Miscellaneous Questions #23

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:

How can I tell if the ammunition I have has corrosive primers?  
What special procedure is needed to clean firearms used with corrosive primed ammunition?
What is the difference between "case capacity," "powder capacity" and "bore volume" how are they measured?
What is the difference between a "tangent nose" and a "secant nose" on a bullet?
In some advanced ballistic calculations there is the term Rt/R ratio that is used in drag analysis.  How are those radii measured/computed?  
What are the NRA's "firearms finish conditions?"
How important is trimming cases to the "trim length?"
Can you give me some tips on cleaning my cases?
Should I full-length resize or only neck size my rifle cases?
What is the BC of an airgun pellet?  
What effect does the number of shots have on group size?  
How often should I clean my firearms?  
How stringent are the NATO ammunition specifications?  
What is better.  A direct gas impingement or a gas piston AR rifle?  
How  good are the moderately priced rangefinders?  
What can I do about light primer hits hit with my M1 Carbine?  
How can I tell if the chamber of my  AR is .223 SAAMI or 5.56 NATO?
Can you suggest some inexpensive, reactive targets for youngsters?


Q. How can I tell if the ammunition I have has corrosive primers?

A. If your ammunition is military and was manufactured before 1950 there is a good chance that it is corrosive, and most Russian and Chinese currently produced ammunition in military packaging is corrosive, while their commercially packaged sporting ammunition is non-corrosive.  I have a chart that's gives the known dates of US military non-corrosive ammunition that you can view by clicking here. Civilian ammunition manufacturers began offering non-corrosive primers in the 1920s, but most military ammunition continued to use corrosive priming mixtures of established reliability up in to the 1950s.

A simple way to test if primers are corrosive is to first use a case primed with a known non-corrosive primer and then fire the primer with the muzzle about 1 inch from a clean piece of mild steel.  Repeat with a second strip of metal using the suspected ammunition after pulling the bullet and dumping the powder.  Place the metal strips in a humid area for 24 hrs.  The corrosive primer will show substantial rusting. Be sure to clean your barrel after the test.

Q. What special procedure is needed to clean firearms used with corrosive primed ammunition?

A. Corrosively primed ammunition has been in use for a long time and is still in service in many countries because it originally had better storage stability than early non-corrosive primers.  I have fired many thousands of corrosively primed ammunition over the years without any ill effects.  However, proper cleaning is essential--especially if you live in an area with any humidity.  Corrosive primers leave a residue of potassium chloride (KClO3) salt in the bore after a cartridge is fired. This residue is hygroscopic and attracts and holds moisture from the humidity in the atmosphere which causes rusting.    This can cause serious damage to both the bore and action of the firearm unless they are cleaned carefully after firing.

Because the corrosive salts are hydroscopic the best cleaning agent for them is hot soapy water, but even plain water works.  You don't have to pour water down the bore or over the action, simply use the soapy water as you would normal bore/firearms cleaner and follow with a cleaning with normal bore cleaner to prevent any after-rust.  "Murphy's Oil Soap" is commonly used.  While many people have satisfactorily cleaned firearms that have used corrosive primers with normal bore cleaners without any problems, a water based cleaning is preferred.  Ed's Red is claimed to work well.  The thing is to ensure that you thoroughly clean and lubricate ALL surfaces.

Many modern firearms have chrome plated bores and gas system parts which resist the corrosive primer residue problem.  However, this does not obviate the need to properly clean these parts when using corrosively primed ammunition.

Q. What is the difference between "case capacity," "powder capacity," and bore volume, and how are they measured?

A. Case capacity is general defined as the volume of water a fired case (with the primer still in place) filled to the case mouth.  Powder capacity is similar but it is the volume of water a case can hold with a bullet seated normally.  Using a fired case tailors the measurement to you rifle's chamber.  While bore volume could be measured with water, it is easier to mathematically compute it.

To determine case capacity, weigh the fired case.  Then fill the case with water to the case mouth and weigh it again.  The difference between the two is the weight of the water.  To get case capacity in cubic centimeters (cc) divide the water weight by 15.4.  To get it in cubic inches (in3) divide the weight by 252.4.

To determine powder capacity weigh the fired case, and the bullet.  Then fill the case with water to the case mouth, carefully seat the bullet to the proper depth and weigh it again.  The difference between the two is the weight of the water.  To get powder capacity in cubic centimeters (cc) divide the water weight by 15.4.  To get it in cubic inches (in3) divide the weight by 252.4.  

To get the bore volume you must know the groove diameter, and the distance from the muzzle to the base of the seated bullet.  The effective bore volume is given by the formula below which assumes that 1.5% of the bore diameter is taken by the lands.

B = .773 x T x D2

Where 

B is the bore volume in cubic inches (to get cc multiply by 16.387)
T = distance from muzzle to base of the seated bullet in inches
D = diameter of the bullet in inches.

The "expansion ratio which is an important part of internal ballistics cane be determined with the formula

Expansion ratio = (Powder volume + bore volume) / Powder volume  (Use same units for powder and bore volume)

Q. What is the difference between a "tangent nose" and a "secant nose" on a bullet?

A. A tangent nose shape has a radius the blends smoothly with the cylindrical portion of the bullet's body, that is the radius used has its center point on a line that is tangent to (at right angle to) the start of the cylindrical body.  With a secant nose shape the center point of the radius of the nose is offset some distance from the start of the cylindrical body.  Secant nose shapes are more aerodynamic but can cause issues with bullet seating depth and standoff from the barrel's lands.  The are also some "hybrid" designs that combine both secant and tangent radii.

Tanget Nose Radius Secant Nose Radius

Q. In some advanced ballistic calculations there is the term Rt/R ratio that is used in drag analysis.  How are those radii measured/computed?

A. The "Rt/R" ratio is the ratio of the radius of the ideal tangent nose to the radius of the existing secant nose of a bullet, or in other words a measurement of the "secantness" of the bullet's nose.  For a tangent nose shape the Rt/R = 1.

R = [L2 / ( D - T )] + [( D - T ) / 4]

Where

L = length of the ogive from the cylindrical portion of the body to the tip in inches
D = bullet diameter in inches
T = the diameter of the bullet tip (metplate) in inches
R = radius of the curve of the ogive in inches

The fly in the ointment is determining the secant radius used.  Unless you have the manufacturer's bullet drawings or access to an optical comparator with radius capabilities you are pretty much out of luck.  However you can use a nose radius chart based on calibers.  One was originally published some time ago and it is reproduced here in reduced size.  If you email me by clicking here I will send a full size printable copy as a 7Mb zipped MS Word file.  This method is not exact but it will get you close enough.  Note that the measurements in the chart are in are in "calibers"  so you will need to multiply  the chart's indicated radius by the bullet's diameter to use in the Rt/R formula.  In using the chart, place the bullet's tip on the point of the curve with the bullets base offset and then move the bullet's base towards the curve.  Repeat until you find a match.

Radius comparison chart

Q. What are the NRA's "firearms finish conditions?"

A.  The descriptions are listed below.

NRA Current Firearms Conditions

NRA Antique Firearms Conditions

Q. How important is trimming cases to the "trim length?"

A.  Much discussion has been done on the subject of case trimming.  It is very  important for straight walled cartridges that headspace on the case mouth, like the .30 Carbine, but with cartridges that headspace on their shoulder or rim the main reason for uniform case length it to help obtain a uniform crimp on the bullet or uniform neck tension.  There are those who claim that if cases get too long they can cause a blowup because the case neck will be forced into the projectile by the mouth of the chamber causing a very tight crimp.  I take issue with that on two points.  First, if a case or case neck was so long that it hits the chamber mouth you would feel the resistance as you closed the bolt or the bolt would not fully close.  Second, with bottle necked cases, if one looks at the case and chamber drawings you will note that there is a lot of space in the chamber neck for long case necks.

As an example the .223 Rem.  SAAMI specifies a maximum case length of 1.760" with a minimum of 1.740" and a recommended trim length of 1.750"  The corresponding chamber dimension is 1.772" to 1.787". A case would have to be over .012" above maximum case length to cause a crimping issue.  Having gauged several hundred fired and sized .223 cases the longest I have ever found was 1.767".  The dimensions for the .308 Win show similar tolerances.

Now this is not to say you shouldn't check and trim your cases, especially those with multiple uses, but other than for crimp purposes or headspacing there is no need to become paranoid about it.  Note that you should check the length, and trim if necessary after full length sizing, as working the brass can cause a change in length.  As an example using 5.56 brass the cases lengthened on average .006" after full length sizing.

Q. Can you give me some tips on cleaning my cases?

A. If fired cases are physically dirty (not just tarnished) my usual procedure is rinse the dirt off with warm soapy water, dry, and then resize/decap the cases, trim, and swage primer pockets if the cases are once fired military brass.  This allows the primer pockets to get cleaned out too when tumbling.  I then tumble them.  While corn cob or walnut shell media works fine in a vibratory tumbler, I have developed a preference for the steel pin media that is used wet (usually a mix of water, citric acid, and a detergent like Dawn or  "Formula 409" and that clean the cases inside and out as well as the primer pockets.  However, the pin media works only in a rotary tumbler but it is generally much faster than corn cob or walnut dry media..

If you use GI cases you need to swage the crimp out of the primer pocket AND ensure that there is no trace of the crimp jammed down into the primer pocket.   as in many instances there will be a sliver of the crimp stuck in the primer pocket that would interfere with primer seating resulting in high primers.  This is where the steel media really shines (no pun intended) as it removes any slivers and deburrs the edges of the swaged primer pocket without altering anything.

One advantage of pre sizing the cases after cleaning and before reloading that you can eliminate the sizing die on a progressive press which lowers the effort needed to run the press.

Once you've loaded your ammo you can tumble the ammo in corncob media with a tablespoon or two of paint thinner for a minute (30 sec in a vibratory cleaner) to remove any residual lube or finger prints before boxing.  

Below are some tips for using the steel pin media.

Pin media is dirty from manufacturing process and usually has a few minor burrs on it.   For best results run the tumbler with just the pins, water, and some detergent before use.

For the liquid case cleaning solution use

For a drum that is about 8" in diameter and 9" long (a.k.a. a "15-17 pound" tumbler, or very roughly a 2 gallon of water capacity) , use the following guidelines. Use a proportional amount for smaller or larger tumblers.

  1. Fill tumbler with about 1 gallon of the cleaner mix described above. (About 8 lb of water)

  2. Add 5 Lbs of pins to the drum.

  3. Add 4 lb of brass.  Tumble nickel plated brass separately and reduce time by about 50%.

The basic proportions BY WEIGHT are:  1 part brass, 1.25 parts pins, 2 parts liquid cleaning solution.

  1. Tumble 3-4 hours. 
  2. Pour out as much liquid as you can without losing any brass or pins. 
  3. Fill drum with clean water, and separate brass by hand or use an media separator with water. 
  4. Rinse your brass several times with some lukewarm water. Not getting a good rinse can leave water spots on the brass, and the more you rinse the brass and pins the better your results will be next time.
  5. Dump brass onto a towel and let dry thoroughly or place in a warm 150 –180 degree oven.  When completely dry use a media separator again to ensure all the pins are out of the cases.

Q. Should I full-length resize or only neck size my rifle cases?

A. Full length sizing is generally only needed if the ammunition will be used in a semiautomatics, or in multiple firearms.  If it is going to be used in a single manually operated firearm you will get longer case life, and frequently better accuracy if you neck size only.  Most manufacturers offer neck sizing dies, and some of them come with various diameter collets for a truly custom fit.  I have had excellent results with the LEE neck sizing dies.

Q. What is the BC of an airgun pellet?

A. The ballistic coefficients of airgun pellets are difficult to compute accurately because of distances involved and their low velocity, shape, and mass which results in very tiny values - in the range of .007 to .025 for common .177 pellets.  A table of approximate G1 BCs is available at http://www.photosbykev.com/wordpress/userfiles/pelletdata.htm.

Q. What effect does the number of shots have on group size?

A. Statistically, the greater the number of shots fired, the larger the group will be.  The table below gives the approximate change in group size as the number of shots increase.

# Shots Size multiplier
3 1.00
5 1.22
10 1.53
20 1.83

Q. How often should I clean my firearms?

A.  While having a clean gun is never bad, you don’t have to clean things to white glove perfection all the time.  While there are a couple of gun writers and other idiots who seem to brag about the fact that they've never cleaned their guns (any gun), the question is WHY would you do that rather than clean them.  Sure, maybe they still (for now) work, but what happens if that "one more little spec of gunk" seizes things up just as the cape buff or VCA (violent criminal actor) takes a bead on you.  In addition gunk can work its way into the action and interfere with the functioning of the safety on many firearms.

My personal recommendation is that unless the firearm got wet or dirt covered, after each use, you field strip the firearm, clean the bore (discussed here), and remove all visible heavy gunk and fouling from the visible moving parts.  Then relube, reassemble, perform a functioning test, wipe down the outside metal surfaces, and the go about your business.  Depending on the amount of use, I usually total strip things down and detail clean and lube between once a quarter and once a year.

A reminder.  Do not store your firearms in "gun cases."  They can trap and hold moisture and promote rust.  Invest in a safe or even one of the fairly inexpensive heavy sheet metal gun lockers.

Q. How stringent are the NATO ammunition specifications?

A. Not very.  As an example for 5.56 mm ammunition no velocity specification is given but rather a minimum projectile energy figure of 1500 Joules (1100 ft lb).  Ballistics require that with a 300 m zero that the 600 meter point of impact must be within +/- 300 mm (+/- 11.8") of the reference ammunition and that the maximum ordinate of the  trajectory for a 300 m zero be not more than 250 mm (9.8").  Port pressure is given as not less than 88Mpa (12,700 psi) and chamber pressure is given as 420 Mpa (61,000 psi).  Accuracy is given as a 600 m group size whose vertical and horizontal standard deviations are less than 225 mm for ball ammo  (about a 20" diameter group or about 3.3 MOA.

Q. What is better, a direct gas impingement or a gas piston AR rifle?

A. Despite all the hype to the contrary the direct impingement gas system works just fine--assuming you keep the rifle lubed and somewhat clean.  Under conditions of high volume full auto fire the direct impingement system does heat up the action very fast, and when using a suppressor the action gets very dirty, very fast due to the extra back pressure, unless a gas system cut off is use.

The piston systems keep the action cool and clean since hot gas isn't blown back into the action, but they can present their own issues.  If the upper receiver isn't purpose built and designed  for piston operation, it requires a rather thin fragile piston to fit.  In addition unless the bolt carrier is properly designed it can put downward torque on the carrier causing wear and stress to the upper and lower receivers.

There are some who claim that piston guns are not as accurate as the direct impingement guns but I have never seen any real studies done.

When it comes right down to it both systems run just fine, but a properly designed piston system is my first choice.

Q. How good are the moderately priced rangefinders?

A. Rangefinder performance depends on a lot of things including beam size, beam power, the return pulse sensor, the reflective properties and size of the target being lased,  stability of the unit, lighting conditions, and the environment.  They work by bouncing a laser beam off of the target and measuring the time it took for the beam to make the round trip.  Really accurate, long range rangefinder are neither inexpensive nor usually pocket sized and can run from $2,000 to$13,000 or more which put them way out of attainability for most folks.  

There are generally two types: standalone handhelds and binoculars with built in range finders.  The binocular units are about twice the cost of the standalone, but if you are looking for both a rangefinder and binoculars they might be an option.

In real life you will usually only be able to get readings out to 70-80% of the advertised max distance of consumer grade units under most daytime conditions (bright light) on a 2 MOA reflective target.  The really inexpensive pocket models ($100-$250 street price) are probably limited to 200-400 yards under ideal conditions, and less under typical field conditions.  For $300 - $500+ you can get some fairly decent hand held units that will work to 600-700+ yards or so fairly well.  The Leupold Rx 800 and 1000i series seem to get excellent reviews as does the Bushnell Elite "1 Mile ARC" unit but things are changing fast.  Binocular units will generally run between $1,500 and $2,000 for a good unit.  With any of the rangefinders ensuring a solid, non vibrating rest when using will help accuracy immensely.

Q. What can I do about light primer hits hit with my M1 Carbine?

A. Light hits in the M1 carbine are almost always due to a short firing pin or the wrong hammer spring.  The firing pin should have an overall length of no less than 2.953" and the maximum length is 2.957".  In addition check the hammer spring for the number of coils.  It should have 26 -26.5 coils and not the early 22.  If either part does not meet spec it should be replaced.

Q. How can I tell if the chamber of my  AR is .223 SAAMI or 5.56 NATO?

A. One would think that manufacturers would make this clear but many do not.  Short of making a chamber cast and measuring it the only way to easily tell is to use the Michiguns .223/5.56 gage available from http://www.m-guns.com/tools.php .  This $50 tool will stick in the throat of a SAAMI chamber but drop free of the NATO chamber.

Photo by Michiguns

Q. Can you suggest some inexpensive, reactive targets for youngsters?

A. Interesting, and fun targets for young shooters (of all ages), that don't requires lots of clean up are easy to find if you are creative.  Among the fun targets I have used are:

   charcoal briquettes
    Necco wafers
    ice cubes
    eggs
    popsicles
    cookies


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Disclaimer

As far as I know all the information presented above is correct and I have attempted to ensure 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 2016-12-26