What Is Recoil

Webster's dictionary defines "recoil" as "...to fall back under pressure," or in the instance of a firearm "the action of recoiling," or "the kickback of a gun upon firing."

Recoil is one of the unpleasant side effects that comes with shooting. In some cases, as with heavy magnum guns and loads, it can be severe, even painful. In other cases, it simply contributes to the shooter's inability to shoot a perfect score through flinch.

Recoil can be measured in two ways: one in the sense of physics expressed in foot-pounds of energy or momentum, commonly known as "free recoil," and another way through the shooter's perceived or "felt" recoil, frequently referred to as "kick," such as "That gun really kicks!"

Since the introduction of gunpowder and the development of hand-held firearms, men have been trying to develop various methods and means of reducing recoil in firearms. The objective was to eliminate or lessen the "kick" of a particular gun, but examined from a historical perspective it can be said these efforts have reached epic proportions since the turn of the 20th Century.

First, the development of fully-automatic machine guns, and then semi-automatic firearms fostered the search for improvements in recoil reduction for small arms used both in the role of shoulder fired guns, and as light weapons mounted in aircraft and other vehicles. To this end, early experiments were conducted with both precise laboratory controlled trials and haphazard field expedient methods.

For example, on the Browning Automatic Rifle, better known as the BAR, field expedient attempts included filing ports in the solid tube flash hider to create a crude compensator to lessen felt recoil and increase control.

Later, these efforts were realized in the Cutts Compensator seen on the Thompson Submachine Gun. The Cutts Compensator neutralized the muzzle climb of the heavy Thompson, and the same device was sold in the civilian market for use on shotguns. According to Julian S. Hatcher the Cutts Compensator reduced the recoil of shotguns anywhere from 15 to 30 percent.

A lot of factors come into play when we are discussing felt-recoil or the kick of a particular firearm, although it is an admittedly subjective area of evaluation. Most any long gun will "kick" differently to different shooters, and there is solid scientific reason behind this.

If the gun fits the shooter to the point the rifle, or shotgun, is pressed firmly into the shooter's shoulder, the shooter's body weight is essentially added to the weight of the firearm and the felt recoil is lessened.

If, however, the gun fits another shooter badly when the gun may be held loosely, or in an awkward manner, this allows the firearm to gain velocity through movement independent of the shooter's shoulder and its impact against his body will be felt as a severe blow.

Was the actual recoil any different between the two shooters, assuming the same gun and load was used?

No, but you will have a hard time convincing the second shooter his shoulder received the same blow the first shooter managed without difficulty.

To the second shooter, this particular gun will always "kick like a mule" while the first may find it a "sweet shooter."

Julian S. Hatcher wrote what probably has to rank as the greatest treatise on the subject of recoil in his book, Hatcher's Notebook, and in it he identifies the three elements of a firearm's recoil.

These three factors all contribute to the production of recoil when a firearm is discharged.

The first according to Hatcher is "...the reaction which accompanies the acceleration of the bullet from a state of rest to the velocity it possesses when it leaves the gun, that is, to its muzzle velocity."

The second element of recoil that Hatcher identifies is "...the reaction which accompanies the acceleration of the powder charge in the form of a gas to a velocity in the order of half the muzzle velocity of the bullet."

The third factor influencing recoil from firearms is "...the reaction due to the muzzle blast which occurs when the bullet leaves and releases the gas, which rushes out and gives the same kind of reaction or push that propels a rocket or a jet plane."

A lot of attention has been paid to this third factor by pistolsmiths and shooters competing in Unlimited IPSC competitions. It has given rise to a specific type of handgun — the comp gun. The comp gun is a highly modified semiauto pistol that always possesses a muzzle mounted compensator.

Compensators are not new, like we said earlier, the Thompson Submachine Gun had a production compensator as early as 1928, and compensators have been used on a variety of competition handguns. We've even seen them on Olympic grade rapid fire pistols shooting the .22 Short cartridge, but it was IPSC competition that really prompted the development of first the single port, then the double port, and now today — the triple port compensator, as a means of increasing the control of the handgun, while also lessening or redirecting the felt recoil of the handgun.

The most popular caliber in IPSC is the .38 Super cartridge. It is a caliber that in the older single column magazine, 1911 style pistols offers both increased magazine capacity and "Major Caliber" power factors.

It used to be the .38 Super was loaded with 160 gr hard cast bullets and a sufficient charge of powder, like Accurate No. 7, to make Major. Today the movement is toward extremely light bullets and even higher velocities.

The problem, of course, is managing these loads at safe pressure levels, and that is extremely difficult to do. One of the reasons the shooters keep moving to yet ever lighter bullets is the reduction in felt-recoil, and the increase in gas pressure that accompanies these light bullet major loads. The light bullets "recoil" less in terms of barrel jump, while the increased pressure can be utilized more efficiently by today's triple port compensators.

But, we may be getting ahead of ourselves just a bit at this point, because recoil in handguns is a little different than it is in long guns.

The barrel of any handgun is almost always above the hand, in some cases well above the hand, meaning the handgun in recoil will pivot off the center of the shooter's wrist. Lowering the barrel in relation to the wrist will lessen the leverage the gun exerts on the wrist and that would lessen the recoil the handgunner would experience, but the slide on an auto-pistol must always clear the top of the hand during its operation, so the barrel remains above the hand on any auto-pistol.

This leverage on the wrist forces the barrel upward in an arc, and there is little the shooter can do physically to counteract this force, other than gripping the gun with a tighter and firmer grasp.

The advantage to the compensator is the baffles of the compensator deflect and redirect the high velocity muzzle gases. The deflection of gases redirects some part of the recoil energy and makes it easier for the shooter to control the pistol in the manner desired. It doesn't eliminate the recoil, but a good compensator will vector the recoil through jet effect in a manner that exxentially reduces the force through redirection.

The move toward lighter bullets is also understandable for the simple fact that all other factors remaining the same lighter bullets recoil less than heavier bullets.

The use of light bullets versus heavy bullets in terms of recoil is exactly what Hatcher was talking about in his first element of recoil. It naturally follows from Newton's third law of motion that to every action there is always an equal reaction. So, if a heavy projectile is launched from a firearm the resultant equal reaction is going to be heavier recoil.

Some will question what Hatcher's second element of recoil is all about, but what he was discussing here is the force generated when gas expands.

What Hatcher did was study the inside volume of a loaded .30-06 cartridge and compare that against the volume of the barrel and firing chamber on a 1903 Springfield rifle. Using these figures he compared the various volumes behind the bullet as it traveled down the barrel after firing and came to the conclusion the propellant gas in this situation was moving at a velocity 46.75% of the muzzle velocity of the .30-06 projectile.

Gas has mass, and this mass attains velocity, so it naturally follows that it will produce both momentum and recoil, the question remains to what extent does this element of recoil influence the shooter, and how great is its effect?

There are other factors that will influence felt recoil that have nothing whatsoever to do with free recoil, muzzle blast being the most prominent factor. Many experts feel muzzle blast, both in terms of noise and flash, contributes more to shooter flinch than any actual element of free recoil.

Have someone drop a plank behind you while standing on a concrete floor and see if you don't jump as a natural reaction to the noise! Muzzle blast has the same effect on many shooters and it is a difficult reaction to learn to control. Yet, muzzle blast has little or nothing to do with free recoil. Muzzle blast is a phenomenon that occurs outside the gun. The gases have left the muzzle and the projectile is long gone when this blast registers its effect upon the shooter.

All of these elements are part and parcel of recoil, and shooters are going to have to deal with them as long as we have metallic cartridge firearms. Certain aspects of recoil will remain even after ballistic science moves on to futuristic rail-guns and needle-like projectiles that reach velocities greater than 30,000 fps. The laws of physics remain true, but by then perhaps computers will help the shooter anticipate the sequence of events as they occur in microseconds and provide the appropriate counter-measures.

In the meantime, we will continue to work on improving our flinch and shooting ever tighter groups.

EFFECTS OF ATMOSPHERIC CONDITIONS ON INTERIOR BALLISTICS OF AMMUNITION_

Experienced handloaders know that the performance of ammunition is affected by conditions in the surrounding atmosphere. They know, for example, that handloads developed in cool weather, using the usual subjective pressure signs to establish the maximum charge weight, sometimes show signs of excessive chamber pressure when they are fired on a hot day. The effects of atmospheric conditions on the interior-ballistic performance of ammunition can occur at four different stages in the life of the ammunition, namely: (1) during storage of the components before the ammunition is loaded; (2) during the process of loading; (3) during storage of the loaded ammunition; and (4) at the time of firing.

Temperature is the environmental factor that has the greatest effect on ammunition performance. It can affect the performance at all four stages in the life of ammunition, and it effects the flight of projectiles from gun muzzle to target as well. The exterior-ballistic effects of temperature on the trajectory are accurately predictable, but the interior-ballistic effects are not. The general trend, of course, is that lower ammunition temperatures are associated with lower muzzle velocities, and higher ammuni tion temperatures with higher muzzle velocities, but the magnitude of the velocity change that accompanies a given temperature change can vary quite widely from one situation to another.

There are, in fact, instances in which the trend is reversed. There are also exceptions to the rule that lower temperatures produce lower chamber pressures. At very cold temperatures, some powders, particularly some double-base powders having a relatively high nitroglycerine content, will be found to produce relatively wide round-to-round variations in velocity and pressure, and the highest pressures observed at cold temperature may then be higher than the highest pressures observed at normal or high temperatures.

The general trend is shown in a short Memorandum Report written by Barbara Wagoner of the U. S. Army Ballistic Research Laboratory, titled CHANGE IN MUZZLE VELOCITY DUE TO A CHANGE IN PROPELLANT TEMPERATURE FOR SMALL ARMS AMMUNITION. The detailed data on which the report is based are not given in the report. They involved ammunition of various calibers from

5.56mm to 30mm, with both single-base tubular-grain (IMR) propellants and double-base spherical-grain (Winchester) Ball propellants. The data pertain only to ammunition for rifles and machine guns, and do not necessarily apply to handgun ammunition or to shotshells. The temperature range covered is from -65o F. to + 165° F. There is relatively wide scattering of points on the graph shown in the report, but the trend line that was established can be said to represent the typical relationship between muzzle velocity and temperature for ammunition of the types included. The following equations were derived from the information given in the report.

To find the muzzle velocity (VT) at any temperature (T) from -65o F. to +165o F., given the muzzle velocity (V70) at 70o F., the equation is:

Similarly, to find the muzzle velocity (V70) at 70o F., given the velocity (VT) at temperature T, the equation is:

The limitations of any such equations intended to predict the change in velocity corresponding to a given temperature change are illustrated by the following example. A test of three different primers was conducted at the former U. S. Army Frankford Arsenal, using the military equivalent of the .308 Winchester cartridge. All of the bullets were the same, all of the cartridge cases were the same, and all of the charges consisted of 43.0 grains of IMR 3031 powder. The tests were conducted with ammunition conditioned at temperatures of -70o F., +70o F. and +165o F., with the following results:

INSTRUMENTAL VELOCITY, FPS

PRIMER

TEMP.

DIFFER

TEMP.

DIFFER-

TEMP.

TYPE

-70O

ENCE

+70O

ENCE

+165O

R39

2686

+55

2741

+35

2776

T53

2576

+134

2710

+77

2787

FA26

2528

+164

2692

+59

2751

These results demonstrate the difficulty of trying to find a general formula that will accu rately predict the effect of ammunition temperature on muzzle velocity. As we see, a difference in even one component, in this case the primer, may produce distinctly different results in the relationship between ammunition temperature and muzzle velocity.

The effects discussed above pertain specifically to temperature of the ammunition at the time of firing. Temperatures during storage of the loaded ammunition, and/or the ammunition components, can also have significant effects on performance of the ammunition. The powder is the component most affected by storage temperature. All smokeless powders consist mostly of nitrocellulose, but double-base powders contain nitroglycerine as well, whereas singlebase powders do not. Nitrocellulose is actually an unstable chemical compound, and it must eventually decompose into its constituent elements and/or into simpler chemical compounds. The life of smokeless powders is extended by the addition of a chemical stabilizer, usually diphenylamine, to the powder during manufacture. Powders that decompose very slowly are said to have good chemical stability. The chemical stability of powder depends not only on its nominal formulation, but also on the care exercised during its manufacture. For example, the chemical stability of a powder is compromised if the acids used in the production of the nitrocellulose are not thoroughly removed in later stages of the manufacturing process.

The early effects of chemical decomposition on ammunition performance are that average velocity and pressure are reduced, and round-to-round variations in velocity and pressure are increased. The powder may at this stage also show a reddish-brown "dust" on the surface of an undisturbed powder bed, and the "dust" will be found mixed in with the powder granules. The "dust" is an oxide of nitrogen, and it is toxic if inhaled or ingested. Moisture in the environment to which the powder is exposed will accelerate its deterioration. As decomposition progresses still further, the powder will attack and corrode metal containers in which it is kept. If loaded into ammunition at this stage, the powder will produce misfires, and the acidic products of the decomposition will attack metallic cartridge cases from the inside and ruin them.

At normal temperatures, the decomposition of well made smokeless powder proceeds so slowly that we may see no change in its ballistic performance during twenty or thirty years of storage. The rate at which the decomposition takes place is strongly dependent on temperature. There is no simple way to predict very accurately the rate at which a particular type and lot of smokeless powder will decompose, but if certain simplifying assumptions are made, we can apply a simple rule of physical chemistry to illustrate the kind of relationship that exists between storage temperature and powder life. It is a fact that the rate of many chemical reactions is approximately doubled for each rise of 10o Celsius (18o Fahrenheit) in temperature. If that rule is applied to the decomposition of smokeless powder, it indicates that a powder which is expected to have a life of 20 years when stored at a cool 60o F. in a powder magazine, would be expected to have a life of about 14 years at 70o F., 9 years at 80o F., 6 years at 90oF., and 4 years at 100o F.

Still another effect of storing powder at high temperature is to drive out some of the moisture, as well as some of the volatile solvents such as ether and/or alcohol and/or acetone, that normally remain in newly manufactured powder. This change in the powder is partly reversible and partly irreversible. Once driven out, the volatile solvents cannot be recovered, but the moisture content of the powder will increase or decrease upon exposure to the air, more or less, depending on the relative humidity. This property of giving up moisture or taking on moisture from the air is called hygroscopicity, and of course it is undesirable. Single-base powders are usually more hygroscopic than are double-base powders. The moisture content of newly manufactured singlebase powders is approximately one percent by weight, and it is somewhat less for double-base powders. Powders are normally packaged in hermetically sealed containers, and the moisture content will remain constant until the seal on the container is broken.

Another type of stability in propellants is called ballistic stability. It refers to changes in the powder that are not chemical in nature, but like chemical changes, they affect adversely the interior-ballistic performance of the powder, and like chemical changes they are irreversible. Unlike the adverse effects of chemical deterioration, which tend to lower both velocity and pressure levels, changes related to ballistic stability cause increases in chamber pressures, usually without a commensurate increase in muzzle velocity. These changes in ballistic performance are brought about by changes in the distribution ofthe deterrent coating, which has been applied to the outer surface of the powder granules during manufacture to give the powder the quality of progressive burning. Deterrent coatings work to achieve progressive burning by decreasing the linear burning rate of the powder granules during the early stages of powder burning. To work effectively, deterrent coatings must remain on and very near to the outer surface of the powder granules. At very high storage temperatures - generally above about 140o Fahrenheit - the deterrent coating used on some propellants may migrate inward from the surface toward the center of the grain, where it can no longer serve effectively its intended purpose of reducing the initial burning rate of the powder.

When exposed to the atmosphere, powder will lose moisture or gain moisture, depending on the relative humidity of the air to which it is exposed. There is for each powder type, and for each value of relative humidity, a particular level of moisture content that is called the equilibrium moisture. The equilibrium moisture is that level at which the moisture content of the powder will remain unchanged, provided the corresponding level of relative humidity remains unchanged. The following table gives the approximate equilibrium moisture for two types of powder that have been used in loading .30-06 military rifle ammunition. The singlebase powder is a tubular-grain IMR type, and the double-base powder is a tubular-grain type containing about 20 percent nitroglycerine. The entry M. & V. under D. B. Powder indicates that the percentages below include both moisture and volatile solvents.

RELATIVE S. B. POWDER

HUMIDITY, MOISTURE,

PERCENT PERCENT

10 0.20

30 0.55

50 0.80

70 1.05

90 1.55

Decreases in moisture content will increase pressure and velocity, while increases in moisture content will decrease pressure and velocity. The magnitude of the effect that variations in moisture content will have on the ballistic performance of the ammunition will depend on the particular powder, the particular cartridge in question, and other factors not discussed here. The typical magnitude can be illustrated, however, by figures for the powders mentioned above, if loaded in .30-06 military ammunition. The following table shows the effects of a change of 0.1 percent in moisture content of the powders.

Type of Powder:

Change in Charge Weight (Grain) for Constant Velocity:

Change in Velocity (fps) for Constant Charge Weight:

Change in Pressure (psi) for Constant Charge Weight:

SINGLE-BASE DOUBLE-BASE

0.30

0.30

Another effect of powder temperature on the ballistic performance of ammunition becomes of some importance at the time of loading. That is the effect of temperature on the gravimetric density (sometimes called bulk density) of the powder. The gravimetric density of powder is the ratio of the weight of powder which fills a particular volume (under carefully prescribed conditions of filling), to the weight of water that would fill the same volume. The gravimetric density of a powder is important because it determines the amount of powder that will be dispensed from a volumetric measure of a particular capacity. Of course it is also important because it determines the weight of charge that can be accommodated by a particular cartridge case, and the amount of airspace or the degree of compression corresponding to a given charge weight in a particular cartridge.

Gravimetric density depends on several characteristics of the powder that are not discussed here, but it has been found in general that increasing temperature of the powder causes a reduction in the gravimetric density. For the single-base tubular-grain (IMR) powders that have been loaded in military ammunition, the increase in gravimetric density amounts to about one percent for a temperature change of 20o F. In practical terms, this means that the powder charge dropped by a volumetric measure set for 50.0 grains at 65o F. would weigh only about 49.5 grains at a temperature of 85o F. The corresponding changes in ballistics for the .30-06 are about 25 fps in velocity, and about 1200 psi in pressure. This is of some importance in loading plants, where the charges are dispensed from volumetric measures that are not easily adjusted, and it is important to maintain uniform velocities throughout a lot of ammunition. It is for this reason, among others, that the temperature of the powder and the surrounding atmosphere in a loading plant must be maintained at a reasonably constant level.

It is good advice also for handloaders to avoid, insofar as possible, conditions that can seriously affect the performance of the powder when it has been loaded into ammunition. Powder should be stored in a cool, dry place preferably in the hermetically sealed containers in which it was packaged by the manufacturer. If powder is brought into the loading area from a cold environment, such as an unheated building in wintertime, it should be left in its sealed container for several hours until it has come to room temperature before it is opened. If it is not, then condensation of moisture on the powder granules will in the short term reduce the gravimetric density and impede free flowing of the powder from a measure, and in the long term it will increase the moisture content significantly as the surface moisture is absorbed into the granules.

For good reasons, some of which have been mentioned before, it is likewise good advice to protect loaded ammunition from excessive humidity, and from extremes of temperature, and from frequent temperature changes. This is especially true of handloaded ammunition, because the individual cartridges of handloaded ammunition (and indeed of most non-military factory-loaded ammunition as well) are not hermetically sealed. Such ammunition "breathes" during temperature changes, because the air inside expands at high temperature and leaks out into the atmosphere, and it contracts at low temperature, drawing in air from the surrounding atmosphere. The powder inside the cartridge is thus exposed constantly to fluctuations in humidity of the surrounding atmosphere. Whatever care was used in the storage of components, and handling of them during loading, the good effects can be lost if the loaded ammunition is carelessly stored. Modern ammunition components are remarkably tolerant of adverse storage conditions, and will perform adequately under a wide variety of circumstances, but care in handling of the components and the ammunition at every step is nevertheless worthwhile.

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