Switch- Barrel headspace requirements

[duster]

It would seem that switch-barrel rifles excuding the Savage would have special headspace requirements due to barrel thread wear, barrel shoulder wear, recoil lug wear etc.
Of course wear can be reduced with anti-sieze compounds and other lubricants, but sooner or later a rifle set up with minimum headspace is going to have a problem chambering cartridges.
Is the answer to chamber on the loose side, or have the chamber cut a little deeper when necessary, or is some other generally accepted way this is handled?

 

[EddieHarren]

If you are causing "wear" on barrel threads, action faces, recoil lugs and barrel shoulders, when you switch barrels, you are doing something very, very wrong.

 

[duster]

If you are causing "wear" on barrel threads, action faces, recoil lugs and barrel shoulders, when you switch barrels, you are doing something very, very wrong.

Actually, no I am not having a problem and I did not mention "action faces"since that area is not subject to wear. And threads do wear which is the reason that nice white assembly lube I use has turned a gray color in the threads when I remove the barrel. I also believe recoil lugs and barrel shoulders will wear or set-back with repeated assembly/disassembly.
Of course the amount of wear or set-back will vary greatly depending on factors such as the the type of thread lubricant used, how many times the barrel is installed, how tightly the barrel is torqued, use of a hardened lug, etc.
I am not talking about hunting rifles with several thousands headspace to start with. I am talking about a precision chamber such as in a benchrest or tactical rifle, where the headspace may be set at Zero to .001" Even if there is only .0001" combined wear each time the barrel is changed, eventually the chamber will be short.
My question was aimed at obtaining some practical knowledge on this topic. My question was aimed at how is such a problem prevented. I am asking are there any special headspacing requirements, specific assembly practices, or other ways of dealing with this eventuality?
Very possibly I am making more of this potential problem than the topic deserves.
Sorry but your answer did not contain any practical help. Thanks

 

[EddieHarren]

Well, first of all, I would like to know why you think the recoil lug and barrel threads will wear and the front of the action will not. They all three are part of the process of installing and changing a barrel. I don't think you will wear any of these parts out due to changing barrels. I have been building and shooting BR, Tactical and live Varmint rifles for nearly 20 years. I have a Light Varmint BR rifle that has had at least 15 barrels on it over this period of time. Some of these barrels I still use. I can unscrew one and screw another on and never see noticable headspace. I think you are anticipating a problem which does not exist. As for the "white lubricant" turning grey, put some Iosso bore paste on a white patch, rub it on the outside of a SS barrel. The patch and the Iosso paste will turn black. It does not mean that the surface is being worn out.

 

[duster]

Ok. To start Eddie thanks for sharing you experience and knowledge on this topic.
As for the front of the action, at least on my rifle, the recoil lug is pinned to the action so it is not free to rotate. Since there is no rotation movement between the action face and the lug so I would not expect to see any wear.
I would like to know what kind of clearance you give to headspacing? How tightly do you screw the barrels on? Do you go by torque when installing barrels or do you tighten by means of witness marks on the action and barrel after the initial set up? What calibers have you chambered switch-barrel rifle barrels in? Do you feel that the big calibers need to be screwed on tighter than small calibers?
Thanks in advance for your response

 

[EddieHarren]

You have a PM!

 

[J E Custom]

It would seem that switch-barrel rifles excuding the Savage would have special headspace requirements due to barrel thread wear, barrel shoulder wear, recoil lug wear etc.
Of course wear can be reduced with anti-sieze compounds and other lubricants, but sooner or later a rifle set up with minimum headspace is going to have a problem chambering cartridges.
Is the answer to chamber on the loose side, or have the chamber cut a little deeper when necessary, or is some other generally accepted way this is handled?

I am by no means an expert on this subject but I do believe it is not a good pratice
to switch barrels with out headspacing every time because there will be a change
in headspace at some point.

If you make up a barrel to an action with a set torque and match mark the two then remove
it and re torque it at the same torque it will go past the match mark by a small ammount.IT may
only reduce head space a small amount (.0001) but over time it will be enough to require
touching the chamber with a reamer to bring the head space back to the origanal specs.

I see no real advantage in switching barrels on any rifle because of the work required to do so
and the cost of re zeroing and possible accuracy changes buy changing headspace and scope
position every time you switch.

If you need another caliber then build another complete rifle for very little more.

And to answer your question- If you start with a head space around .003 you can probably
get away with a few switches but brass and accuracy will suffer from the excessive headspace
untill it reaches around ,001.

Each barrel of a different caliber even if they are the same contour will have different harmonics
and the scope/sights will have to be changed every time you switch barrels.

I know that some will not agree but thats what makes the world go around.

J E CUSTOM

 

[NesikaChad]

If I may:

A typical receiver runs about 38 to 45 on the Rockwell Hardness C scale. The actions are typically made from Chromoly or a heat treatable stainless steel.

Barrels typically run about 25-28 on the Rockwell C scale. They are typically made from either chromoly or a 400 series stainless.

Now it may be easy to say that threads are "wearing" and affecting headspace. Lets pretend that is in deed the case (which it isn't). If a thread wears, meaning there is material being transfered off of the flanks of the threads to be suspended in the lubricant used on the threads during assembly how would this affect the dimension between the face of the bolt and the datum line on the shoulder of the barrel's chamber? (that's longwinded talk for headspace) The answer is it wouldn't. All it's going to do is make for a progressively poorer and poorer thread fit between the barrel and the action. The distance from the shoulder hasn't changed any unless there is wear there too. (I personally am not buying it)

HOWEVER!

If a guy is assembling a barreled action and then taking it apart over and over again with the force of Apollo (assembly) it is reasonable to state that the threads are yielding a bit which will cause an elongation of the barrel tennon and/or receiver ring and this will certainly affect headspace.

Remember, the barrel is only 28 HRC max (only one barrel maker I know of hardens barrel steel to this number) As hard as that is, it's still quite soft and subject to a tensile yield.

Just something to consider.

Cheers,

Chad


Chad Dixon
LongRifles Inc.

 

[eddybo]

Recoil lug is pinned, then the only wear to consider is between the lug and tennon shoulder. I would think that you will wear the barrel out from shooting it before you have headspace problems. Wear on the threads only affects fit, not the distance between the boltface and datum of shoulder. It also seems that headspace would only become shorter if this were taken to the extremes, like shoot one switch, shoot another round switch. I would not even check headspace on it unless I was having problems chambering cases. With normal use I would say a non-probalemo

 

[duster]

message deleted due to double posting

 

[eddybo]

7 chages.....wow your headspace must have been zero to begin with. I have one that has been twisted on and off more times than I can recall with no problems but each had .003 clearance.

 

[duster]

If I may:

A typical receiver runs about 38 to 45 on the Rockwell Hardness C scale. The actions are typically made from Chromoly or a heat treatable stainless steel.

Barrels typically run about 25-28 on the Rockwell C scale. They are typically made from either chromoly or a 400 series stainless.

Now it may be easy to say that threads are "wearing" and affecting headspace. Lets pretend that is in deed the case (which it isn't). If a thread wears, meaning there is material being transfered off of the flanks of the threads to be suspended in the lubricant used on the threads during assembly how would this affect the dimension between the face of the bolt and the datum line on the shoulder of the barrel's chamber? (that's longwinded talk for headspace) The answer is it wouldn't. All it's going to do is make for a progressively poorer and poorer thread fit between the barrel and the action. The distance from the shoulder hasn't changed any unless there is wear there too. (I personally am not buying it)

HOWEVER!

If a guy is assembling a barreled action and then taking it apart over and over again with the force of Apollo (assembly) it is reasonable to state that the threads are yielding a bit which will cause an elongation of the barrel tennon and/or receiver ring and this will certainly affect headspace.

Remember, the barrel is only 28 HRC max (only one barrel maker I know of hardens barrel steel to this number) As hard as that is, it's still quite soft and subject to a tensile yield.

Just something to consider.

Cheers,

Chad


Chad Dixon
LongRifles Inc.

Chad thanks for the Rockwell info on receivers and barrels that was interesting.
I totally agree with your dissertation on thread wear not effecting headspace.
What my experience does not agree with is your explation of how headspace could change.
Tha gun is chambered for the large Weatherby cartridges. Barrels are torqued to 50 ft.lbs., which I do not consider the force of Apollo. Thread lube is used each time. The barrel was changed 4 times last year and 3 times this year. Now the bolt will not close on the proper "GO" gauge for the respective barrel.
Obviously between barrel shoulder/ Lug wear and/or barrel tennon/receiver stretching something has changed.
My origional question was should the gun be set up with "special " loose headspace to delay /avoid this eventuallity?
Bullets could be jammed into the rifling to "fire form" cases.Sizing dies could be backed off to fit the larger chamber then cranked down as headspace is reduced, for whatever reason.
My own experience closely parallels that of JE Custom.
I am starting to think switch-barrels are not worth the trouble.

 

[duster]

7 chages.....wow your headspace must have been zero to begin with. I have one that has been twisted on and off more times than I can recall with no problems but each had .003 clearance.

Thanks for your input eddybo. I am thinking that the kind of clearance you have should be considered minimum clearance in a switch-barrel rifle. Apparently mine was set to zero or very close to it, which I firmly do not believe is practical in a switch-barrel.
Of course fine gunsmiths like to set headspace as close as possible for increased accuracy,and take great pains to do so. I just do not believe this is a prudent practice in a switch-barrel.
Tight headspace in a switch-barrel ends up being a problem when FL resized cases will not fit back into the gun.

 

[Locknload]

Duster,
It sounds to me like you have something less than a true switch barrel gun and the other folks trying to help you have, at best, limited experiance building a true switch barrel rifle.

I believe what you probably, actually wanted, was a take down rifle with an extra barrel. Am I right? You wanted a rifle that you could if desired, change calibers during a hunt.

I built such a rifle and helped build a second rifle that was never finished due to the death of the master gunsmith. The first rifle is built on a M-70 Classic Action in 338 Win Mag / 7mm Rem Mag. The second gun was built on a M-700 Rem action with a thick locking lug welded to the front of the action ala M-70. That 700 is in 280 AI and the second barrel in 22-250. It is in the estate.

This started as a problem put to me by a master gunsmith, who's shop I often worked in as a hobby. Can a switch barrel rifle be built that would be a marketable and a viable product (expense, time, sale price, etc) to offer from a small shop.

When I explain this to you, you will have all of your answers.

The problem with your "switch barrel" setup is that you do not have a way to stop the rotation of the threads at the same point every time and lock up the barrel in the action. The way you correct the problem is to make a spring loaded, tapered plunger, that goes down through the recoil lug, into the bottom of the barrel shank and is round at the tip. The hole in the shank matches the plunger. Once the plunger drops into place, a set screw is screwed down tight onto the plunger's end, locking it into the barrel and the barrel into the action. The barrel is at the same time matched to two witness marks, 1/8 turn out, which your eye will align every time.

The setup requires a spanner wrench with a tit welded on one jaw to match another hole in the bottom of the barrel. Sometimes it requires two people, one holding the action and the other using the wrench to unscrew and screw in the barrel.

How does it work? .56 inch 338 Win Mag and .70 inch with the 7mm Rem Mag.

I think you might already understand from your experiance that as long as the barrel is only tightened to point x every time, there is no headspace difference and the wear with a good greese is negligable. Will it wear, yes, because all swithch barrel guns do eventually wear, but as has been mentioned here, it is most probable the bore will wear out first. Th switch barrel guns will also not take a lot of abuse.

Those who do not understand the process have not thought out the string as to what two scopes with matching QD rings mean to such a gun. Mine returns to zero every time for both barrels.
Best,
Ed