Updated Oct 05
Updated Oct 05| Note: Before attempting any subsonic load development, read the page "Safely Develop and Load Subsonic Ammunition". The loading of subsonic ammunition should not be undertaken lightly. Done incorrectly or by the incautious or inexperienced, it presents a significant risk of grievous bodily harm to the shooter. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Things aren't going as well as I had hoped with the .30 caliber bullet found in a cigar box at a gunshow. Despite my first report the darn thing has defied all attempts to get it to fly straight. The flight stability calculator says it should be stable but reality sometimes doesn't match up with the math. Near as I can figure the base problem with this bullet is the weight is distributed all wrong, being mostly at the back end, which causes it to try to swap ends in flight. So the search is back on. Subsonic ammunition presents a constant battle between opposing forces. With velocity fixed below 1000 fps, terminal energy can only be increased by increasing bullet weight. Increased bullet weight eventually runs smack dab into stability problems because the longer and heavier bullet is more difficult to stabilize. Bullet weight can be increased without increasing length by changing the shape from pointed to round nosed or even a full wad-cutter shape. However each of these changes presents limitations. Wad-cutter shapes, with their square nose, cause feeding problems in rifle actions and round nosed bullets do not perform terminally as well as we would like them to. At low velocity, a conventional spitzer shaped bullet will simply push tissue aside without doing much terminal damage. Somewhat like pushing a pencil through the target. A blunt bullet nose with a sharp edge will result in the greatest terminal damage because it cuts tissue as it passes, leaving the largest possible permanent cavity and thus wounding potential. Conventional jacketed hollow point bullets are designed to operate within a narrow velocity range and most will not expand reliably at low velocity. Typically, any jacketed rifle bullet will not expand at subsonic velocity. This presents a big problem to tactical users of subsonics. Bullet expansion and the resulting permanent cavity are the primary wounding mechanism required to prevent the perpetrator committing further aggressive acts. A low velocity impact is a very different event than most marksmen are familiar with. Without a high velocity shockwave to compact material in front of the bullet, it can push aside material and penetrate much deeper. Col. Hatcher discovered a 150 grain .30-06 bullet penetrates sand more deeply at 600 yards than at 200 yards. The full metal jacket service bullet doesn't expand at either distance but lower impact velocity at long range allows the bullet to push sand aside as it penetrates. An easy demonstration of this phenomenon is to thump a pile of sand with your fist compared to pushing a finger into the sand. As a result, impact energy is not as important to subsonic terminal ballistics as it is for conventional velocity bullets. Subsonics rely on shape to cut a wound path. From personal observation, lighter weight subsonic bullets have more than enough energy to penetrate a soft target and it can be advantageous to sacrifice some weight and energy to gain stability and trajectory. |
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For commercially available jacketed bullets in a .30 caliber subsonic system, the 168 grain BTHP flying backwards is the best compromise between shape, weight and length. It feeds reliably in a bolt-action rifle and gives the best possible penetration and terminal performance (without expansion) for a conventional bullet. Flight noise is an issue even at subsonic velocities. The slow speed and lack of ballistic crack actually make it very easy to track the flight path of a subsonic bullet back to the shooter. Studies have shown that a teardrop shaped projectile creates the least flight noise. A BTHP bullet travelling backwards is as close to a teardrop shape as can be had in a commercially available bullet. The only fly in the ointment is that subsonics are most accurate with a flat base so it can be difficult to get backward HPBT's to shoot accurately. A HPBT flying backwards is NOT the most airodynamically efficient shape. The same projectile flying backwards will drop considerably more over a given distance. |
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With a bit of experimentation, it quickly becomes obvious that conventional jacketed bullet designs offer hopelessly compromised terminal performance. The only solution is a purpose built subsonic bullet. Consider this .45 caliber bullet, designed by Dr. Richard Gunn, to get an idea of what a purpose built subsonic might look like. Weighing 480 grains this large bullet has a stability factor of 2.5 in a standard 1:16 twist. Note the flat, sharp edged nose for maximum wound track and the flat base for accuracy. A mold is available from NEI Handtools. |
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| Lapua offers a subsonic 200 grain jacketed, rebated boat-tail, spitzer bullet that seems to defy all logic and subsonic knowledge. Uncoated an unlubricated it never sticks in a dirty bore. A stability factor of only 1.36 suggests it shouldn't stabilise from anything less than a 1:8 twist yet it works just fine in the more common 1:10 twist. The only issue with this bullet is its minimal terminal performance. This thing will penetrate about 26 inches in wet paper and come out in good enough shape to be reloaded and used again. |
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A quickie discussion on stability factors (SF) is indicated: The stability factor of a projectile is a unitless value calculated from the projectile's length, weight, diameter, velocity and rotation (twist) rate. At conventional velocities, a stability factor of less than 1.0 indicates an unstable projectile. A stability factor between 1.0 and 1.2 is marginally stable and anything over 1.3 is fully stabilised. STABILITY CALCULATOR |
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From experience it is clear these stability values do not hold true for subsonic projectiles. Low velocity projectiles require a much greater degree of stability (at least 2.0). The chart to the right displays data for .30 caliber bullets fired at 1000 feet per second from a 1:10 twist barrel. The stability factors marked red are stable bullets. Black are unstable. There are two cases of bullets that do not follow the stability rules. The Nosler 165gr Ballistic Tip should be stable like the Hornady 168gr Amax but it is not. It is marginally stable fired backwards but completely unstable fired forwards. Note: The plastic tips of the Ballistic Tip and Amax bullets were cut off to reduce their length and increase stability. The Lapua 200 Subsonic should not be stable but it is. This bullet was designed for subsonic flight and is very well balanced fore and aft meaning it doesn't require as high a stability factor. A supersonic projectile rotationally stable out of the muzzle will continue to be stable downrange because forward velocity drops faster than rotational velocity. This is not true for subsonics which often display a stability that decays with distance. That is a bullet stable at 100 yards may wobble or tumble at 200 yards. Not being a ballistician I can't explain why this is but I have witnessed these effects so many times during testing that new bullets are always tested for flight stability out to 200 yards. A bullet stable in flight may become unstable upon impact. The drastic reduction in velocity causes the bullet to yaw wildly. A stability factor of 2.2 or greater is required to maintain directional stability after impact. Post impact stability is definitely important when attempting to shoot through cover like glass or metal. Another odd effect often seen with subsonics is they can maintain directional stability even though they have lost spin stability and have yawed to a great degree. That is an unstable bullet can still be reasonably accurate. I have shot subsonic groups that maintained 1.8 to 2.5 MOA with several of the bullets impacting completely sideways. |
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Lastly, a subsonic projectile can be stable flying backward and unstable flying forward. This appears to indicate the center of gravity has an effect on stability. I suspect this is the reason the Lapua subsonic bullet is stable when the stability factor calculation says it shouldn't be. Lapua's bullet is quite stumpy and the rebated base both cause the center of gravity to be quite far forward, improving stability. However this is not always true. The Nosler 165gr Ballistic Tip is not completely stable in backwards flight even though the majority of the mass is at the front. The unknown 200gr cigar box bullet is also not stable in backwards flight. The best bullet to start experimenting with in .30 caliber would be the 180 grain roundnose. As an example, Hornady's 180gr RN (#3075) has a stability factor of 3.42. The Engel Ballistic Research 180gr RN bullet is a bit longer but still has a SF of 3.18. Most bullet makers offer 180gr RN designs. Be careful utilising heavier bullets, not all of them are suitable for the 1:10 twist rate. The Hornady 220gr RN has a SF of 2.04 while the Sierra 220gr RN only has a SF of 1.74 meaning it would require a faster twist rate to stabilise. Subsonic terminal performance is not as closely related to weight as their supersonic cousins. Subsonic bullets depend on their shape to cut a wound path. This means medium weight bullets can perform as well as the really heavy stuff and they are much easier to stabilize to boot. We have done a lot of testing with 150 grain bullets because they are easy to stabilize and still provide impressive terminal punch. |
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The author has used .311" bullets in a .308" bore without a problem. The other way around has not been tested yet but some loss of accuracy would be expected from the loose fit. Conventional jacketed bullets require some sort of lubrication to avoid sticking in a dirty bore. Don't be fooled by apparently uncoated factory subsonic ammunition. Older EBR advertising noted they used pure animal fats for lubrication and the Lapua bullet appears to have a very thin guilding jacket and minimal size driving bands that allow it to slip down the bore without sticking. Otherwise, it is imperative to coat jacketed subsonics with animal fat or plated moly powder. Bullets do stick in dirty bores (it has happened to the author) and if not noticed will cause a ruined bore at the next shot. Cast bullets are naturally lubricated and do not require further coating. |
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Expansion is the most difficult task for subsonics. The Lapua subsonic bullet appears to have a very thin guilding jacket, giving it the potential to deform more easily on impact. It doesn't expand but can bend and flatten depending on what it hits. FMJ-BT designs do not deform. There is only once class of bullet that has a hope of expanding at subsonic velocity and that is a cast lead hollow point. This is where the failed cigar box bullet comes into play and some subsequent learning happens. Browsing tables, I stumbled across an old cigar box containing 100 examples of what I thought at the time might be the perfect subsonic bullet. The gas check points to a supersonic origin, probably for use in an Enfield rifle. However multiple driving bands, short length and a huge hollow point make this a mathematically high stability subsonic bullet (SF 2.39) that should expand at low impact velocity. The 200 grain weight gives maximum impact energy in this caliber without having to resort to a speciality high-speed twist rate bore. |
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Initial tests into wet clay demonstrated a bit too vigorous expansion. Typically the front half expanded violently, breaking apart, leaving the back end intact, (approximately 100 grains) to penetrate deeply. However further testing in wet phonebooks showed the bullet didn't reliably expand. Instead it tended to turn sideways and elongate and bend slightly. This effect was probably more due to the bullet's flight instability. We recently acquired a .300 Whisper with a 1:8 twist barrel and will continue to test this bullet to see if it can be coaxed to perform at subsonic velocity. |
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From terminal testing we think the hollow point mouth needs to be a bit larger to initiate expansion at low velocity and the hollow point needs to extend deeper into the bullet body to ensure maximum expansion. Cast lead Cigar Box Bullets recovered from wet clay at 50 yards. Remaining weights ranged from 100 to 107 grains. Unfired 200gr Supu Bullet at right. Note the gas check indicating its supersonic origins. |
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Additional testing has been performed with the Lapua subsonic bullet modified with a blunt nose and hollow point. Initial results (right) in wet paper indicate some potential for success. However more work needs to be done. Another route is to forget expansion and go with an agressive cutting shape like a lead semi-wadcutter complete with flat point and sharp shoulder. The .300 Whisper came with a box of 170 grain bullets of exactly this nature. Mark White, from Sound Technology, is a proponent of induced tumbling upon impact by way of a spoon nosed bullet. This is common military technology where use of hollow point and expanding bullets are forbidden. FMJ bullets will tumble on impact if they are not completely symmetrical at the front end. These bullets are stable in flight due to the high rotational velocity. The effects of spoon nosed bullets as practiced by Mr. White will also be explored in future updates. For now, read his article The Use of Sound Suppressors on High-Powered Rifles. |
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Recently an LE issue Level 2 kevlar vest fell into our possession so we of course decided to shoot holes in it. We were a bit surprised to discover that .30 caliber subsonic rifle bullets are quite capable of penetrating level 2 body armour. Both 150gr FMJ and 200gr Lapua bullets penetrated the vest and an additional 16 plus inches of wet paper. Penetration of kevlar is dependant on several factors: Shape, weight, velocity and frontal surface area. With velocity fixed, a subsonic must have a pointed nose and sufficient weight with minimum frontal size. For example, 52gr subsonic 223 bullet did not penetrate the vest. It was actually caught in the second of 21 layers. |
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