Miscellaneous Questions #19

This section contains brief discussions of various ballistics and shooting related topics as requested by correspondents. If you have a question you have been trying to find an answer to (keep 'em ballistics and shooting related--see your minister for the mysteries of life) email me by clicking here and I'll do my best to find the answer for you and if it is of general interest, publish it here. If you can contribute additional input to one of the answers I'd would appreciate hearing from you too.

Check back frequently as new topics are always being added.

On this page:

What does detonation mean and does it happen with small arms ammunition?
What are the ballistic coefficients for lead round ball projectile?
What do you mean by "Manual of Arms?"
How much does using a Ransom Rest decrease group size when testing handguns?
Who invented the Foster shotgun slug?
How can I make "sub-sonic" ammo?

Q.  What does "detonation" mean and can it happen with small arms ammunition?

A.  There have been 2 common but improper uses of the word "detonation" among shooters.  The first meaning simply refers to the normal action of the propelling charge.  The second meaning is an apparently severe or destructive action of the charge.  However, in long established technical usage, "detonation" means a highly distinctive kind of explosion different from either of the above.

After decades of study and instrumented test, the functioning  of explosives and detonation is understood in considerable depth.

Typical smokeless powder propellants burn at and very near the surface. The speed at which the burning surface moves inward depends partly on composition of the propellant and any deterrent coating, and more on temperature and pressure.  It varies from about 0.5" f/s for high energy solid propellants at about 2000 psi to about 5 f/s  at typical firearms pressures, although such burning or deflagration as it is called, can proceed at pressures well above 100,000 psi. A large amount of gas is formed and flows away from the burning surface.

On the other hand a detonation, if started under proper conditions and with sufficient intensity, spreads with great speed. There are two conditions necessary for detonation. When they are met, an extremely thin shock wave moves through the explosive, followed closely by a somewhat thicker chemical reaction zone and then by the reaction products. The products move in the same direction as the detonation, a characteristic of detonation contrasting it with deflagration in which the reaction products flow away from the burning direction. In typical high explosives the speed of the detonation is 15,000 to 27,000 f/s, and the pressure in the reaction zone is 1,000,000 to 2,000,000 psi.  This great speed and pressure varies somewhat with outside conditions but its order of magnitude is always the same.

Except in certain primary explosives, a detonation is not easily achieved. In practice the initiator must have comparatively great sharpness and be correctly placed, if the explosion is not to fall short of detonation or even fail entirely. The explosive must be packed to at least a minimum density throughout to propagate the detonation. In general any high energy material can be detonated; but it is the necessary conditions that may be difficult.

Attention has been given to the question as to whether detonation is possible in a small arms cartridges. The general opinion of authorities who have investigated the matter thoroughly has been that it is not. It appears that under the conditions in the cartridge, the explosive column is too short for the reaction to gain enough speed to make the great leap over to detonation. The possibility has been considered of detonation from some radical alteration in state of the charge, such as pulverizing from brittleness in extreme cold, but even then the gun would give way long before a detonation could be reached. In any case, no case of detonation of a small arms cartridge has ever been established so far as is known.

When discussing firearms pressures it would be convenient to have some single word for an unusually steep pressure rise, but "detonation" is not that word, if the user desires his report to be considered with respect.

Extracted from an letter  by E. H. Harrison to the author and which was published in a somewhat modified format in the June 1965, American Rifleman

Q. What are the ballistic coefficients for lead round ball projectiles?

A. To properly describe the ballistic qualities of round projectiles you really need to use the GS drag model.  Since most ballistics programs do not work with the GS function I have converted the GS to approximate G1 BCs.  

Note:  Using the incorrect G1 model yields  between about 12 and 25 percent less drop depending on the range than the correct GS model as can be seen from the table below.  Data based upon a .58 caliber round ball at 1200 f/s.  1" sight line.

Q. What do you mean by learning the "Manual of Arms?"

A. While the common meaning generally refers to rifle drill (present arm; right shoulder arms, etc),  in the modern technique it refers to the operation, and care of a firearm and it is part of the triad of marksmanship, gun handling, and mindset.  To be a proficient shottist one should be able to operate the firearm (load, actuate its controls, and know its modes of carry), know how it operates and any operational quirks, be able to clear routine malfunctions, and to disassemble and maintain it.  You should become so familiar with your firearm(s) that you don't have to give any conscious though to the above.

Q. How much does using a Ransom Rest decrease group size when testing handguns?

A. The answer depends a lot on the skill of the shooter.  When used with revolvers or fixed barrel pistols the accuracy you get from the rest is pretty much dependent on the ammunition.  However, when used with autopistols the results can be less than what one would like unless the pistol is fitted very tightly which can effect reliability.  Due to small random displacements in the slide/barrel on the frame the group size can increase because the pistol is aligned by the frame.  Frequently autopistols will show better accuracy fired over a sandbag or off hand by a good shooter using the sights than from a Ransom rest.

As an example, a friend and I once experimented with a well worn GI 1911.  I tightened the barrel to slide fit but left the slide to frame fit deliberately "GI" loose.  Over a sandbag, my friend who was a member of the "Presidents Hundred" shot 2.3" groups at 25 yards  using his match ammo, while using the ransom Rest the gun would only shoot about 4.5" groups.  We then fitted a National Match barrel to the pistol in the same manner and only minimally tightened the slide/frame fit (There was still a little slop.)  This gun shot 1.5" groups at 25 yards over sand bags and 2.9" groups from the Ransom Rest. 

Q. Who invented the "Foster" shotgun slug?

A. The Foster slug was invented by a Karl Foster around 1931.  It is characterized its deep hollow base design which places the center of mass very near the tip of the slug, much like a badminton shuttlecock. If the slug begins to tumble in flight, body drag will tend to push the slug back into straight flight. This gives the Foster slug stability and allows for accurate shooting out to ranges of about 75 to 100 yards, depending on the barrel  Most Foster slugs also have "rifling" that consists of thin fins pressed into the outside of the slug. Contrary to popular belief, these fins do not impart any spin to the slug as it travels through the air, because the slug's passage through the air  pushes a shockwave ahead of it that creates a vacuum on the side of the slug, where the fins are located. The actual purpose of the fins is to allow the slug to safely be swaged down when fired through a choke, although accuracy will suffer when such a slug is fired through tighter chokes. While designed for smooth bore barrels they can be used in rifled barrels but lead greatly.

Q. How can I make "sub-sonic" ammo?

A.  Sub-sonic ammunition is simply ammunition whose bullet velocity is below the speed of sound (nominally 1050 to 1100 f/s).  For handguns with their small case capacity simply load a full weight bullet so the muzzle velocity is in or below that velocity range.  For rifle cartridges the problem is a little more complex.  Because of the large capacity of the typical rifle cartridge case and the small charge weight volume there is a lot of empty space in the case which leads to erratic ignition and velocity.

Because of the friction of jacketed bullets it is very easy to get a bullet stuck in a rifles bore with very light loads, and subsonic rifle ammunition usually has to be tailored to a specific rifle.  

Testing with one "commercial" subsonic load in a variety of rifles with barrels from 18" to 24" produced a range of  927 f/s to 1248 f/s and barrel length had little relationship to the velocity

The easiest method to get subsonic rifle ammunition is to us a full weight cast bullet and the appropriate charge, making what is basically a duplication of the old "guard" cartridges.  If you have to use jacketed bullets they should be lubed either with a dry film lube or be moly-coated to reduce friction.

Some general guidelines are...

• Use a fast burning powder such as Red Dot, N310, N312, Bullseye, Clays, HP38, or Titewad,  or . There are other suitable powders available but these are the best to start out with.   Red Dot is probably the best powder to work with because it gives the highest load densities.  Try to use a powder that will keep you at a loading density of 30% or greater.
• Lube your jacketed bullets by lightly tumbling the bullets in Lee Liquid Alox, diluted 50-50 with mineral spirits, or use moly coated bullets.
• Don't crimp the bullets
• Consider using military brass.  It is thicker and has a slightly reduced capacity which helps load density.
• Consider  using magnum primers for more consistent ignition and drilling the flash-hole out to 9/64 inch (3.5mm) for a large-rifle primer. The enlarged flash hole allows as much of the primer flash as possible to reach the powder. This is especially important in full sized (.308 & .30-06) rifle cases.
• Distinctively and permanently mark cases used for subsonic loads so they won't be used for standard loads.

.308 Win sub-sonic load using 150 gr .30-30 flat point bullet.  Bottom of case is colored and the case just above the extractor cut is knurled for positive identification.

Because of the many variables involved I'm hesitant to give any loading data.  But...

You use this data at your own risk.  I am not responsible for any damage arising from the use or misuse of this data.

In the 5.56 the lightest load that will exit the barrel reliably with M193 Ball 55-gr. bullet is 4 grs. of Unique or 3 grs. of Bullseye, which is subsonic. You must start with lightly oiled bore and wipe the bore again every ten rounds to keep from "sticking" one in the bore, as you loose all bore pressure as the bullet passes the gas port, and if you don't accelerate the bullet enough it will stick between the gas port and the flash suppressor. Loads of 5 grs. of Unique or 4 grs. of Bullseye will exit more reliably, but are louder.

In the M14 for use with the issue 12-inch twist barrel the 110-gr. FMJ .30 carbine bullet and 5 grs. of Bullseye or 6 grs. of Unique works well. You don't need to turn the spindle valve off, as the White-cutoff system doesn't bleed off all the muzzle pressure. The M80 or M118 boattail bullets do not stabilize very well and aren't very accurate at subsonic velocities in the 12-inch twist and require at least 10-inch twist. You can use a 150-gr. flatnosed .30-30 bullet with good results by  using 6 grs. of Bullseye or 7 grs. of Unique. This charge will work OK with an M80 or M118 bullet if you have a 10-inch twist. You can also use 9-11 grs. of #2400.  Reportedly 6 gr (the absolute minimum load) of Red Dot and up to 7 or 8 grs will work well using 165 or 168 gr jacketedbullets 

With jacketed bullets you don't really want to go much below about 1150 f/s as you may stick a bullet in the bore with subsonic loads unless you use lubedmoly coated bullets or l, and start with a really clean bore.

At one time special subsonic cases were made in .308 that had about half the rear of the case solid to reduce capacity and these worked much better than using regular cases, but they had their own problems.  Unfortunately the company is no longer in business.

For some further information on subsonic loads see the following websites.

http://members.shaw.ca/cronhelm/DevelopSubsonic.html

http://guns.connect.fi/gow/arcane1.html

http://guns.connect.fi/gow/arcane2.html

If you have a rifle in .223, .308 or .30-06 another option is to use one of the cartridge adapters available from http://www.mcace.com/adapters.htm .  These will allow you to shoot .30 Mauser, .30 Luger, .30 carbine,.32 ACP, and .32 Colt/H&R.  I have played a lot with a .308 one in .32 ACP and while not as convenient as a real .308 cartridge it is very quiet and accurate and just the ticket for pot shooting or back yard pest elimination. I also had the opportunity to try one of these in a suppressed .308 rifle and the only noise was the striker falling. If they could come with a way to keep the sub-caliber rounds firmly enough in the inserts to allow magazine feeding they would be close to ideal.

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Disclaimer

As far as I know all the information presented above is correct and I have attempted to insure that it is. However, I am not responsible for any errors, omissions, or damages resulting from the use or misuse of this information, nor for you doing something stupid with it. (Don't you hate these disclaimers? So do I, but there are people out there who refuse to be responsible for their own actions and who will sue anybody to make a buck.)

Updated 2008-07-18