaded barrels; and shooting well enough. If you don't thread the barrel, how would you attach a moderator? Or does this barrel live in a shroud? Or, will you use hearing protection?

You do not have to thread to 1/2-20. A larger thread diameter would likely produce a better, more stable result.

Anyway, I understand that you want to apply the principle of "first do no harm". Nothing wrong with being cautious.

 

[steve123]

I'm not a machinist but I had a thought. What would happen if a adapter was made to be threaded to fit onto the moderator on one end, then the other end was made for a press fit onto the barrel? Then the barrel cooled in a freezer, and the adapter heated so it'd expand, then pressed onto the barrel.
I imagine the barrel would become slightly chocked as well as the adapter would be considered permanent.

Consequences of?

 

[Scotchmo]

I'm not a machinist but I had a thought. What would happen if a adapter was made to be threaded to fit onto the moderator on one end, then the other end was made for a press fit onto the barrel? Then the barrel cooled in a freezer, and the adapter heated so it'd expand, then pressed onto the barrel.
I imagine the barrel would become slightly chocked as well as the adapter would be considered permanent.

Consequences of?

Loctite bearing mount (641, 680, etc) and a snug slip fit.

 

[PowderBurnt]

Can't we just go back to smoothbores, the way god intended?

 

[subscriber]

Scotchmo's suggestion would not induce any stress in the barrel; while a light press fit would. If the length of a light diametral interference were long enough, it should provide adequate holding power, with minimal choke on such a heavy barrel in .177. But achieving a precise light press fit is much more work than a slip fit with Loctite.

I am not sure that a choke of 0.0002" would matter, or if you could measure it. Start with, how even are the barrel land and groove diameters, all the way down, breech to muzzle. If that is on the order of +- 0.0002", just make sure the muzzle is the "tight" end. On the other hand, I think button rifled barrels should have the projectile travel in the same direction as the button, from a friction and fouling perspective - due to induced steel crystal "lay".

44 years ago I learnt how to calculate the strain on interference fits "by hand". Now I am pretty sure there is software out there that can do it - such as this online calculator:

Interference (Press - Shrink) Fit Calculator

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