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What Happens When You Fire A Gun |
Page 2/3
A significant complication to consider is a problem with jacketed bullets. In conventional jacketed designs, the only thing that bonds the core to the jacket is residual stretching of the jacket by the core. Therefore, the only thing that keeps the core and jacket bonded during the initial stage of bullet acceleration (before obturation begins) is bullet-to-core friction.
As obturation occurs, the core pushes against the jacket and stretches it, thereby pushing it against the barrel. The core is thereafter intimately bonded to the jacket and eventually the engraved rifling creates a mechanical bond that prevents the core from turning independently from the jacket.
However, as the bullet begins to engrave into the rifling, it is possible for one side (or area) of the jacket to experience more resistance to forward movement. When this differential force exceeds the strength of the jacket and the frictional jacket-to-core bond, the bullet can deform asymmetrically. This will destroy accuracy. It is at least possible that some types of conventional bullets are more susceptible to this problem than are others.
This might explain why long-range accuracy is so difficult to obtain with conventional match bullets launched at unusually high velocity. (It also is possible that core heating in flight is to blame or that deformation at the muzzle is to blame or that some other factor or combination of factors is to blame but for some reason or reasons, such bullets seldom produce fine accuracy when launched faster than about 3,100 fps.
Bullets are heated during firing. Several factors contribute. First is acceleration. The faster and the longer a bullet is accelerated, the more it is heated. We see the same effect when we shoot at a mild steel target with a rifle bullet and observe either a crater or a hole punched in the steel. The extremely rapid deceleration generates sufficient heat to melt both the bullet and the steel of the target.
Second, friction between bullet and bore generates significant surface heating. With cast bullets, as the acceleration force increases, frictional heating increases and, at some point, it will generate so much heat that the bullet and barrel cannot carry the heat away from the bullet surface fast enough to prevent partial melting. When that occurs, accuracy plummets and bore leading results.
Conversely, if peak chamber pressure is insufficient to fully obturate a cast bullet, the bore will not seal and escaping hot propellant gases can melt the bullet surface. This also will lead to leading, and because such a bullet is not apt to center well in the bore, accuracy usually is poor.
Third, with some bullets, some small amount of incandescent heating will occur on the bullet base. In most (if not all) loads, the propellant granules forced against the bullet base will act as a very efficient insulator and thereby will limit such heating.
Fourth, deformations associated with rifling engravement and obturation will significantly heat the bullet. The greater the acceleration rate, the greater this heating will be.
Finally, actually primarily, the shock wave from the primer can compress the granules against the bullet and thereby adiabaticly (a process that occurs without loss or gain of heat by the substance concerned) heat the trapped air, which will slightly heat the bullet.
Perhaps interesting to some readers is the fact that anything that reduces bullet-to-bore friction will also reduce heating from most of these sources.
In any case, the bullet reaches the muzzle with the shank mirroring the bore (in any normal barrel muzzle pressure is far below obturation pressure; if it is not, the load will be wildly inaccurate!). It usually will be somewhat shorter than before firing. It always will be warmer than before firing (the rearmost portion of the shank surface will be at the highest temperature of any portion of the bullet).
Obviously, the bullet will be engraved with rifling, which usually will extrude material behind the bullet base from the bottoms of the rifling grooves in the bullet. Commonly, the bullet nose (and boat-tail) also will be somewhat shortened.
If, at the onset of the firing process, the bullet is poorly centered in the bore or is poorly aligned with the bore as it engraves the rifling and begins to obturate, it is apt to be asymmetrically deformed — one side will be pushed farther forward, which will deform the nose, shank, and base.
This list of things that happen to a bullet during firing is reasonably complete.
THE CASE
As the striker hits the primer, it drives the case forward in the chamber (to the extent that headspace will allow). On some cases with poor headspace control (e.g., 35 Whelen), the striker can drive the case into the chamber with enough force to move the shoulder back and thereby increase headspace.
As the primer pellet explodes, it generates considerable additional force that works to drive the case forward (and will do so, if headspace control is inadequate) and the primer backward (the primer moves until it is supported by the bolt). For cases using small primers, this force is about 750 pounds; for cases using large primers, this force is about 1,500 pounds (cases that have small-diameter flash holes generate greater force).
As the powder charge ignites and chamber pressure begins to build, the hollow portion of the case stretches, to fill the available space. When chamber pressure reaches approximately 3,000 psi, the case walls begin to push against the chamber. Thereafter, as pressure progressively increases, case-to-chamber bonding becomes progressively more solid.
When the force inside the case becomes sufficient, the case body yields and the case head begins to move rearward. The amount of pressure the case can withstand before the case head begins to move rearward depends upon case wall thickness and hardness (in the area near the body-to-web transition), difference between web diameter and primer pocket diameter, and how long pressure stays above the threshold pressure (where case begins to yield).
In most typical loads used in modern guns, chamber pressure will always be sufficient to drive the case head rearward and thereby stretch the case walls. Depending upon case wall thickness and degree of wall thickness taper near the body-to-web transition, the case can elastically stretch some amount — typically several thousandths inch. When this stretching exceeds the elastic limit for that case, the case walls will permanently thin and lengthen. This damage usually occurs about 0.1-inch forward of the floor of the case web.
After chamber pressure becomes sufficient to initiate case wall stretching, the case head soon hits the bolt. As pressure progressively increases, it pushes the case head progressively harder against the bolt. Hence, the bolt will progressively compress and the action will progressively stretch until chamber pressure peaks. (Actually, owing to inertia, the case head will continue to push against the bolt, as the bolt continues to retreat, for some time after chamber pressure has peaked.)
Eventually, the bolt will exert more force against the case head than chamber pressure exerts against the case head. At that instant, the bolt will begin to slow, eventually to stop, and then to reverse direction. As chamber pressure continues to plummet, the energy stored in the bolt and receiver will drive the bolt back toward the resting position. In practice, the bolt will hammer the case into the chamber with considerable force; often sufficient to set the case shoulder back enough to assure that case headspace length is shorter than chamber headspace length. As a result, the action will open freely.
While all of this is happening, the chamber also is stretching in both length and diameter. This contributes some to case stretching; it also supplies the energy that allows the chamber to hammer back against the case body, so that it is reduced in diameter enough to assure free extraction.
Depending upon case shape (body taper, shoulder width, shoulder angle), case construction (hardness, thickness, etc.), load variables (pressure peak and duration), action design and barrel design (over the chamber), many results are possible. First, the action can open freely and the case can extract freely. Second, the bolt can turn freely (because case headspace is shorter than chamber headspace) but the case can hang up in the chamber (because relaxed case body diameter is slightly larger than relaxed chamber diameter (the case is an interference fit). Third, the bolt can turn hard (because case headspace length is longer than chamber headspace length) but the case can extract easily (because case body diameter is smaller than chamber diameter).
This partly explains why various chamber and gun designs show different symptoms with loads at similar peak pressure. The classic comparison is the 22-250 versus the 22-250 Ackley Improved (AI). With the former, this rebounding bolt can easily drive the case into the chamber far enough to move the shoulder and solidly wedge the case; with the latter, the shoulder is many times more resistant to being moved and driving the case into the chamber the same distance accounts for many times less increase in case diameter at any given location. Hence, pressure that causes sticky extraction with the 22-250 will show perfectly free extraction in the AI version.
THE GUN
How much the case head moves the bolt face rearward depends upon the following factors:- Shape of pressure curve — a wider curve means more movement (owing to inertia, the case head never has time to move as far as it would if the same peak pressure were applied in a static situation. Therefore, the longer the pressure stays close to the peak, the more the bolt will compress — the farther the head will move);
- Peak chamber pressure — force and movement are directly proportional;
- Distance between bolt face and locking surface — distance and movement are directly proportional;
- Cross-sectional area of bolt — area and movement are inversely proportional; and,
- Cross-sectional area of receiver — area and movement are inversely proportional.
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