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  1. #11
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    My uni mentor and later very good friend Prof Mike Hxxxxxxx and I did some research into silencers while writing the Sten book, using the student facilities here at the main lab on the Banbury Road here in Oxford. It was really about why the Sten silencer, developed by the National Physics Lab is still regarded as probably the best and still regarded as the yardstick by which others are judged. Woodsy is dead right in that the silencer casing MUST be able to absorb the total amount of gas generated by the fired propellant. The problem in ascertaining this volume of now uncompressed gas seemed insurmountable but true to form, he'd worked out a way of getting the answer. Not mathematically/theoretically but actually using an ingenious method he'd last used at senior school in York! Anyway, we worked it out that as a rough guide, the volume of air space in the silencer (excluding the baffles etc etc don't forget) should be 20 volumes of the barrel. Forgive me if I'm slightly wrong as I'm spouting this stuff from memory. We also established that because the gas exits the muzzle at 29 degrees (nope, professor Venturi had already worked this out for us. It's not 30 degrees nor 28, but 29!) there can be nothing between the muzzle and a direct line drawn to where............ anyway. It all gets complicated when you have a vented barrel of course because then, there is already pressure building up BEHIND the muzzle - which George Patchett solved.

    Dr Philip Dater a US specialist used to visit us fairly regularly and was always a most interesting person to speak to. I wish he could have spoken to Prof Mike as they were both physicists who took the mystery out of the subject by using plain language. For example, when talking about a sudden heat ring build-up, Dr Phil explained it by saying '......just accept that it is fxxxxxg hot' Well, you don't get plainer than that! Interesting thread

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    Quote Originally Posted by Woodsy View Post
    Suppressor design (despite what some manufacturers would have you believe) is quite simple and is just a combination of good baffle design coupled with sufficient internal volume to absorb the muzzle gases. Some baffle designs are better than others but none are 'magical' and even plain washers will do a good job in a proper design. The manufacture of a good suppressor needs to be accurate to ensure correct alignment with the bore, and robust enough to take a moderate knock.
    Was exactly what I was driving at with...
    Quote Originally Posted by browningautorifleicon View Post
    Suppressors are a number of principles and when applied they usually work to an end. How effectively depends on how many rules you break during construction.
    Two people discussing what they've built over time may disagree about specific details but if their general principles are the same it will work. Sub sonic ammo is mandatory for quiet and quiet is only to make it "Not sound like gunfire to people used to hearing gunfire"... A bolt action is best. All the gasses are contained that way. Yes, self loaders get extremely hot. Then they get loose and when you tighten them and they cool, you need a pipewrench to loosen them.
    Regards, Jim

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    Contributing Member Flying10uk's Avatar
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    Yes an interesting thread and a couple of questions that I would like to ask, please. Are silencers/suppressors a "controlled item" here in the U.K. now, because they attached to the barrel and in the case of a deac would they need to be pinned/welded permanently to the barrel. Occasionally deacs do come on the market that have a suppressor fitted although I've never purchased such an example.

    Question 2: How much larger would the hole through the centre of the baffle need to be than the calibre of the gun; would the hole be only fractionally larger or would there be a reasonable amount of clearance? So if we take the Sten as an example, which we all know is of 9mm calibre, would the hole through the centre of the baffles be in the region of say 9.15mm i.e. very minimal clearance or significantly larger say a 10mm dia hole?

    Thanks for the information chaps.

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    Quote Originally Posted by Flying10uk View Post
    How much larger would the hole through the centre of the baffle need to be
    Not much. Centering is vital so it need just pass. Close is better so the gasses get trapped and don't just blow by. I worked at the size of a couple calibers bigger...if you suffer baffle strike, all bets are off.
    Regards, Jim

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    Contributing Member Flying10uk's Avatar
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    Ok thanks Jim, so it is a high precision item then.

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    Ah..... In Q1 thread 13, you asked '...are silencers/supressors controlled items in the UKicon. Mmmmmmmm. Silencers per-se are outlawed. End of story. But supressors or what we now call 'moderators' are permitted subject to blah blah blah. Because by definition, while they 'diminish' the noise they do not silence it. This is the reason why the readily available (but NOT so readily available now.....) .22" P-H sound moderator is described as a moderator, it is not a silencer because the internal volume doesn't equate to 20 or so volumes of the barrel. Please don't quote this 3 liner as gospel though.

    As for the moderators being proofed because they form part of the barrel and........ Nope, I don't understand it either. Nor, clearly, does the proof house understand the physics of what they have effectively passed into their monopolistic rules

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    Senior Member Woodsy's Avatar
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    Quote Originally Posted by Flying10uk View Post
    Question 2: How much larger would the hole through the centre of the baffle need to be than the calibre of the gun; would the hole be only fractionally larger or would there be a reasonable amount of clearance? So if we take the Sten as an example, which we all know is of 9mm calibre, would the hole through the centre of the baffles be in the region of say 9.15mm i.e. very minimal clearance or significantly larger say a 10mm dia hole?
    Experience has taught me that the optimum size for the baffle holes is 1.5mm larger than bullet diameter. The reason is that most bullets (particularly rifle) exhibit a certain amount of yaw (wobble about their axis) as they exit the muzzle which takes a few metres to settle down, so that amount of clearance is needed to prevent contact. With over barrel suppressors the accuracy potential for many rifles is actually increased due to the extra support given to the last 150 - 200mm of barrel by the suppressor, and the reduced recoil. The over barrel types feature an internal muzzle brake to direct a lot of the muzzle gases into the back expansion chamber, and also have a snug fitting bush (usually engineering plastic) at the back end which increases support. For semi-autos and full-autos, the rear bush needs to be metal as the barrels often get too hot for the plastic bushes. Such firearms also often need to be tuned in the gas system for reliable operation because of the effect of the internal muzzle brake and the different barrel pressure curve. Semi-auto locked-breech pistols also need a floating muzzle system to negate the extra weight of the suppressor which can prevent sufficient barrel recoil for the pistol to operate reliably. Pistol suppressors need to be quite large to be effective and that size negates their usefulness, so such units tend to be much smaller and less effective.


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    Quote Originally Posted by Flying10uk View Post
    A recent new thread on a silenced Sten made me recall a home made silencer which a chap showed to me years ago, late 1970s/early 1980s. The chap made it because he wasn't able to purchase a ready made example during the war and although I never thought to ask him exactly why he needed it, he was a gun enthusiast all his life and did serve in the Home Guard. As I recall it, the outer body of the silencer was made to attache to the muzzle of the gun and the other end of the silencer had a removable end plate. The chap showed me the baffles which he had turned on his lathe and baffle packing pieces. These baffles and baffle packing pieces were really just like steel washers; the baffle packing pieces had larger holes than the baffles. The other item which the chap had made was a baffle alignment rod which, as the name suggest, was a steel rod.

    The chap explained that when he wished to fit the silencer to gun he would first attache the outer body tube, empty without any baffles or packing pieces, to the muzzle. He would then insert the alignment rod into the bore of the gun which extended out of the silencer body. Then he would fit one baffle followed by one packing piece onto the alignment rod and into the silencer body. When he had fitted all the baffles and packing pieces the silencer end cap was screwed on to "clamp" the baffles into alignment with the bore. Finally the alignment rod was removed.

    I have no personal experience of silencers and so don't know if the manner in which the chap designed his own version is similar to how commercial versions are designed. Do the baffles and packing pieces always have to be fitted after the silencer body has been fitted to the weapon? I don't recall what calibre the silencer was designed for.

    I have wondered, in recent years, if the chap concerned may have also been a member of the Auxiliary Units as well as the Home Guard but have not seen any proof of it.
    That is somewhat similar to the old Maxim silencers which at least in some cases, came with a polished steel rod that was slid down the bore of the silencer when not fitted to the rifle to discourage anything from migrating into the bore of the silencer. I believe some of the smaller models used coarse steel wool between the baffles and that was probably the issue there. Being an interrupted thread they were easy to take on and off quickly.

    As the Maxim models were around from well before WWI, your old acquaintance might well have been familiar with them.

    There has been talk that Maxim silencers were used in WWI, but no direct evidence that I'm aware of. A silenced semi-auto .22 would have been most effective in that situation and not difficult to acquire in the UKicon even then.
    Last edited by Surpmil; 03-21-2017 at 12:31 AM.
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    Advisory Panel browningautorifle's Avatar
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    Quote Originally Posted by Woodsy View Post
    Experience has taught me
    I agree with all that Woodsy, like I said...two that discuss it and have some experience come very close without building things the same. There was a formula that I'd read somewhere, a .22 bore needed a .24 hole, a .25 needed a .27 hole...a .30 needed a .32 hole... We have to use aluminum mostly.
    Regards, Jim

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    One of the problems with an exit hole can be resolved by the material that is placed behind it. One VERY effective material is LINOTEX (or LINATEX). There'll be others of course....... If the hole size is too great the linotex bellows outwards slightly (visible with a high speed camera) under the impact and gas pressure and then reasserts itself. Eventually it will cause a larger ragged hole. If the exit hole in the end cap is too close to bullet size, the bullet acts as a blade and quickly shears a neat ring of the rubberised heavy linatex away. But while the larger hole and linatex remains more effective for longer, in both instances they will both degrade........

    Interesting thread. NZicon'er Bill Mxxxxxxx designed a system based on formula ? racing car exhausts where the baffles are on spring loaded rods and allowed to move forwards and rearwards slightly, to reciprocate under the forward facing and variable gas pressure/load and effectively act as a shock absorber to the gas. After all, springs are just another means of absorbing energy (and storing it of course) which we NEED to do with our silencers. You need to meet up with Bill again Woodsy.

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