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Gun Glossary - Letter I
Index of Firearm & Gun Terminology

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Letter - I Page Updated: 06 March 2003

IDPA: Abbreviation for International Defensive Pistol Association.

IGNITION: The action of setting the powder charge on fire.  In former time this source of ignition could be a spark from a flint, a felt lit on fire, a match or even a torch.  In modern firearms the primer ignites the powder charge.

IMR: Abbreviation for Improved Military Rifle.

IN THE PANTS:  Type of holster or concealment method used for concealed carry of a firearm where the firearm is inside the waistband of the pants or trousers.  Abbreviated ITP.  Also know as IWB.

INFANTRY: n. Roman Infant meaning those with-out voice. Modern body of soldiers who fight on foot, as distinct from CAVALRY or other branches of an ARMY. In ancient times the relative military value of infantry fluctuated. The Romans are believed to have made the most effective use of the foot soldier, but with the decline of Rome, cavalry became dominant in war, remaining so until firearms were introduced in the mid-14th cent. Armed with muskets, and then rifles, troops fought in mass formation until the early 20th cent., when trench warfare and automatic weapons affected deployment. Aircraft, TANKS, and ARTILLERY supported a massive use of infantry in WORLD WAR II. Despite the innovations in weaponry since then, strategists continue to regard the infantry as the indispensable factor in military victory.

INSIDE THE WAISTBAND:  A holster design to be worn inside the waistband of pants.  While they are somewhat less convenient, IWB holsters generally improve concealment.  Many people hate IWB holsters and think they are a modern form of torture.  I recommend that you try before you buy.  Also know as "ITP Rigs" or In The Pants.

INTERNAL BALLISTICS: the ballistic science of what goes on inside of the gun. This aspect of ballistic science includes Burn Rates, Pressure, Expansion Ratios, Load Density, Chamber Capacity, Gun Powder Capacity, Sectional Density and the propellant Charge Weight to Bullet Weight ratio.

For additional information on Ballistics go to The Gun Glossary Letter B - or - Click Here.

For additional information on Internal Ballistic see the article in the detail box below.

Internal Ballistics
A (Very) Short Course in Internal Ballistics
Written by John Schaefer

Internal ballistic, or the science of what goes on inside of the gun, is a very complicated topic. This page is not designed to give you a degree in the field but rather to provide some background data to help you understand the subject.

Definition of Terms

Barrel Length - In interior ballistic work this differs from the "barrel length" use in general measurements. It is measured from the face of the muzzle to the base of the seated bullet or base of the case neck.

Burning Rate - A arbitrary index of the quickness that burning propellant changes into gas. Burning rate is controlled by the chemical composition, the size and shape of the propellant grains, and the pressure at which the burning takes place. IMR 5010 powder is very slow burning and Bullseye is fast burning.

For a chart of comparative powder burning rates for current commercial powders you can click here.

CUP - Copper Units of Pressure. In this pressure measuring technique a hole is drilled in the chamber and a piston fitted that presses on a calibrated copper slug (or crusher). (Some set ups also drill the cartridge case which changes the results compared to non-drilled cases.) The whole assembly is held in place with a yolk. When the cartridge is fired the piston presses on the crusher and deforms it lengthwise slightly. 

Measurement of the length change is used with a lookup table supplied with each lot of crushers to determine the peak pressure. The total deformation is effected by both the peak pressure and by shape of the pressure curve around the peak pressure. This method tends to give readings slightly lower than the actual peak pressure. A typical reading might be stated as 47,500 CUP which is supposed to be close to the actual psi measurement. This method has been largely superseded by piezo-electric units which replaced the copper slug with a crystal which changes its electrical properties in response to pressure.

Pressure gun (26k jpg)

Typical Dual Measurement (Copper & Piezo-Electric) Pressure Gun

Expansion Ratio - The ratio of the capacity of the powder chamber plus bore (in grains of water) to the capacity of the powder chamber (in grains of water).

Loading Density - The ratio of the weight of the powder charge to the capacity the powder chamber.  It is usually expressed as the ratio of the charge weight to the capacity the powder chamber in grains of water. (See below.) Generally, the more fully the powder charge fills the case the more consistent and accurate the load will be.  On the other hand if the loading density is too low, (too much free space in the case) it can cause erratic ignition, change in the pressure curve (moving the peak towards the muzzle), or even overly rapid burning ("detonation") of the powder charge. (One reason manuals list minimum or starting loads.)

PSI - Pounds per square inch. It is often seen designated as PSIA. This designation is now used to signify a measurement of chamber pressure taken with a piezo-electric device. Piezo-electric units operate in a similar fashion to the copper crusher units but use a reusable crystal "crusher" that changes its electrical properties in response to pressure. When connected to suitable recording equipment the entire pressure pulse history can be recorded or displayed. The peak pressure recorded by a piezo-electric peak device usually reads about 5,000 psi higher than the figure determined by the copper crusher method.

Powder Chamber Capacity - As with most interior ballistics capacity measurements it is usually expressed in grains of water. It is determined by measuring the weight of water that a fired case from the test firearm can contain with a bullet seated to its normal depth. Note that this varies with different bullets or seating depth as well as the dimensions of the chamber, and the brand of case.

Sectional Density - The is the ratio of the bullet's weight (in pounds) to its diameter

Charge Weight to Bullet Weight ratio - This is the ratio of the weight of the powder charge to the weight of the projectile.

What Affects What

Among the things that affect the internal performance (pressure and velocity) of a given cartridge & bullet are:

  • Capacity of the powder chamber (a function of chamber size and case construction)
  • Relative burning rate and burning characteristics of the propellant powder used
  • Amount of propellant used and how much it fills the powder chamber
  • Diameter, weight, and the bearing length of the projectile
  • Length and interior dimensions of the barrel
  • Uniformity and speed of ignition of the propellant powder (Primer and loading density related)
  • Temperature of the propellant prior to ignition

Some Powder Basics

The grains of smokeless propellant powders are made in various sizes and shapes to give the best results in the firearm they are designed to be used in. Powder for the 16" naval cannon has grains almost an inch in diameter and 2 inches long while typical modern rifle powder has grains only 2/32 or 3/32 of an inch long and maybe a 1/32 of an inch wide. The smaller the grain the faster it burns. Modern powders are available in tubular grains, spherical grains, and flakes.

Most tubular grained powders have a hole through the center of the grain to allow burning to take place from both the inside and outside surfaces. Without the hole, as the grain burned its surface area would decrease and the amount of gas would decrease as the grain burned. Powders of this type, called degressive powders, give peak pressures when the bullet is close to the breach and a low muzzle pressure. By putting a hole down the center of the grain the burning area stays more constant and the maximum pressure point is moved to a point where the bullet has moved further down the bore. These types of powders are called neutral burning.

Another way of controlling the burning rate is with the use of a coating. The coating on the powder grain tends to hold back the rate of burning until the coating has burned through. These powders, called progressive burning give a peak pressure at a point in bullet travel even further along. Coating technology is important in small arms powders since without it the grains would sometimes have to be larger than practical.

The majority of commercial small arms powders are made of either a nitrocellulose base, referred to as single base powders, and typified by the IMR powders, or of a nitrocellulose and nitroglycerine mix (usually from 3 to 39 percent), called double base powders, which include powders like Bullseye, 2400, and Hi-Vel #2. Double base powders have a higher energy potential than single base powders. Some modern powders referred to as "triple base" now incorporate an additional ingredient called nitroguanidine, but these are not in wide distribution except military powders and some commercial grade non canister powders, but they can yield a substantial increase in velocities while maintaining normal peak preasures.

As a technical point of interest, conventional small arms propellant powders have an energy potential of about 178 ft lb per grain of weight. However, in practice this figure is not even closely approached because of various energy losses--such as the energy used in forcing the bullet into the rifling, overcoming bore friction, heating the barrel, and giving the powder gases their high velocity, etc.-- that are inherent and unavoidable. According to The World s Great Rifles, by Roger Ford (1998, Brown Packaging Books, Ltd., London) iIt is estimated that just one-fifth of one percent of the energy produced when a cartridge is fired goes to rotating the bullet, while friction in the barrel accounts for another three percent (20 to 30 percent goes to propelling the bullet, 30 percent goes in heat to the barrel, and 40 percent goes in muzzle blast

Contrary to media myths, smokeless powder is not an explosive but rather a highly flammable solid. (Note however that we are talking about smokeless powders here. Black powder IS an explosive and a small quantity ignited even in the open simply explodes.) As an example, one of the large powder grains used in the 16" naval cannon can actually be lit while held in the hand and then blown out. Even individual grains of modern small arms powders burn very slowly. But, when several grains or a pile of them are ignited together the heat generated by each burning grain makes the adjacent grains burn even faster. A very small pile of smokeless powder burned in the open may seem harmless but the resulting rapid ignition can cause such a rapid build up of heat that a person may not be able to get away fast enough to avoid injury. Inside of a cartridge case the same process happens but on a much shorter time scale.

Bang!  (What happens when the trigger is pulled.)

When you pull the trigger and the primer fires, the intense flame created by the priming compound fills the interior of the cartridge case and ignites the entire powder charge. The more nearly the powder charge fills the case the less the powder gases generated by the burning powder can expand without doing work and the more the heat generated accelerates the burning process as described above. (Thus a full or nearly full case of powder is more efficient than a partially filled case.) The increasing pressures generated by the burning powder pushes the bullet down the barrel. If the bullet is heavy, tightly held into the neck of the cartridge, or if it is a tight fit in the rifling then the confinement of the powder is accentuated and the burning is proceeds more quickly than if these conditions were not present--in other words you get higher pressures more quickly.

The graph below is a representative pressure curve of a typical cartridge (in this case from the M193 5.56mm cartridge using 846 ball powder). Notice how the peak pressure rises quickly and then tapers off. The location of the peak pressure and the shape of the pressure curve is determined by the burning characteristics of the powder used and the loading density. Slower burning powders tend to have a flatter curve while faster powders a steeper curve.

The gas port pressure curve on this chart shows a critical part of internal ballistics for cartridges used in gas operated firearms. Gas operated weapons are generally tailored to a narrow range of powder burning rates and characteristics. If the port pressure is too low the weapon will fail to function and if to high the weapon may function too forcefully or rapidly causing extraction or cycling problems. A poorly researched change of powder from an IMR type to a ball powder and the subsequent change of port pressure was what caused all those nightmare stories about the M16 during the early days of the Vietnam war.

typical pressure curve (4k gif)

Another very good graphical representation of what happens is shown on page 322 of "Hatcher's Notebook", (by Julian S. Hatcher, 3rd edition, June 1962, Stackpole Books, ISBN: 0811707954) but the graph is a little data dense for good reproduction here. It is similar to what is shown above but it is based upon the .30-'06 cartridge, and along with the pressure curve it shows bullet velocity and time of bullet travel.

As can be seen from the above graph, the longer the barrel the more time the powder has to work on moving the bullet, so thus you get higher velocities with a longer barrel--all else being equal. However, while not often reached with high powered rifles there is a point at which additional barrel length does not increase velocity, but rather causes a decrease.

As the bullet moves down the bore and the gas pressure behind it decreases there will come a time--with a sufficiently long barrel-- that the bore friction and the air pressure in front of the bullet will equal the pressure behind it. At that point velocity will start to decrease. With the .22 rimfire that point is reached in about 14" - 16" of barrel. Beyond that length no velocity increase occurs, but the extra length may be useful for other things such as increased sight radius or legal requirements.

If we have to keep barrel length reasonable, the way to get more velocity is to either change the burning characteristics of the powder so it gives higher pressures over a longer time (as is done with the powders in the new "enhanced performance" ammunition now available from Hornady and Federal), or to simply burn more powder by using a bigger cartridge case.

By using a cartridge case with greater capacity we provide for more chamber volume with the same bore volume, thus the expansion ratio (see definitions above) becomes less. As a result, the powder gases have been through less expansion by the time they reach the muzzle and muzzle pressure is higher. As the muzzle pressure is higher, the average pressure along the bore is higher and the bullet has a higher velocity. There is a trade off for this. Because the muzzle pressure is higher, more energy is carried off into the air unused (as flash and muzzle blast) and thus the efficiency of the load (the getting of energy out of the powder charge) is less.

The Hacksaw Effect
(Sometimes referred to as "get shorty.")

The often asked question of "How much velocity will I loose if I shorten my barrel "x" inches?" is tied quite closely to the expansion ratio, although the type of powder also plays a part. Thus there is no clear cut general answer as to just how much velocity is lost per inch of barrel shortening. Rifles with high expansion ratios tend to loose less velocity as the barrel is shortened than do rifles with low expansion ration. Thus a .300 Win Mag will loose more velocity per inch of barrel than say a .30-'06 and it is not uncommon for a smaller cartridge in a long barrel to out perform a big bottle cartridge in a short barrel as some folks found out to their dismay when they chronographed their 22" razzle-dazzle magnums next to their buddies convention calibers with 24" barrels.

Because of the fact that "high velocity" cartridges generally have low expansion ratios (big cases in relation to bore diameter) one can make some generalizations. The chart below will give you some idea of what can be expected if you cut your barrel shorter within the range of 20" to 26". Remember, your mileage may vary from this chart. Note that this chart does not hold true if we are comparing different barrels of different lengths. In that case it is quite possible, due to certain barrel design parameters, for a short barrel to give higher velocity than a longer one with the same ammunition.

Velocity Range D V per inch
0-2000 5-10
2000-2500 10-20
2500-3000 20-30
3000-3500 30-40

Mechanical Calculation of Internal Ballistics

There are some "manual" methods of computing a fairly close approximation of the actual internal performance (i.e.: chamber pressure and muzzle velocity) of a given combination of cartridge, firearm, bullet, and powder but their details are beyond the scope of this page. The "Powley Computer," a slide rule type device has been available for many years and gives rather good results when used with the "IMR" type of powders. It is based on some mathematically derived "constants" which provide a close approximation to the results derived by some rather complex mathematical gyrations. Several computer-based adaptations of this program are available from different sources.

At one time there was available a "do-it-yourself" pressure measuring device using a "crusher" type tool. Rather than utilizing a drilled chamber this unit was mounted on a firearm in place of the telescopic sight and when the firearm was fired the recoil impulse compressed the crusher which could be measured and compared to a calibrated table. Unfortunately, the unit was tricky to set up, required very precise measurements of the firearm's weight, and if memory serves me right worked best when the firearm was unrestrained in recoil (as in hanging from support wires). It was also only really suitable for rifles.

The Oehler Model 43 "Ballistic System" is equipped with a strain gauge pressure sensor. While you can reliably compare the pressure generated by your handloads with the pressures generated by factory ammunition fired in your gun, or compare the pressures from different handloads in the same gun, it does not generate SAAMI pressure measurements.

The best way to develop internal ballistics data still remains careful load testing using an instrumented pressure test barrel.

Rules of the Road

Rule 1 - Don't do anything stupid.

Rule 2 - For a given load a 3 percent rise in velocity requires a 6 percent rise in chamber pressure.

Rule 3 - For a 3 percent change in case capacity chamber pressure changes by 6 percent. Remember that case capacity varies drastically between brands of cases and that bullet seating depth also changes case capacity.

Rule 4 - Changing ANY component can drastically effect chamber pressure.

Rule 5 - You DO NOT need to wring the last possible foot-second of velocity out of your ammunition--it won't do anything for you. An accurate/moderate velocity load is better than an inaccurate/fast load. (See the external ballistics page)

Rule 6 - Temperature affects chamber pressure. While the effect differs with each powder, over the range of about 0 F to 125 F most modern commercial powders are fairly stable showing pressure variation of up to 3000 psi from loads developed at 70 F. Out side of this range the effect is still there but not as linear.) While most current ball powders handle temperatures changes well some types have exhibited a very non-linear response especially at temperature extremes outside of the above range and can result in catastrophic changes in pressures at temperatures much higher than the original temperature. Loads with any powders should be carefully worked up if their use in extreme temperatures is expected especially if at near maximum (Click here for a chart of temperature effects.)

Rule 7 - Don't do anything stupid.

Remember, Watcha-U-Back
(...or, once again, don't do anything stupid)

Unless you are well versed in internal ballistics leave the development of loading data with new powders to the experts with the proper tools, and don't play around with unknown powders. More than one firearm has been destroyed when the wrong powder was used without proper knowledge. As an example, in the 50s and 60s there were a lot of blow ups caused by people trying to use the powder from GI .30-06 blanks in pistols and rifles. 

What these poor souls didn't realize was that the "EC" powder in those blanks burned so fast that it was also used as a grenade filler. These folks turned their firearms into "grenades" with quite small charges of this powder. By the same token many powders of vastly different burning rates look similar. If it's not in the original can, don't use it!

Leave new load development to the pros and stick with the data in reliable loading manuals.

Special Thanks to John Schaefer

The article above is from Fr. Frog's web site at Web URL: and is being used with his express written permission.

Article Copyright 1997-2002 John Schaefer - All Rights Reserved

INTERNATIONAL DEFENSIVE PISTOL ASSOCIATION:   IDPA.  A competition shooting organization that stresses Defensive Tactics and the used of stock or duty type firearms typically drawn from concealment and where the targets are typically engaged with the uses of all available cover.  Defensive pistol shooting as a sport is quite simply the use of practical equipment including full charge service ammunition to solve simulated "real world" self-defense scenarios.   Shooters competing in Defensive Pistol events are required to use practical handguns and holsters that are truly suitable for self-defense use. No "competition only" equipment is permitted in Defensive Pistol matches since the main goal is to test the skill and ability of an individual, not his or her equipment or gamesmanship.   See

INTERNATIONAL SHOOTING UNION [ ISU ]: the organization that sponsors and controls all international firing competitions between nations. The ISU headquarters is in Weisbaden, West Germany.

International Firearms Terms: Click here for translations of common gun terms

INTERMEDIATE CARTRIDGE:  A cartridge designed to allow controllable automatic fire from a rifle, yet more power than a sub-machinegun. The first such intermediate round was the 7.92 x 33 mm developed by the Germans during World War II.  It was primarily used in the Machine Pistol or MP-43 / MP-44 and the Sturm  Gewehre or Stg.-44.  It is referred to as "intermediate" in that it is more powerful than a pistol round, like a 9 mm Parabellum, but less powerful than a full-power rifle cartridge, like the 7.92x57mm.

INTERNATIONAL STANDARD PISTOL: is similar to the caliber .22 weapons used in domestic NRA competition. The barrel length is limited to 15 centimeters and the trigger pull is limited to 1000 grams.

INTERNATIONAL WOUND BALLISTICS ASSOCIATION:  The IWBA is devoted to the medical and technical study of wound ballistics, including evaluation of literature in the field as well as encouraging and promoting new work in wound ballistics.   IWBA is an IRS 501(c) (3) non-profit scientific, educational, and public benefit California Corporation.  On the web at URL:

INTRINSIC SAFETY DEVICE: These are safety devices which are permanently attached to the handgun, either during manufacture or by the user. These include such devices as loaded chamber indicators, magazine disconnectors, manual thumb safeties, transfer bars (a type of drop safety), grip safeties, built-in locks and personalized guns.

IRON SIGHTS: Metallic sights on a gun.  The term is used to differentiate them from optical sights (scopes).

ISOSCELES STANCE:  Common and natural handgun shooting stance where the extended arms form an isosceles triangle, giving the stance its geometric name. For detailed information on the Isosceles Stance see the detail box below.

The Isosceles Stance

The Isosceles is the basic stance that most people will take instinctively -- thrusting the gun forward with both arms straight out, shoulders perpendicular to the target, and elbows locked.  The arms and shoulders make an isosceles triangle, hence the name.  This stance is the fastest to assume and does not depend on handedness and eye dominance, a crucial factor, if like myself, you are cross-dominant.  It is important not to merely thrust the gun forward and shoot, but to lean your entire upper body forward and curl your hips to flatten out the curve of your lower back.  This allows for maximum recoil absorption, even with large caliber handguns.  Like any other martial, mental or physical skill, to master this stance and to become fast accurate and natural with this technique or other shooting stance you must practice.

Practice your draw and presentation with an unloaded firearm or a dummy gun.  It is a good idea to remove the ammo and magazine from the training area.  Do a chamber check and verify empty, every time you pick up the gun.  Someone else or even you may have loaded it. 

** To be cross-dominant means that you are right-handed, and yet rely on your left eye to aim, or vice versa.  It is more common to be left-handed and left-eyed, or right-handed and right-eyed, but it is not unheard of to be cross-dominant as well.  And switching eye dominance is no simpler than switching hand dominance; if your brain is wired to rely on your right eye, there is little you can do to change this.

There are 3 common hand gun stances.  For more information see Shooting Stance.

ITP:  Abbreviation for Inside The Pants (holster).

IWB:  Abbreviation for Inside The Waistband (holster).

IWBA: Abbreviation for International Wound Ballistics Association.  See above.

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