Th• Botilenge chronograph "hook-up" before tho shot ha* b*«n fired.
of the gun and continuing to the left, say, in increments of ten feet each, running up to at least eighty feet. The right hand vertical side of the sheet should be marked off in units of velocity. Fire a series of shots at 78 feet in the usual manner and plot the result as one point of a velocity-distance curve. Then move the terminal target up to about 38 feet from the muzzle, fire another series of shots and plot a second point on the cross section paper. Now, connect the two points and extend the line to the right until it reaches the zero, or muzzle point, and you will have the approximate muzzle velocity. A more accurate result can be obtained by firing the series at a greater number of different distances but two points will prove fairly accurate; at least much more accurate than guessing.
A Boulenge chronograph, in good condition and carefully adjusted, can measure such short intervals of time as that between the time the trigger is pulled and the time the bullet leaves the muzzle of the barrel. Such a fine adjustment is hardly necessary for routine testing of ammunition, but is a virtue in testing powder lots. The powder companies will take the velocities of hand loaded ammunition for any hancfloader of an experimental turn of mind, at a very reasonable charge.
The Aberdeen chronograph is also used to a limited extent for measuring bullet velocities. This apparatus takes the form of a synchronous motor, mounted with the shaft in a vertical position. To the shaft is assembled a shallow pan or metal dish with straight sides, carefully turned and balanced. This pan carries a strip of thin paper equal in length to the 95 Inside circumference of die pan. Instead of two weights, two points arc located, one just over the other and close to the pan wall. These points are connected to separate electrical circuits, one passing through the muzzle wire and the other through the disjunctor. The bullet, on breaking the muzzle wire, causes a spark to jump from its point to the wall of the pan, burning a tiny hole in the paper and when the bullet strikes the disjunctor, a second spark jumps through the paper. The distance between the two holes is an indication of the instrumental velocity. It can be seen that the accuracy is dependent upon the pan rotating at a proper and uniform speed and if it dees this, the paper can be (and is) marked off with lines so that a direct velocity reading may be taken from it.
A synchronous motor is one especially wound to maintain a uniform number of revolutions per minute, regardless of fluctuations in the line voltage. Actually, it does not do this but will lose or gain more or less speed as additional load is taken off or put on to the line the motor is running on. What it does is to adjust itself to these changes and return to its normal speed. These temporary fluctuations in speed are of no consequence for all ordinary purposes, but they are fatal to the taking of accurate velocities. An Aberdeen chronograph would be of little use if hooked up on a city line, especially a power line. If the lady next door turned on her electric stove just as a shot was fired, the reading of velocity given by the instrument would be worthless. These chronographs arc at their best when operated by a separate generator of their own and on a line which is not used for any other purpose. Under such a condition, very accurate results can be obtained with them.
In working out their tables of chargcs, the powder companies use new, primed cases purchased from the commercial ammunition companies or, in the ease of the Cal. .30-06 cartridge, from Frankiord Arsenal. These cases are primed
96 with the primers that the manufacturer uses in loading his own ammunition and most of these primers are different from those sold for reloading purposes. The flash holes in the cases are made of a correct size for the primers used, but the use of a primer of different make from the cartridge case can give a very different order of ignition to the powder charge. All cartridge eases of the same calibcr arc not of the same capacity; some have thicker side walls and thicker heads than others. As the outside dimensions must be the same, within very close limits, any variation in the thickness of the metal in the cases will mean a variation in their internal volume. If two cases of different thicknesses are loaded with the same charge of powder, one will have a higher density of loading than the other and will conse-quendy develop a higher pressure. Between the differences in cases and primers, it is a matter of chance if a hand-loader gets the same velocity and pressure from his reloaded ammunition that the powder manufacturer got, even though the handloadcr is meticulous in the preparation of his ammunition. The nearer the load is to the maximum recommended load for any cartridge, the more marked will be the effect of variations from the conditions under which the load was worked out. Any difference in bullet weight, diameter, shape, or hardness will also affect the ballistics.
In addition to variations in components, there are the guns to be considered. Chambers vary in size and shape and ammunition fired in an arm having a tighter chamber than the test gun will develop a higher pressure. The same is true if the bore and groove dimensions, and especially the throating of the barrel, arc tighter or smaller.
Powder for testing purposes is kept in rooms or magazines, where the temperature is maintained at a uniform level, and tests for velocity and pressure are made with the powder at 70° F. Any increase in the temperature of the powder will cause it to ignite more easily and to burn more rapidly, thereby causing a rise in the chamber pressure above the expected point. Exposing ammunition to the hot
97 sun long enough to heat up the powder chargcs will usually produce some surprising results and doing this with some cartridges has been known to increase the pressures over 10,000 pounds per square inch. Maximum charges, which are much in the nature of "proof charges" to begin with, will certainly become more dangerous if warmed up before firing.
The powder boys know these things and state very clearly in their folders and booklets of tables of charges that the figures shown are those obtained with the arms and components that they used. They further recommend that charges below the maximum recommended chargcs be used for the best accuracy and that in rifles with tight chambers the heaviest charges should be reduced several grains in weight. This is excellent advice to follow.
The tables of charges published by the powder companies give either the seating depth of the bullets or the over-all length of the cartridge for each load listed. This is done to show the condition under which the ballistics were developed. In the moderate and reduced loads, the exact seating depth of bullets is not of great importance, but it bccomes of increasing importance as the charges approach the higher levels. With die heaviest charges shown in these tables, the bullets should never be seated deeper than the dimensions given in the tables. With full charges in such cartridges as the .220 Swift, .257 Roberts, or any others in which the cartridge is a close fit in the chamber, especially at the neck, even a few thousandths or an inch increase in the seating depth of the bullets will cause an appreciable increase in the chamber pressures.
If the reader believes from the foregoing remarks that the writer is a timid soul or an alarmist, he is mistaken. Variations in components such as primers, flash holes, volume of cartridge cases, etc., are matters a handloadcr seldom thinks of. To the average person, all cases of the same caliber are alike and a primer is a primer; just a little dingus that makes the cartridge explode. There is a lot more to them than that and while their variations can cause lower pressures, as well as higher ones, than arc indicated in tables of recommended charges, their effect in a minus direction has been skipped over because it doesn't involve the matter of safety. Even when these variations operate to increase pressures they arc not likely to result in dangerous pressures if the handloadcr observes carefully all of the information given in the tables.
In manufacturing arms and ammunition there must be manufacturing tolerances and all arms and ammunition of the same caliber are not cxacdy alike nor are the components that go into the manufacture of the ammunition alike. Bullets will vary a litde in diameter and weight, and primers, however good they may be, will not be absolutely uniform from one lot to another.
In the establishment of pressure limits for diiferent arms, experience has dictated a limit for each caliber, make or type that should not be exceeded. This maximum pressure limit is not necessarily close to the bursting point of the gun. As a matter of fact, no one knows what the bursting point of a gun is. One arm may stand a prodigious charge of powder, while another of the same make, model and caliber lets go with what appears to be a perfecdy normal charge. Maximum pressure limits are established, however, to allow a reasonable margin of safety, to take care of the unavoidable variations in ammunition and ammunition components. One primer out of ten thousand or more may give an exceptionally hot flash and over-ignite the powder charge. A cartridge case may have a hidden flaw that can not be seen by the inspectors, or an over-size bullet may get into a cartridge, to say nothing of errors in powder charges. The margin of safety is left to protect the shooter who may happen to shoot a faulty cartridge once in a while and in loading ammunition the handloader should guard against any encroachment on diis margin of safety; it is the most important part of a firearm. All is not gold that glitters and all ammunition components are not alike just because they look alike. To load ammunition intelligendy, especially the heavier loads, one must know what he is dealing with.
Maximum loads are those that exceed the limits of pressures prescribed by experience and intelligent ballistic determinations. They arc over-loads and are therefore dangerous. Sometime, someone lit on the not particularly bright idea that some arms are stronger than others using the same caliber of cartridge and that the ammunition manufacturers load their ammunition to be safe in the weakest arm of each calibcr. It is true that some arms arc mechanically stronger than others but regardless of the arm, it is the cartridge case which has to hold the gasses in. The idea that the ammunition manufacturers load for the weakest arm of each caliber is pure fiction. There are plenty of imported arms in use which have such narrow margins of safety that they ought not be fired with normal loads, fust as one example, do you believe that any cartridge manufacturer loads his ammunition to be safe in the pot metal Spanish guns with which the American market was flooded shordy after the War? Of course not! And plenty of this junk has popped open like a jack-in-the-box with pcrfccdy normal ammunition. Ammunition is loaded by the factories to the 8 highest level of pressure that wili be reasonably safe under the varying conditions of use it is likely to be subjected to,
IN GOOD ARMS THAT ARE IN GOOD CONDITION. The effect' of the improvements that have been made in alloy steels and the additional strength of arms resulting from their use has been to increase the margin or factor of safety and to decrease the number of accidents that occurred with older arms. In spite of this increased strength, accidents will occasionally occur with factory loaded ammunition.
Before we get too far away from the subject, let's take a look at the .30-06 cartridge. This cartridge was developed as the standard military cartridge for the armed forces of the United States. It was especially made to be used in the too Model 1903 Springfield rifle and that rifle is one of the strongest dicre is. Every rifle taking the .30-06 cartridge that has been brought out since the Springfield made its debut has been built to hold the cartridge. The cartridge has not been loaded to suit these rifles. This is true of many other calibers as well. The Springfield rifle has been loaded with prodigious loads of many kinds of powder and has withstood them in laboratory tests, yet these rifles occasionally blow up in service with apparendy normal loads—and so do all other rifles.
Now, it is well for the reader to bear in mind that there is a big difference between a wrecked and a blown up rifle, or any other type of firearm. A wrecked rifle is one that is destroyed by any cause when fired. A blown up rifle is one that is wrecked because of a mechanical or structural weakness in the arm itself. For example, a cartridge case may give way at the head and blow a firearm all to hell, but such a blow is the fault of the cartridge case or load and not due to weakness in the arm. If the shoulders supporting a breech block shear off or a firing pin blows out with a normal load, that is the fault of the arm. The difference is a technical one and of litde interest to the shooter who has his puss parked alongside of the action when it lets go, but when you hear of any rifle not blowing up under excessive pres-
sure don't try to duplicate the load used with the belief that the rifle used in making the test remained intact. Even if it did there would be no object in duplicating the experiment.
A crude test of primer performance. Two "groups" made by firing the primers from the inside of the cup. Not« the deep penetration of the left hand group. This indicates violence but the primers in the right hand group were, by far, the best ignitors of the two.
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