Posted by Vlad
on August 24, 2009, 9:40 am, in reply to "Silent survival arms partt one"
The tip of a fly rod makes considerable noise when whipped. The tip of a whip can crack mach 1 and make a muzzle blast type noise. Below this speed the noise drops rapidly and a lot.
The noise caused buy fast objects does not appear instantly at mach 1, according to the Finnish site on firearms muffling. (URL below) Such noise rises rather linearly at a modest slope up to about 1000 fps, where it assumes a nearly vertical slope to rise with increasing velocity to a peak at about 1300 fps and remains at that level for higher velocities.
The noise at 1300 fps is about 10,000 times as intense as it is at 950 fps. That, in any book, is a whole lot.
The noise is related to bullet diameter, of course, but speed affects it much more. A low altitude aircraft passing over at 950 fps makes an earth shaking noise, but at 1300 fps it makes a blast which will shatter windows, and smack you like a nearby mortar blast wave. Size of the slower plane can add up to a large noise. Speed of the faster plane can have an explosive effect on noise.
Therefore the truly silent weapon is limited to an exit velocity of no more than about 950 fps, to account for any velocity variations due to reloading inaccuracies under primitive conditions, or sound velocity changes due to extremes of air temperature.
Transferred impact energy controls the instant injury capability of bullets. Provided there is enough energy (and inertia) to penetrate about 6 inches at least, then diameter helps transfer energy to the target. Thus even a mach 2 military bullet of 30 cal, which does not tumble nor expand, is often not noticed in a wounded soldier until he sees blood running out, for the bullet went clear through causing some damage, but not pain.
Bullet wounds occur so fast that pain is not present until some time after. A common event where a hunter is hit by a deer slug is he will exclaim to others "I'm hit" and then behave rather normally for up to a few minutes, if neither heart, aorta, spine nor head were hit.
Energy transfer increases with the square of velocity, but only linearly with weight of the bullet. Therefore, being limited to 950 fps, the only variable that can raise impact injury is bullet weight.
Transferred energy is desired, and that is better with larger diameters. A given weight bullet can be accelerated to speed with lower pressure, the larger its diameter. A doubling of diameter requires only 1/4 the average pressure to get equal velocity, neglecting friction and 2nd order effects. 1/4 the pressure will greatly reduce muffler size, to get equal silence.
Indeed, a common 30 caliber rifle, with a long (WWII era) barrel of about 30 inches needs no muffler at all, if one fires 32 automatic pistol bullets, or 32 short revolver bullets in it using an adapter. It will be about as quiet as a pellet gun fired at medium pressure. Put a muffler on it and it can be quiet enough that all the shooter notices is the clunk of the firing pin striking. Such bullets exit at about 800 fps, which is still quite deadly, and very quiet. Barrel friction reduces bullet speed after the first 6 inches or so of travel and heat transfer reduces exit pressure to very low values. This is a good weapon without silencer and can be used in all 3 types of survival situations above. Adapters cost less than $20 each. They do not stay in the chamber but stay with the cartridge, so switching to full power is an instant thing. Adapters are available for most rifles, having the same bore diameter as pistol cartridges, but low power pistol cartridges might need be reloaded for some calibers. But I digress...
Acceleration for a given weight bullet and a given pressure, rises with the square of its diameter.
All this stuff put together, means that small caliber bullets will have less impact injury potential, as well as need larger muffler volume ratios to silence them.
Muffler size means how many times the bore volume does the muffler volume have in it. Pressure is reduced almost in direct relation to this ratio. For example a muffler with 100 times the volume of the bore, will drop a 10,000 psi muzzle exit pressure to about 100 psi. I don;t have data on how much noise drops as pressure does, but suspect it is probably as the square, so the above would likely make a 10,000 fold drop in exit noise intensity.
Stuffing in mufflers helps only a little bit. Things happen too fast for much to be done with stuffing, fancy fins, etc. More volume is easier to get, and more reliable.
Stuffing can get loaded with half burned powder which can accumulate and burn all at once in a muffler fire. (This is when the accumulated half burnt powder which cannot be shaken out of stuffed mufflers, ignites and fire comes out both breech and far end which can last several seconds, and even distort the muffler or explode in some).
However, although large diameters are nice, the common 22LR can be quietened to equal the best, and with practical size mufflers, due to its small gas volume. It also has great penetration ability due to its small frontal area upon impact. So nature is not tilted entirely to favor large diameter bullets for all uses. The 22LR would require head shots for instant disability of the target.
Such silent 22 bolt action rifles will have a max effective range of about 75 yards, and at that distance be capable of penetrating skull bone due to the small diameter. Beyone that range drop with distance becomes large enough to cause large vertical errors for modest range errors.
The cal 45 auto is a good large diameter cartridge, especially in rifles. It will also have about that same max effective range as the 22LR, but can cause considerable damage to non vital area body hits. It will definitely not go un-noticed, but would also not likely instantly disable the target.
The problem with all truly silent weapons, is that the bullets travel only about 800 fps average over their 75 yard range, and bullets are dropping fast beyond that, causing otherwise trivial errors in range estimation to cause large errors in vertical impact. Wind also has a longer time to act on slow bullets. All these things tend to cut their practical range limit to less than 100 yards.
Due to the extreme quietness of both the muzzle blast and bullet flight noise with a truly silent weapon, one can shoot again and again with little fear of incoming. Neither of these weapons will have any muzzle flash to give away their location at night, to unaided human vision.
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