Sound moderators

[onehole]

Hi,now heres a question that may spark some difference of opinion.Lets talk specifically about one calibre for the moment "308" what would you say is the correct clearance or put it another way for the size of the exit hole for a sound moderator in this calibre ?

To kick off I use an Ase Utra compact with a 8.5mm exit hole and this conforms to what "Jackson Rifles" info says is suitable.In this format my rifle presents no problems and is spot on accurate but doing the maths leaves only .88mm or 35 thou clearance,thats .44mm or 17 thou appx around the head on its way through!!!!now is that acceptable?must be I guess but just seems very close to me?

Would be good to here some "smiths" comments with regard to this as they would relate to this measurement more than most.

I will have some more questions following the answers to this one.thanks Onehole.

[Scotch_egg]

I would like as tight exit hole as possible. Now if everything lines up you can work with tight tolerances.

 

 

A good friend of mine who we call The Dingle recently purchased a .308 howa. He already owned a nice tikka .243 with a T8 mod upto .25. Now he comes down to my place with the new rifle and asks if this here mod will work. I quickly point out the difference in hole size. Dingle is quick to reply so if I put a round through it will open up nicely?

 

I managed to stop him shooting a round through the mod but he went and put the 8mm drill bit down it when he got home!!! Darn thing shoots though....

[Ronin]

Generally the less clearance the better sound attenuation..............,,,however,,,

 

 

 

 

and this is a BIG, however.

 

 

 

If the threads are not cut concentric with the barrel bore or the shoulder isnt cut at 90 degrees or even worse in the case of reflex style mods, the bore is not in the centre of the barrel (trust me ive seen some custom barrels that are .020" off centre - it isnt easy drilling a 30 cal hole 28 inches deep and getting it totally straight)

 

 

All the above will cause the mod to be off centre, a thou or two out at the crown isnt a problem, but transfer that to the mod "crown" and your boreline is out .020" - .040" or even more - which is why all moderators are slightly larger than calibre.

 

I prefer to see at least .020" clearance all around (thats .040" total) if not more - as there is always the issue of the mod working lose at the range when repeated unchecked fire may twist the mod a little loose and disaster strikes.....

[eldon]

Thought I read somewhere that the general idea was 1mm (39 thou) bigger than the bullet size. Bigger will work but then attenuation isn't as good. Be interesting to see a comparison between say a 243 rifle with a 308 mod and a 243 mod. I wonder how much real difference it would be to the mark 1 earhole.

[MJR]

Think I can answer that one. When I was looking for a mod for my .243 I already had a .308 on my ticket so I wanted one mod to fit both rifles. The Guy I bought the mod off was very helpful(Mike Dickinson)and had both calibres of mod in stock of the same make. We duly tried the .308 and the sound reduction was acceptable. Then we tried a .243 mod and the difference was marked, a further reduction in sound was very evident. No scientific equipment was used but the difference was there. I stuck with the .243 mod.

[onehole]

So nobody screaming out that these clearances are too tight then.Right,so if I want to send a 6.5 head down a 7.5 hole this should be OK then,assuming machining etc is spot on?

If you think not then please explain why as if its OK for these tiny clearances for the 308 why not in the 6.5 scenario?I have been told by an extremly respected "smithy" this is a big no no??? and the moderator must have a bigger exit hole.

Any comments please,cheers Onehole.

[eldon]

Onehole, it all sounds good to me.

 

Concentricity is the key. In theory if everything was perfect you could go a lot tighter but that is theory.

 

As the saying goes, "in theory, practice and theory are the same thing, but in practice they are not".

 

I personally would feel ok with a 7.5mm hole on a 6.5 bullet as long as you were machining and not using a top quality chinese tap and die set and a file.

 

Cheers MJR, your findings are noted. Obviously there would be a difference, I just wondered how much. I have both a 243 and a 308 reflex but they are different threads so I couldn't try it for myself.

 

Onehole, out of curiosity, what size would your respected smithy suggest then for a 6.5 mm bullet?

[deerman]

As many have said already, it is all about build quality, i.e. concentricity, perfect alignment of all baffles with one another and with the bore, etc. We had a customer shoot a .308 round through one of our .270 moderators with a bore of .310. As concentricity, alignment of all baffles with one another and with the bore were perfect, the .308 round went through all the baffles without touching any of them. You do the maths and calculate how much tolerance there was between .308 and .310. Hardly any I would say. The other major factor is the quality of the thread on your rifle, which in his case was as near perfect as possible. It has already been discussed here in the past that perfect threading is paramount. There is no point in trying to align a near-perfect moderator to a wonky rifle thread and shoulder. For your 6.5mm, a bore of .290 or .30 would be plenty, provided that all the above are spot on. Just my two pence worth.

 

Best,

 

George

[onehole]

Onehole, out of curiosity, what size would your respected smithy suggest then for a 6.5 mm bullet?

 

He didn,t.but this goes back a long way but will ring him up and ask the question again to see what he says.What was supplied with rebuild was a 8.5 holed T8 as that was my choice of mod at the time.I am just wondering now if the next size down T8 has a smaller hole than 7.5 and would have been the reason for recommending the bigger one?Not a problem anyway it all works superb.cheers Onehole.

[baldie]

You will rapidly find, on anything but a titanium mod, that the gas will cut the bore to the size it is most comfortable with anyway. Its not wise to go too tight, even if the thread and mod are perfectly concentric. Anything tighter than .020" per side will have a bad effect on accuracy on a brake that is 2" long, never mind a 10" moderator.

Bullets are not perfect. You are always going to get one that will "yaw" on exit, due to poor forming etc. That is disastrous in a tightly bored mod. I,ve tested mods with large and small holes on the same gun, and my ears have never been able to tell any difference.It will be completely irrelevant to the animals you are shooting at. Far better to run a safe rig.

[onehole]

Thanks for replies gentlemen and yes a safe rig is key factor I agree.I was going to run 140 vlds on that 6.5 and from memory that was a concern with regard yaw on those long ones,guess the bigger hole was the best choice.cheers Onehole.

[andy h]

here is a clearance chart for baffle/cap hole sizes.

[onehole]

Thank you Andy H a useful piece of info that,thanks for sharing it.I am tempted to use my spare 7.5 holed Ase utra for my 6.5 now as I do not run vlds through it just 120 and 123 Amax.Only problem is now getting a 3/4" fine thread adapter for it and in addition to that making absolutely certain the adapter screws in dead centre!!!!Think I can do this if I get an adapter threaded up and fit and true everything up in my lathe using a revolving centre in the tailstock and tighten up keeping pressure on the centre'

On that note about fitting these threaded adapter,s how do the professional make sure they are screwed in dead centre as from my limited experience they flop around quite a bit before they lock in??Anybody care to comment please,thanks Onehole.

[andy h]

Have you had a look at this American forum just on making silencers/moderators.

 

http://www.silencertalk.com/forum/viewforum.php?f=10&sid=94ef46e6edd1c6eca429d509e4ffbf65