Thread size & barrel diameter

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Look deer

Well-Known Member
Hi all


Just looking for some advice from the more experienced:

I am getting a heavy barrel CZ thread cut, and the moderator i am looking at is threaded 1/2 UNF.

So here's my question: Is it advisable to thread cut a heavy barrel to this or should I be looking at M18x1 etc

Would appreciate your advice

Thanks
 

Sharpie

Well-Known Member
Is it advisable to thread cut a heavy barrel to this or should I be looking at M18x1 etc

Some people would do this, perhaps because they already have a moderator with this thread, or want to standardise on the same thread as other rifles already owned.

But if you are starting from scratch, much better to use a larger thread.

5/8 UNF is the larger common imperial thread. M14x1, M15x1 and M18x1 are often found. M14x1 is pretty standard on Tikka, Sako etc.

The finer pitch of the metric threads mean they are less likely to loosen, and less metal is cut away in the thread.

Don't over-do it though, there needs to be a decent (and well cut) shoulder for the moderator to line up on.

Also consider using a spigotted thread, which is the technically superior method, though not particularly common. M17x1 spigotted is pretty much the standard.

If it is .30 calibre I'd never do it, because there would be hardly any metal left between the bottom of the thread and the rifle bore. About 50/1000" if I've got my sums right, assuming the thread is cut perfectly central to the bore.

There is also the possibility that any internal stress in the barrel could cause the threaded portion to swell when so much metal is cut away, which would do nothing for accuracy. CZ use hammer forged barrels which have the highest internal stresses.
 
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palo

Well-Known Member
My cz varmint 22lr is threaded 1/2 unf and it performs well.
I was told all rimfire moderators came with this thread but I dont know if this is true or not.
 

Sharpie

Well-Known Member
My cz varmint 22lr is threaded 1/2 unf and it performs well.
I was told all rimfire moderators came with this thread but I dont know if this is true or not.

You are quite right, if this is a rimfire then 1/2" x20 UNF would be the standard thread and is the sensible one to use. I was talking about centrefire rifles where the decision is more complicated.
 

Tim1

Well-Known Member
Also consider using a spigotted thread, which is the technically superior method, though not particularly common. M17x1 spigotted is pretty much the standard.


Agreed, I would definitely go with a spigot.


For what it's worth, all my sporter profiles are M14 spigot. I don't have any heavy barreled rifles.

Kind regards,

Tim
 

saddler

Well-Known Member
I see 1/2" UNF threads on a lot of .30-cal rifles

Makes about as much sense as an ashtray on a motorbike


I had my heavy barrel .223 threaded 9/16 x 24...as that was the largest dia. thread the factory could do at the time...


1/2" UNF is the thing for .22lr bunny guns - anything else = :cuckoo: :rofl:
 

bewsher500

Well-Known Member
I have a:
.17M2
.17HMR
.22WMR
.22LR
.222Rem
.243Win

all sporting barrels, all threaded 1/2" UNF

​plenty of meat on the bore end of the .243
 

Whitebeard

Well-Known Member
Also consider using a spigotted thread, which is the technically superior method, though not particularly common. M17x1 spigotted is pretty much the standard.


Agreed, I would definitely go with a spigot.


For what it's worth, all my sporter profiles are M14 spigot. I don't have any heavy barreled rifles.

Kind regards,

Tim

Tim spigotted is definitely superior but you also have to have the corressponding counterbored mod, not all mods are machined for spigotted installations.

Ian.
 

bewsher500

Well-Known Member
what is "spiggoted"?

I was thinking it is the non threaded part between thread and barrel. but this description doesn't fit my mental picture
 

bewsher500

Well-Known Member
oh....

why is that superior?
surely the best contact points for an overbarrel mod is the thread and the rear bush?
Does the mod contact all along the "spigotted" part?
 

Sharpie

Well-Known Member
why is that superior?
surely the best contact points for an overbarrel mod is the thread and the rear bush?
Does the mod contact all along the "spigotted" part?

The thread is not a contact point. It is just the means to apply pressure to the contact and alignment area, which is the shoulder, whether it is spigotted or not.

To some extent a slightly loose thread is superior, because it allows the assembly to align properly at the contact point i.e. the shoulder. The spigot is a tightly toleranced feature, and a mod. manufacturer who properly understands the feature will ensure that the mating hole is similarly precise.

The spigot is not a contact feature, it is for alignment. It assures concentricity, which neither the thread nor the shoulder do.

It also acts as a safeguard, a secondary alignment feature, in case the mod. comes loose.

The spigot also extends the distance between the thread, and the shoulder. The longer this distance the more precise the alignment. By tightly tolerancing the spigot and its alignment with the shoulder the precision of the thread becomes less important, indeed a looser thread is desirable.

A similar result might be achieved with a very long threaded portion, but this would be impractical and have a great risk of jamming, and not allow the slight float necessary for the mating with the shoulder.

Those who don't offer it perhaps are not capable of working to the necessary tolerances, or use unsuitable materials for the thread, e.g. aluminium alloy.

The rear bush on a reflex design is not an alignment feature, it is there for safety, in case the mod. comes loose, or is knocked. It should ideally have a slight clearance with the barrel to ensure it doesn't touch and alter barrel harmonics, hence it does nothing for alignment just safety.

It is an antiquated design feature, introduced with the original BR Tuote design e.g. T8, which was designed for military applications, automatic fire, cost effective loosely toleranced mass production (welded together from inexpensive materials). On e.g. the T8 the bit where the thread is attached is rather flimsy, attached only by a welded three-legged spider of thin sheet metal. The bush serves to protect this weak feature from being bent by a modest knock.

Ase Utra pointedly eliminated it in their Northstar reflex design, which they designed primarily for the UK market where they were missing out because of our peculiar obsession with reflex designs.

The large expansion chamber of the reflex mod. is important when designing for sustained rapid or automatic fire where large internal volume is important. For our purposes it is an unnecessary weighty irrelevance that merely serves to cook the barrel.

It continues to be incorporated by "designers" who either do not understand the basics, or continue to provide it cynically because that is what the market demands.

Now back to the point,

Here is a useful table of recommended thread sizes and lengths for non-spigotted barrels, depending on barrel diameter, to ensure a decent shoulder.

Maintenance
 
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Sharpie

Well-Known Member
PS: I should say that Peter Jackson's explanation in the link is slightly confused IMHO.

Perhaps the BR Tuote design is sufficiently imprecise and weak that the rear bush serves a necessary alignment purpose, but that conflicts with every other advice, where the rear bush is supposed to have a (small) clearance with the barrel.

In Jackson's explanation the purpose of the spigot is not properly understood, and if fitted as described it will not be fully exploited.

IMHO.
 

Look deer

Well-Known Member
Thanks for all the guidance

So- just to be clear- if I'm buying a moderator I need to specify that it is for a spigotted barrel?
 

Sharpie

Well-Known Member
Thanks for all the guidance

So- just to be clear- if I'm buying a moderator I need to specify that it is for a spigotted barrel?

Yes absolutely. Whilst you could screw a none-spigotted mod onto a spigotted barrel it wouldn't be properly supported, and the moderator thread would be damaged, even jammed. There is a way of making it work (a sleeve over the spigot to bring the shoulder forward), but its not ideal.

A mod. designed for a spigot will not fit onto an ordinary barrel thread at-all.

If you want to follow this route, is best to:

Choose your preferred moderator.

Contact the manufacturer to confirm that they can supply it threaded for spigot

Obtain an installation drawing from the manufacturer and ask your 'smith if he is happy to thread and spigot the barrel according to the detailed drawing. He may have a reason why the choice is unsuitable, e.g. fluted barrel, not enough meat etc.

If you are having the barrel shortened too, be very cautious. Many barrels are not straight internally. Though the bore is concentric at muzzle and breech (barrel profiled between centres) when its cut down you may discover the muzzle is no longer concentric, sometimes badly so. This can be a big problem.

Only then buy the mod. and get the barrel cut.

The disadvantage of the spigot is that your choice of moderator manufacturers, and secondhand moderators etc. will be more limited than if choosing a more standard thread.

Those that do offer a spigotted fitting often have interchangeable threaded sections, meaning if you ever change rifle for one with a different thread, you would only have to change this bit. Much cheaper than buying a new. mod. Worth considering.
 
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