Letters to SAR: July 1998
By Dan Shea
The Marines Respond
Subject: Marine corps Participation in the Objective Individual combat Weapon (OICW) Program
Ref: “Marines Won’t Take OICW” article, the S.A.D.W. Column in Small Arms Review, Vol 1 No. 5, February 1998
1. The referenced article suggests that the USMC has made a decision against the adoption of the OICW. A decision to either adopt or not adopt the OICW has not been made. Furthermore, a decision of that nature would be premature due to the stage of the weapon system development.
2. The USMC feels that this is a promising Advanced Technology Demonstration (ATD) program. We have supported it with Science and Technology funding, Joint Services Small Arms Program (JSSAP) participation, program support by participation in the OICW Integrated Process Team (IPT), and have assisted the Army in their drafting of the Operational Requirements Document (ORD) for the program.
3. While the USMC has concerns about “Cost vs. Performance”, Life-Cycle Costs, and tactical fit, there issues are currently being addressed through the OICW programs IPT process. The Marine Corps is confident that the answers to these issues will be forthcoming and will provide sufficient information to make an informed decision on the future of the OICW in the USMC.
4. Lastly, regardless of the final Marine Corps position on the OICW, the USMC is aware that many of the technologies demonstrated during this ATD may have broader applications. For example, the fire control and fusing technology translate to the Objective Crew Served Weapons (OCSW).
D.G Haywood
Program Manager, Infantry Weapons, Marine Corps Systems Command
Dear SAR,
The current issue of SAR (Vol 1 #7) contains Mark White’s well written and informative article, “Suppressed Sniper Rifles: 101”. I thoroughly enjoyed the article and found it to be thought provoking and educational. However, there are five areas where I would like to take issue.
He stated that the speed of sound varies from “1,089 to 1,114 fps in dry, 65 degree F air, depending on who one listens to.” There is a common misconception that the speed of sound in air varies with humidity, barometric pressure, and a host of other variables. Unfortunately, this is not true, and it varies only with the square root of the absolute temperature (degrees Kelvin or Rankin) at altitudes below 36,000 feet. At 65 degrees F the speed of sound is 1,123 ft/sec. (Reference Minzer et all, “The ARDC Model atmosphere, 1959,” USAF Cambridge Research Center, TR No. 59-267). The reason that the speed of sound changes with significant changes in altitude is related to the change in ambient air temperature at high altitudes. It is true that the speed of sound varies with the medium (water, pure gases other than air, etc). While it is true that acoustic impedance and sound attenuation vary with humidity, this is unrelated to the speed of sound.
He praised one suppressor manufacturer for maintaining a tight bore aperture throughout the suppressor (0.007” clearance each side of the bullet). Although this will significantly reduce one component of the sound signature (Blowby), it is an invitation to having bullet strikes depending on the ammunition. At the moment a projectile exits the rifling, it starts a transition from being shoved through a high friction tube rotating about its physical center to flying in a low friction medium and rotating about its center of gravity.
This introduces some yaw for a number of feet before it stabilizes, the degree of yaw varying with the projectile shape and rotational velocity. The same effect is apparent when a jet fighter is catapulted from an aircraft carrier. Some subsonic rifle caliber loads, particularly with boat tail projectiles, do not have adequate rotational velocity and will yaw badly. I am referring specifically to Lapua .308 subsonic ammunition, which is notorious for bullet-baffle contact on a perfectly aligned suppressor even with 0.050 inch clearance each side of the projectile. The degree of yaw is apparent when viewing high speed photographs. The major suppressor manufacturers (Knight, AWC, Gemtech) allow adequate clearance each side of the projectile to compensate for yaw in conventional ammunition.
I agree with the author on the stability of the two point suppressor mount. It does provide excellent suppressor alignment provided the bore is concentric with the outside of the barrel. Our experience with shortening rifle barrels to a more optimal length is that the bore is not concentric with the outside of the barrel except at the breech and original muzzle (and sometimes not even at the muzzle). As a result, when using a two point mount on a shortened barrel, it is necessary to re-contour the barrel to restore concentricity.
I do not wish to get into blast baffle design, but I strongly disagree with two of the author’s statements. First, the high velocity impact of hot, partially burned powder particles will peen the rear surface and diminish the bore aperture (hole diameter) regardless of the material. It is just faster with softer materials. We have experimented with a variety of hardened steels, including hardened tool steel and steels plasma coated with tungsten and tungsten carbide. With time and over-temperature abuse from machine gunners, there will be erosion from almost all materials. The author further stated that asymmetry in the blast baffle will cause inaccuracy. We have not found this to be true, and the major suppressor manufacturers generally have some degree of asymmetry in the blast baffle with repeatable accuracy, decrease in group size, and predictable shift of point of impact with mounting and dismounting of the suppressor. Both Knight and Gemtech have developed quick disconnect suppressor systems that have proven repeatable accuracy with extensive military testing.
Last, the author commented that the bullet flight noise of a “rapidly moving .308 bullet will sound louder than a .22 pistol... “This is contrary to testing done at comparable distances from the source (bullet flight path and pistol muzzle) by both Knight Armament and myself. Generally, the .22 pistol will be significantly louder, varying somewhat with pistol barrel length. The sound level of the bullet flight noise is actually slightly less than the sound of a .22 rifle. Interestingly, testing of military ball ammunition has shown that the 5.56mm projectile is about 3 dB louder in fight than the 7.62.
Respectfully
Philip H. Dater, MD
Mark White’s Response to Dr. Philip Dater’s Letter
I’m reminded of the old bit of humor “If a man speaks in the forest, and if there’s no woman to hear him, is he still wrong?” In this case we were dealing with a device ( a suppressor) designed to reduce gunfire noise from .223 and .308, bolt-action sniper rifles.
The human ear is the standard by which we base our observations, and by definition it is a perfect instrument. Unfortunately, the human ear lacks uniform calibration, to which a finite and universal number can be attached. Electronic instrumentation can provide finite decibel numbers, but at this stage of development those numbers are seriously flawed because most of today’s instrumentation lacks sufficient rise time and sophistication to catch the very brief sound pressure rise, pitch and duration of suppressed gunfire. The thing a human ear can do exceptionally well is listen to different pairs of shots fired in close sequence to determine which is louder. If we hear two shots in the forest, and if there is no instrumentation to back us up, are we wrong?
When we listen to .223 and .308 projectile fight noise in the field it is our distinct impression that the larger diameter bullet makes significantly more noise. We tested a .22-250 Sound Tech can in the Nevada desert at SOF last fall, and a number of sophisticated observers noted that the projectile noise was less than what they heard with a .308. Quantified measurements taken and documented research conducted in Finland back us up on this.
Al Paulson went through the procedure of listening to bullet flight noise by having a trusted accomplice (100 meters away) fire both a .223 and a .308 round one meter from his face. He reports that both events were painfully loud, and (in layman’s terms) likened them in intensity to the muzzle blast of a .22 pistol at one meter. Of course the muzzle blast from a pistol is more massive and contains a number of frequencies for a longer duration. The two sonic cracks coming from a high-powered rifle bullet are of a purer quality and only last for a very brief duration.
When we fire ammunition at the edge of the speed of sound through a calibrated chronograph we notice a sonic crack on some days, but not on others. We’ve done this for over 20 years with thousands of rounds. Even when temperatures and velocities are similar, we sometimes hear a crack and sometimes don’t. We feel that humidity and barometric pressure (and possible some undefined factors) are playing a role in the sonic phenomenon, in addition to temperature. Higher humidity lowers the density of the air because water (hydrogen bonded to oxygen as a gas) is lighter than air (essentially 1 part oxygen and 4 parts nitrogen) as a gas. Lower the density of a medium and you will lower the speed with which sound travels through that medium. Tap on a long steel beam (or a stainless counter in a cafeteria) with a quarter (higher density, faster speed) to get a feel for how much faster sound can travel through a denser medium.
We’ve noted a ‘rack’ at 1050 fps (at 65 degrees F), and conversely, have seen situations where a ballistic crack did not occur at 1150 fps. Our chronographs divide a second into 4,000 parts, and are calibrated from time to time to encourage acceptable accuracy. We are definitely not alone in this. Many of our colleagues notice it as well. Again, same suppressed firearm, same ammunition, same chrono, same temperature. On some days there will be a ballistic crack. On other days there will not. Something is causing this. Is it pollen or air pollution or extremes of barometric pressure? Spend enough time with a .22 rifle equipped with a muzzle can and target ammunition and you will begin to notice differences from day to day.
With regard to particle peening of the blast baffle, we mentioned that in the article and agree with Phil that softer materials will peen more easily. When I worked in a shipyard we used carbide nozzles in sand-blasters, and they did not decrease in diameter. It might be that the softer grades (C-1 & C-2) of tungsten carbide would resist peening without shattering. C-6 carbide is harder, and I would expect it to shatter under a .308 muzzle blast. In any event, a bolt-action rifle will never be cycled fast enough to replicate full-auto fire, hence we did not consider full-auto fire as it relates to blast baffle design.
With regard to tolerance between baffle and bullet, we did not suggest that tighter is better, we merely remarked that at least one manufacturer is getting away with minimal spacing. We also suggested that the cone of dispersion would allow closer tolerances nearest the barrel’s muzzle. With regard to the blast baffle and its effect on accuracy, we have noted that (in our experience) asymmetrical blast baffles had a negative effect on both stability and accuracy. Steeper, more effective baffle geometry seemed to have a greater negative impact. Is it possible to design a steeply asymmetrical blast baffle that doesn’t have a negative impact on accuracy? Perhaps, but we would have to see it and test its performance before issuing a blanket endorsement. Certainly, Knight’s Armament, Gemtech and others are producing accurate systems what work well with asymmetric baffles, so it can be done.
It is critical that a sniper rifle maintains optimal accuracy at all times. It is our feeling that adequate clearance between bullet and baffles, a heavy, rigid barrel, and a heavy, two-point suppressor mount are important aspects in the equation for dependable suppressed accuracy. Some unusual things happen during extremes of heat and cold, and one does not want to discover this for the first time on deployment.
Great minds can differ in their opinions. Different groups of people can develop various systems that operate effectively. Some laws of nature are never going to be changed, but there may be two or more different operating systems that can both work dependably and accurately.
Mark White
Dear SAR,
This morning I was riding into work on the bus, and having missed the express, I got stuck on one taking a whole class of 5/6th grade kids to the Science museum. So have to sit in the back surrounded by obnoxious kids. One kid asked if I have any money in my briefcase, I nod, and he asked if he can see it. I then tell him I have something better, which is the SAR with mini-gun equipped Suburban. In a couple of minutes I have a dozen kids who think I’m God, especially with the tracer-laden centerfold. One asked why I keep calling it a mini-gun, so I show them the picture of the GAU-8 next to the VW bug. This then takes them to a whole new level of hyper. I pity the teacher who had to contend with them after I jumped off at my exit. One kid even yelled bye from a window as the bus rolled past.
On the down side, one kid was of the opinion that assault rifles were much more powerful than a mini-gun. Hopefully they learned something from me.
Wood
Dear Wood,
It is amazing how interested a young mind is when an adult can actually take the time to pay a seconds worth of attention to them.
Your last paragraph seems to sum up one of the greatest problems we have with young people today, and that is the amount of time they sit in front of the electronic baby sitter. ED
This article first appeared in Small Arms Review V1N10 (July 1998) |
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