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What Happens When You Fire A Gun |
What Happens When You Fire A Gun?By M.L. McPherson
©Copyright 2008, The Varmint Hunters Association, Inc.
Synopsis: While most serious shooters have a basic understanding of what happens to the gun when it is fired, it is interesting to consider the details. Here I will try to cover everything of interest in this regard.
I do not intend to cover the intricacies of internal ballistics; I have rather well-covered that in previous articles. In this work, I am more interested in what happens to the gun, the bullet, and the cartridge case when we fire a gun. These subjects are tied together because of the intimate nature of the process. For example, as the rifling engraves onto the bullet, the barrel stretches. Nevertheless, I will try to cover each subject separately and without too much repetition.
THE BULLET
The primer blast can dislodge the bullet. If so, before developing propellant force is sufficient to keep it moving, the bullet can stop. Or, as the bullet slows, inchoate (just beginning) propellant force can become sufficient to stop the slowing and then accelerate it. Either instance is not apt to occur in accurate ammunition — if the primer dislodges the bullet, the load is not apt to be particularly accurate.
In most rifle loads, the primer blast does not dislodge the bullet. It stays put until the force generated by the burning propellant pushes on the base hard enough to overcome the frictional bond between bullet and case neck (bullet pull). At that instant, the bullet begins to accelerate from the case and into the bore.
In most loads, propellant force only has to overcome neck tension and inertia to move the bullet until it touches the rifling. Then, propellant force must be sufficient to engrave the bullet into the rifling.
A significant complication occurs with normal sporting bullets. Such bullets cannot support more than a small percentage of the force applied during the firing process; therefore, the bullet soon will begin to swell at the base (because the base must push against more bullet mass than the portion of the bullet just forward of the base). As applied force progressively increases, this swelling will progress from the bullet base toward the bullet nose so that it includes progressively more of the shank.
This swelling is called obturation and is necessary for accuracy with any sporting bullet and gun. As I have proven by firing perfectly normal 44 Magnum ammunition out of a Ruger 45 Colt revolver — most such bullets will obturate sufficiently to fully engrave the rifling — the only thing that keeps the bullet from swelling indefinitely is support from the barrel (which in turn stretches, as I will cover later). When the “normal” force created by the stretched bore matches the obturation force created by the bullet, the two are stable — the bullet and the bore stop swelling. As the bullet travels down the bore, obturation force initially increases, reaches a peak, and subsequently decreases, in step with chamber pressure. At each increment along the barrel, diameter of the rearmost portion of the obturated bullet shank necessarily matches diameter of the stretched bore.
As the bullet continues down the bore, at some point, force of acceleration falls under the obturation threshold and bullet diameter stabilizes with a cross section that mirrors the bore.
The complications are many. First, with conventional rifling, bridging strength prevents any bullet from fully sealing any bore. At the bottoms of the “grooves” in the bore, the corners of the “lands” on the bullet will never be completely “square.” The rounding will leave tiny vents where propellant gas can escape.
Second, as the bullet is stretching the barrel laterally (increasing bore diameter) and even when it is not, friction between bullet and bore will reflect the normal force between the two. Hence, when chamber pressure peaks, bullet-to-bore friction is rather significant. As the bullet moves toward the muzzle, friction decreases until bullet acceleration force falls below the limit that the bullet will support. Thereafter, bullet-to-bore friction is more or less constant. Seems a miracle that any firearm can be accurate!
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