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Normal Topic Heat vs Pressure vs Velocity effects on barrel wear, consistency and accuracy. (Read 631 times)
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Heat vs Pressure vs Velocity effects on barrel wear, consistency and accuracy.
Sep 3rd, 2018 at 3:14am
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There have been a number of PMs to me in this area. Given the content, apparent concerns and queries raised, I will take the time to give my thoughts based on my experience in firearm and ammunition R and D over the last 20 plus years. To be very clear, the field I worked in concentrated on Firearms for Military/ Law Enforcement use. We had at our disposal several million Euros’ of state of the art metrology, An exhaustive range complex and uninhibited access to more than 30 firing ranges in locations from the High Arctic to Saharan Desert, Tropical Jungle to High Alpine, an extremely well supplied armorer/engineering/ ammunition workshop with a phenomenal team of experts to run it, Access to the R&D staff and libraries of virtually every armaments manufacturer in the US and Europe. Funding came from government agencies, international bodies/NGOs or manufacturers. The various tasks were undertaken without any knowledge of who was paying the bills. To sum up, we lacked for nothing and had no idea of who would be using the results. I was involved as an expert in Special Forces and Sniper weapon deployment, use and techniques. During this period I would estimate we tested or examined several thousand different weapons and put hundreds of thousands of rounds downrange, including on the battlefield, in police actions, game protection work, border security operations and the like.

The info I use is drawn from memory for the most part. In some cases, I may make statements that rely on non-disclosable testing or data. I will not, in any case, make any statement or offer any information which I know to be inaccurate or unsubstantiated by thorough testing/ verification.

This will take some time to put together and I will do it along the lines of the Does you firearm fit correctly series I am posting at the moment.

To set some foundation and to help those who may not know these terms here is a basic glossary for the upcoming items

Bore Capacity - A term used to describe the volume of the bore as it relates to its ability to effectively burn a given amount or type of powder, with a certain combination of components. A cartridge which may be “over bore capacity” with one type of powder, may be perfectly suited to another powder of a different burning rate. Specifically, bore capacity for a given component combination is indicated by the point (or Delta) at which the pressure and velocity curves begin to separate.

Brisance - The shattering or crushing effect of an explosive.

Bullet creep - the movement of a bullet out of the cartridge case due to the recoil of the firearm and the inertia of the bullet. Also called bullet starting or popping.

Compressed Charge - A load in which the seating of the bullet actually causes some compression of the powder. This situation is quite normal when using some of the slower-burning powders commonly used in the large-capacity Magnums.

Duplex Load - A load utilizing two different type of powder; usually a faster powder near the primer, and a slower one for the main charge. Duplex loading is intended to give higher velocities, smoother pressure curves, and greater powder efficiency.

Erosion - The wear, usually in the throat area of a barrel, caused by extreme heat and friction. Erosion occurs in all firearms but is aggravated by rapid-fire, large case capacity, or the use of hotter burning powders.

Expansion Ratio - A mathematical expression for the relationship of the volume of the bore and powder chamber, to that of the powder chamber alone. Expansion ratio is a critical factor in predicting the performance capabilities of a gun/cartridge combination.

Freebore - Essentially, the throat area of a barrel. Normally, use of the term “free-bore” indicates the rifle in question has an unusually long throat.

Gas - The rapidly expanding vapour caused by combustion. As the gas expands in an enclosed chamber (the cartridge case), it generates tremendous pressure. It is this pressure which drives the projectile to the target.

Gyroscopic stability - the ability of a fired bullet to remain stable in flight due to its spin.

Ignition Time - The time interval between the impact of the striker or firing pin on the primer, and a rise in pressure sufficient to start the bullet from its seat.

Leade - The minute portion of a barrels rifling which slopes from the unrifled throat to the full-depth rifling. Although frequently referred to as the throat, there is a definite difference between the two.

Load Density - The weight of the powder charge in grains, divided by the volume (frequently expressed in grains of water) of the case.

Lock Time - The time interval between the sear’s release of the striker or firing pin, and the subsequent impact on the primer.

Maximum ordinate – the maximum height the projectile will travel above the line of sight on its path to the point of impact.

Metal Fouling - Metallic residue left in a barrel after firing. Although the current use of gilding metal has reduced fouling problems, the shooter still needs to keep a close eye on the condition of the barrel. This fouling, normally seen as a copper wash in the bore will have a detrimental effect on accuracy.

Obturation - The sealing of a bore and chamber by pressure. During the firing process, pressure swells the case within the chamber, preventing gas from leaking back into the action. The same pressure, applied to the base of the projectile causes it to swell or upset, filling and sealing the bore.

Overbore - A loose term used to describe a case that has more capacity than it can effectively use with normally available powders.

Point Blank Range - The range to which a shooter can obtain a hit in the vital zone of a target, without holding over or under. Point blank range is influenced by many variables, including target size, initial velocity, and ballistic coefficient. This term is frequently misused to refer to extremely close ranges. In reality, a target may be point-blank range even when it is several hundred yards away, depending on the variables mentioned.

Precession – The phenomenon of unburnt and burning powder and gases being forced past the projectile due to Brisance before Obturation can seal the projectile to the rifling. This usually occurs just as the projectile starts to move forward in the neck as the neck expands due to brisance. It is usually worst with low density, high-pressure loads propelling high SD projectiles in freebore conditions.

Seating lines - the circumferential striae parallel to the axis of the projectile generated on the surface of the bullet by the cartridge case.

Sectional Density - A bullets weight, in pounds, divided by its diameter in inches squared. High sectional density is essential to producing a good ballistic coefficient and deep penetration.

SEE - Abbreviation for Secondary Explosive Effect. SEE is a condition which can occur when slow-burning powders are used at greatly reduced charge weights (poor loading density). Rather than burning in a normal fashion, the powder detonates, as though it were a severe overload. Also known as a “pressure excursion.”

Throat - The unrifled portion of the bore immediately ahead of the chamber, and before the Leade.

Time Of Flight - The time taken by a projectile to traverse two points, or a specific distance. Time of flight is a critical factor to a number of ballistic calculations.

Yaw - The rotation of a bullet at an angle (usually very slight) to its line of flight. Some yaw is almost always present when a bullet is fired, but this usually dampens out within 200 yards if the bullet is properly stabilized and well balanced.

Zero - The adjustment of a firearms sights in order to obtain impact at a desired point in relation to a specific point of aim, at a given range.

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