Plate V

53 Cracks sometimes occur in the bottom of primer pockets. Tins defect is traceable to the use of mercuric primers and is, I believe, limited to folded head cases such as are used in much of our present day revolver ammunition. As primers should never be seated without inspecting the primer pockets first, this defect will be easily observed. Its effect is to permit an excessive amount of gas to reach the primer, driving it back violcntiy against the recoil plate of the revolver, setting back or cracking diis part and necessitating sending the arm to the factory for repairs. The webs can be broken to an extent diat will permit the primer to drive forward without firing at all, then again there may be enough resistance to cause the primer to fire. The broken web can permit a greater amount of flash than normal to reach the charge and cause over ignition of the powder, with attendant high pressure.

Some reloading tools have apparendy been designed with more thought about speed of operation than the elementary principles of reloading, and the operations of dccapping and repriming have been combined with no consideration whatever of permitting proper inspection of the primer pockets before seating new primers. Fortunately, all of these tools permit the two operations to be divorced from one another and the reader should be wary of the tool that doesn't.

Primer Pellet. The primer pellet is the fire producing or business part of the primer and it is made from a mixture of several ingredients, as there is no one substance suitable for the purpose. Small arms primers are fired from the blow delivered by a firing pin and the force available is limited, making it necessary to use at least one substance in the mixture that is very sensitive to shock. This clement which starts the burning of the primer is usually called the "initiator" and it must be a substance that will explode on receiving a sudden shock or blow from a firing pin. Its positive action depends upon a second, or frictional element, which is a rough, hard substance incorporated in the priming mixture. The third element is the "fuel" or that part of the mixture which produces the flame. This flame must 54 be of sufficient length and duration and muse produce enough heat to ignite the powder charge properly. The priming compound, in its action, can be likened to a blue-tip match. The tip of the match is the initiator; the rough side of the box the frictional element, and the head of the match is the fuel which must burn long enough to ignite the wooden match stick. A different arrangement, but the same general idea. Some initiators produce a considerable amount of heat, but becausc of their quick, violent nature the heat does not last long enough to accomplish its purpose. Likewise, some frictional elements arc good fuels, while others, notably ground glass do not burn at all but apparendy become incandescent when the primer explodes and thus contribute in a way to the ignition of the charge.

It used to be believed that ground glass from primers caused a scoring of the throats of rifle barrels. The writer doubts this, for if the glass becomes incandescent, and there is evidence to indicate that it does, the hot particles would be too soft to score the barrel. Besides, ground glass is still used in some of our present day primers and long scries of shots fired with such primers show no injury to barrels. It would therefore seem that thr old prejudice against ground glass as a frictional element in primers was unjustified and diat the scoring referred to was in reality due to erosion. Frosicn has only become of interest to ballisticians in com paratively recent years and while what is known of it is largely from die standpoint of results rather than causes, it is certain diat the condition is in no way dependent upon ground glass in primers.

Chlorate Primers. The first initiator used in small arms primers successfully was fulminate of mercury. As it is too quick and violent in its action to make a good primer by itself, it was combined with potassium chlorate, the latter being an initiator as well as a fuel. Both of these substances have formed the basis of primers for small arms up until only a few years ago and our military primers are still made

55 without fulminate of mercury, but with potassium chlorate as the principal ingredient. No primers arc made with these two substances alone, others must be incorporated with them to produce good priming mixtures. Both chlorate and chlor-atc-fulminatc primers arc excellent from an ignition standpoint, but like all good things, there is some evil in them. Potassium chlorate primers, when fired, leave a deposit in the barrel diat gathers dampness rapidly, causing rusting, therefore arms fired with these primers should be cleaned promptly after firing, preferably with water, as oil or nitro solvents that do not contain water will not dissolve this fouling.

Any primer containing fulminate of mercury is termed a mercuric primer; when fired, the mercury will attack the brass cartridge case and often render it unfit for reloading— and the reloaders never did like that.

All of these eases ha\e some difference in vent size or bevel which would affect the ignition of the charge differently.

The vent site may be different in various lots of the same make and caliber of cartridge ease.

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