What thread size to have a barrel threaded?

[Scotch_egg]

There doesn't seem to be a standard size to have a barrel threaded for sound moderator. What is the general accepted size to go for. I have been told 1/2 inch unf is too small and want to make sure that when I change my kit I will have some compatability.

 

cheers

[Bambi-basher]

I have been told 1/2 inch unf is too small and want to make sure that when I change my kit I will have some compatability.

 

cheers

 

I have ½-20tpi on all my rifles for years with no problems, on a sportier weight barrel you will have difficulty having anything larger, if it is a heavy barrel then you could go to 5/8 or 14/16mm but remember if it is a over barrel moddy and not a magnum then it will not offer any advantage IMHO. For illustration the tensile strength of a M12 thread is around 19 tonnes to give you a idea of how strong a thread is.

 

Best rgds

 

B-b

[njc110381]

All my rifles are 1/2" unf. Unless you have a very heavy barrel, that's what I'd go for. I've heard a small thread like this can reduce accuracy on a heavy barrelled gun, but I'm not sure how right it is. My CZ varmint .223 is cut to it and shoots fine.

[22/250 foxer]

i have just had mine done with a m18x1 thread which is the same as tikka and some sako use on there varmint rifles

[eldon]

The general thought is that especially on hammer forged barrels you want to keep as much meat as possible to retain accuracy. By removing metal you are taking some of the inherent stress out which is not always good.

 

1/2''unf is normally classed as too small for 30 cal, what calibre are you wanting to use?

 

A finer thread has more chance of staying tight than a coarse thread so a metric option maybe more desireable.

 

On my 308 i have 5/8''unf and on the 222 I have 14 x 1 and the 22 is 1/2'' unf so as you can see one thread unfortunately do's not fit all!

 

BB the thread strength is not normally the issue I think it is the effect on the remaining barrel after threading that is the main concern. As you rightly say the thread strength is probably well in excess of the application.

 

Go for the biggest sensible size.

[Alycidon]

My number 5 Shilen is threaded 5/8 x24, marked 5/8EF on the T8. That to me seems a bit small and I certainly would not want to remove any more metal at the muzzel. Problem is that at present only limited mods are available in this thread, I wanted another moddy of the same size for my 20 to give a degree of interchangabilty. I don't want another T8, nothing wrong with the one I have as far as I can see but your hear so many things, I like to be able to pull them apart and spot any probs. So I guess we will just have to see what the future holds. Hopefully we may get a CQB in that size in the future.

 

A

[Bambi-basher]

BB the thread strength is not normally the issue I think it is the effect on the remaining barrel after threading that is the main concern. As you rightly say the thread strength is probably well in excess of the application.

 

Go for the biggest sensible size.

 

Your point is a valid one and makes good engineering sense. Although in practice I am not aware of any problems with using the 1/2unf on 30cals, is anybody aware of any actually failing in use?

 

Best rgds

 

B-b

[onehole]

I think the main problem with going to small is the effect it can have on the possible opening up of the internal bore dimensions thus creating an oversized exit section ie the last 1/2 inch or so and thus affecting accuracy.I believe a cut barrel,buttoned rifled barrel,hammer forged barrels all react differently with regard to disturbing the sleeping stress of a barrel that has not been disturbed{ie screw cut}

Anschutz for example back bore their 17mhr factory screw cut rifles to a point where the barrel retains its original thickness and cant beleive they do this for no reason?I think the way forward is in any instance to have your barrel threaded/cut for the max size possible thus creating little or no effect on its internal pre stressing and internal dimensions.

Remember a projectile is being worn away as it travels along the bore and the last thing you want is a loose section at the last part of its travel??Probably talking a load of bull that which promotes growth and vigour but that final exit and perfect crowning is really key to accuracy.Onehole.

[col48]

Your point is a valid one and makes good engineering sense. Although in practice I am not aware of any problems with using the 1/2unf on 30cals, is anybody aware of any actually failing in use?

 

Best rgds

 

B-b

 

yes, some barrels have been known to open up slightly at the crown, i would never recommend putting 1/2" unf on 30 Cal's

 

but you are the engineer and must know best

[17 Rem]

Your point is a valid one and makes good engineering sense. Although in practice I am not aware of any problems with using the 1/2unf on 30cals, is anybody aware of any actually failing in use?

 

Best rgds

 

B-b

 

Never heard of anyone with a 1/2"x20 UNF threaed muzzle on a 30 calibre barrel having any problem.

I certainly had no problems with any of the .308's I owned and used extensively which had that thread.

[Bambi-basher]

The pressure at the end of the barrel will be were it is least, the bullet will of surely swaged to the size of the barrel in the first 20”? The moderator bush would prevent this happening to a degree providing the tread is well machined.

 

I have a mate that has a 20” barrelled Remmy 700 with ½” thread and it still shoots 2” at 500yrds so maybe the opening up of the barrel can’t be that bad.

 

I don’t dispute that putting a larger thread on the barrel is technically a better option, but in practice does anybody have any evidence of this opening up of the muzzle? Ie it was .308 bore before firing and now is .310 or what ever after?

 

Col, an internet link or the title of a book with this evidence in would be perfectly acceptable from you.

 

That the minute I am unconvinced that the theory actually happens in practice, but are willing to accept the evidence if it is out there.

 

Is JR about?

 

Best rgds

 

B-b

[eldon]

BB why do you need hard and fast proof of this? If you want to do it, do it, its your choice. I think it boils down to whether its a good idea or not, not whether it is possible.

 

There seems to be a general consensus that 30 cal and 1/2unf is not a good combination.

Now bear in mind 1/2 unf will be nowhere near this size at the root of the thread. This will be more in the region of 10mm,without looking at the charts. It will look bigger as you look at the face not the root. Minus the bore size of say 7.62mm leaves us with around 2.4mm. Divide this by 2 and the wall thickness is only 1.2mm

 

Another general theory seems to be that a fat barrel (bore) in the middle doesn't matter but a bell mouth is detrimental to accuracy.

Is the 1.2mm wall going to hold the dimension?

 

If ever you machine a press fitted piece, when assembled you can't get the tension, grip, if the walls too thin as it will conform to its surroundings.

 

Its a bit like certain homeloads, in some rifles they work great and safe, in others maybe not ideal and then theres others that work in most rifles. This is probably a similar thing.

[Bambi-basher]

BB why do you need hard and fast proof of this? If you want to do it, do it, its your choice. I think it boils down to whether its a good idea or not, not whether it is possible.

 

 

I don’t doubt that putting a larger thread on a 30 cal is good practice, however if you want to use one moddy on more than one rifle or you have a light weight stalking barrel on your rifle the option of the bigger thread might not be possible, hence why I am labouring the point.

 

I am aware that a opening up of the muzzle would affect accuracy and that a slightly choked barrel is used by some manufactures to aid accuracy, but what if you really want to use ½ thread for practical reasons on your rifle? Realistically what are the chances of this being successful?

 

My brother has a 308 with ½ unf on the end, I will mic’ it up at some time to see if it has “opened up”.

 

I make the wall thickness slightly more than 1.2mm matters not, but why measure to the root of the thread? Are not the top of the threads adding to the support of the wall thickness? What ever size it is matters not, it depends on how much pressure is being exerted on it, and what level of support the bush of the moderator provides to the barrel when screwed on and is putting the threads under tension.

 

I am not being argumentative for the sake of it; I just like to challenge what seems to be perceived wisdom when it is at odds with what I have seen work in practice.

 

Best rgds

 

B-b

[ejg223]

I'd be a bit worried about bending that flimsy thread if one falls on the rifle in the field.

A long moderator attached acts as a nice lever. I also have a nice bit of play in the back bush

so that wouldn't really help.

 

edi

[eldon]

No BB I dont take your comments as being argumentative and I think its a good point to discuss.

 

The reason I highlighted the 1.2mm which is purely mathematical is to point out the flimsyness (is that a word) of the attachment point. This is the minimum thickness i.e. weak spot of that area and purely from a strength point of view. Yes the thread flanks will provide support. If the thread has a relief cut at the back of the thread which it should have then no additional support is available in that area

 

If the mod is T8 type then this is probably immaterial but if it is front mounted then this could be more important.

 

I once jumped off a wall and on landing the front sling stud pulled out. The rifle spun round and the muzzle dug into the floor and plugged it with mud about 1 1/2'' deep.

 

In this circumstance would you want the 1.2mm or whatever holding the baffles and subsequently the whole mod in line?

 

Quote

I just like to challenge what seems to be perceived wisdom when it is at odds with what I have seen work in practice.

 

Sometimes engineers over engineer and some things work that are paper proven not to

 

On a more practical point you could, although I am NOT advocating its use, only pointing it out for information purposes etc etc,

buy the biggest thread of your rifles in the mod and then machine a boss to step up the smaller barrels. Yes I have seen it done and you only need to run x2+ back bushes with the small barrel(s) bush split for fitting.

 

I definetly wouldn't consider this for a front mounted item!!!!

[Bambi-basher]

The reason I highlighted the 1.2mm which is purely mathematical is to point out the flimsyness (is that a word) of the attachment point. This is the minimum thickness i.e. weak spot of that area and purely from a strength point of view. Yes the thread flanks will provide support. If the thread has a relief cut at the back of the thread which it should have then no additional support is available in that area

 

The amount of material left, even in the under cut area will be small but will be directly supported by the parent material of the original barrel and the thread on the other side of the under cut, this will resist radial forces but not lateral forces.

 

The thing is I can now think of 3 people running 308s with ½ unfs plus the post on here, without apparent problems are they lucky? Or is the theory flawed?

 

Here is a theory for you.

 

Thinking logically the bullet is slightly bigger than the bore and is swaged into the bore/rifling at the start of ignition, the bullet being a soft lead core with a thin soft copper jacket forms the to size of the bore initially putting radial pressure on the bore, but as it moves up the bore it has assumed the exact size of the bore and has lost some of it copper jacket while moving up the bore reducing the diameter of it further:- There fore by the time it reaches the muzzle it is at nominal bore diameter and exerting minimal radial forces on the threaded part of the barrel, further more the bullet being longer than where the under cut and roots of the threads are can not exert full force on these points in addition the larger diameter lands of the threads are supporting either side. The moderator bush or more likely the fact that the threads are in tension because of the tightening effect may all so aiding the thread part of the barrel to resist the radial forces.

 

The burning gas behind the bullet are at their lowest pressure just before the bullet leaves the muzzle, that couple to the fact that the bullet is at it’s smallest diameter now and moving at it greatest velocity would explain why maybe this has little effect.

 

If the mod is T8 type then this is probably immaterial but if it is front mounted then this could be more important.

 

I agree I don’t think you would have a problem even with a loose back bush, as Edi has mentioned no back bush fitted and drop it then you could be in trouble. Just front mounted moddy you would have to be careful although the bigger shoulder by having a smaller thread on the muzzle would help resist any bending effect , but agreed from a mechanical handing point of view go bigger if you can.

 

I once jumped off a wall and on landing the front sling stud pulled out. The rifle spun round and the muzzle dug into the floor and plugged it with mud about 1 1/2'' deep.

 

Had that happen to me once with a parker hale, since then I have inletted a small steel plate into the stock forend; thread for the a machine thread swivel.

.

Sometimes engineers over engineer and some things work that are paper proven not to

muzzle dug into the floor and plugged it with mud about 1 1/2'' deep.

 

In today’s world you can’t be blame for over engineering what with lawyers ect. But like you say lots of thing do not work in practice and while looking great on the drawing board/CAD program. Fun enough I have got to sort one out for work tomorrow just like that.

 

In theory should not a bumble bee be unable to fly?

 

On a more practical point you could, although I am NOT advocating its use, only pointing it out for information purposes etc etc,

buy the biggest thread of your rifles in the mod and then machine a boss to step up the smaller barrels. Yes I have seen it done and you only need to run x2+ back bushes with the small barrel(s) bush split for fitting.

 

Good advice.

 

Best rgds

 

B-b

[eldon]

BB

I have to agree with what you said especially the bumble bee anology. I always think pigeons make flying look like hard work.

 

Hopefully scotch egg this has answered your questions in some way. If not comeback and we'll see what we can do.