03-08-2013, 03:47 PM
UNDERSTANDING AND MITIGATING GUN BARREL EROSION
UNDERSTANDING AND MITIGATING .pdf (Size: 845.68 KB / Downloads: 65)
INTRODUCTION
A gun is a weapon designed to discharge a projectile. The first firearms were invented in China in the
12th century, and the technology gradually spread through the rest of Asia and into Europe. One of the
most important part of the gun is the barrel.
Barrel is the front bore of the gun that directs the projectile. Barrel types include rifled—a series of
spiraled grooves or angles within the barrel—when the projectile requires an induced spin to stabilize
it, and smoothbore when the projectile is stabilized by other means. In the simplest of terms, a barrel is
a pressure vessel. High-performance rifle cartridges can generate 60,000 psi, and a safety factor must
be built into the barrel so a hot handload or some other variable doesn't cause damage. There are two
main barrel types. Most hunting and military rifle barrels are made of chrome-molybdenum, the steel
used in truck axles and other high-stress components.
DRILLING
A straight barrel begins with a straight hole. Drilling a straight hole in a rifle barrel is accomplished
with special drilling machines, generally known as Gundrills. The drill (drill bit) used is a special
application type specifically used for drilling deep holes, known as deep hole drills. Not your everyday
twist drill. The deep hole drill is basically a long, hollow tube with a V-groove formed on the outside.
A hollow tungsten carbide tip is brazed to the end of the tube and then ground to specs, including a V-
groove to match the tube. The face of the carbide tip is asymmetrical so that it will only cut on one side,
and is also ground so that the forces acting on the tip keep the drill centered in the workpiece. The deep
hole drill used is .008-.012" under the finished bore diameter, which leaves room for the reaming
process.
After a bar of steel has been cut to length and both ends faced square, the "blank" is inserted into the
Gundrill. The Gundrill rotates the barrel at 2000-4000 RPM while a stationary deep hole drill is fed into
the material through a tight-fitting bushing. A steady rest rides along the drill to keep the tube rigid
while drilling. Coolant is pumped through the hollow drill at 1000 psi via the tailstock to clear chips
and cool the drill face. The oil and chips are forced back through the V-groove and into the chip tray,
where the oil is strained and returned to the reservoir. The drill is fed into the material at a rate of about
1"/minute, so a 28" blank will take approximately 30 minutes to drill.
FLING
A barrel of circular cross-section is not capable of imparting a spin to a projectile, so a rifled barrel has
a noncircular cross-section. Typically the rifled barrel contains one or more grooves that run down its
length, giving it a cross-section resembling an internal gear, though it can also take the shape of a
polygon, usually with rounded corners.
Rifling has been around for 500+ years. It was invented in Nuremberg circa 1492 and is still the
optimum method of creating precise spiral grooves. As the barrel steel has improved, so has the cut
rifling technique. Rifling is produced using a cutter, sometimes called a "hook cutter," which scrapes
metal out of the bore. The cutter rides in a hardened hollow steel cylinder, or "rifling head," which is
ground just under the reamed bore diameter, and contains a lift mechanism and feed screw.
The rifling head is mounted on long, straight hollow steel tubing so that coolant can be pumped to the
cutter. The tubing is fitted with an adapter so that it can be attached to the machine. The cutter is ground
to fit in a slot milled in the rifling head, and is made specific to a caliber and twist rate. Cuttermaking
requires great skill and a lot of patience. A well-made rifling cutter produces some of the most uniform
groove circles found today, and a well maintained cutter will leave a superior finish that requires
minimal lapping.
MATERIALS USED
The first and most important method to mitigate the effects of erosion is to use the right type of material
for making the barrel. Most high-production manufacturers use a Chrome Molybdenum (Cro-Moly)
steel, and most target or similar-type makers use stainless steel.
Cro-Moly steel is usually designated as 4140, 4145, or 4150 type steel. Cro-Moly is relatively cheap
and readily available, is easily machined, can be hardened by heat treatment, and is easily blacked. Most
factory hunting rifles, as well as military rifles, are equipped with Cro-Moly barrels. Stainless steel
barrels are not true autensitic stainless; the better term would be "rust-resistant" steel. Stainless barrels
are a 416 type, which is a martensitic class, and can be hardened by heat treatment. 416 stainless has a
high Chrome content, and sulfur is added to obtain good machining qualities. It is a more expensive
steel, and does not black well due to the chrome content, but the Teflon process has filled that void.
There are two determining factors when selecting steel for barrels: tensile strength and impact strength.
Tensile strength is defined as the measured force required to break a one-inch cross-sectional area of
steel by pulling at both ends. Basically it measures how much force it takes to pull a rod of steel apart.
Barrel steels should be rated a factor of two over chamber pressures (for a good safety margin), which
is usually a tensile strength over 100,000 lb/in^2. Impact strength is the steel's ability to take a sharp
blow without breaking. The tensile strength increases as the steel is hardened, but the steel also becomes
more brittle (easier to fracture upon impact - or maybe from the explosion you create in the chamber
when you pull the trigger). There must be some elasticity in the steel, and it has been determined that a
26-32 Rc (Rockwell C scale) hardness is the appropriate, safe trade-off.
CONCLUSIONS
The push towards higher muzzle velocities, more energetic propellants, and less vulnerable propellants,
has continued to drive research into gun barrel erosion over the last fifteen years. Advancements in
understanding the different erosion mechanisms have arisen through the development and improvement
of erosion modelling and prediction tools, targeted experiments, and the analysis of eroded barrels from
fielded guns. Based on this understanding, a number of new ideas in low-erosivity propellant
formulation and erosion mitigation have resulted.
Significant effort has recently been directed at understanding the erosion mechanisms for barrels coated
with protective refractory metals. The most plausible mechanism is that microcracks in the coatings,
present from the time of manufacture, propagate due to pressure and thermal stress cycling and
eventually reach the gun steel substrate. Through numerical modelling and analysis of eroded barrels,
a number of investigators have shown that once cracks reach the substrate, chemical erosion, gas wash,
and high interfacial temperatures cause piping of the substrate and eventually undermine the coating.
Segments of coating are subsequently removed by the flow or engagement with the projectile, and at
this point the erosion rate of coated barrels may exceed that of steel barrels. A number of ways to
mitigate this erosion pathway have been suggested, including: development of better coating techniques
to avoid the initial microcracks, .