Traditionally firearm barrels have been manufactured out of steel alone, but users have long desired lighter weight firearms that remain durable and reliably accurate. In order to address this desire, it has previously been known to construct strong, lightweight barrels for firearms using composite materials. For example, some previously known composite gun barrels include an inner tubular structure made from a hard material such as steel alloy and an outer jacket made from a composite material such as a continuous carbon fiber-reinforced polymer matrix composite. This combination lightens the gun while retaining barrel strength and stiffness.
The manner in which these previously known gun barrels have been made includes applying carbon fiber in a wet filament winding operation, wherein dry carbon fiber strands are combined with a resin (such as a crosslinkable epoxy, a polyimide, cyanate ester, inorganic polymer or thermoplastic polymer) in a wet dip pan process, then wound around an inner liner and processed. The composite barrel may then be cured such that the resin bonds the outer shell to the inner liner, followed by attaching the barrel to a receiver and stock.
Such carbon fiber-reinforced composites may provide a suitable balance of thermal properties, mechanical properties, and processing characteristics for many common firearms applications. However, such composite gun barrels can also pose problems not encountered with traditional steel barrels. For example, difficulty may arise in constructing the composite barrel in a manner and quantity around and along the liner that ensures that the barrel does not burst upon firing, achieving satisfactory strength and stiffness in the principal directions (for example, axially and torsionally), providing adequate environmental durability, and dampening the shock wave that propagates when a projectile is fired.
Some of the above issues can be addressed by using additional windings (i.e., more circumferential “hoop wraps”) to improve burst strength and more axially oriented helical windings to improve axial tensile and flexural strength and stiffness. However, adding more layers of windings can lead to manufacturing and curing complications, higher material expense, more weight, and/or a bulkier barrel profile than desired.
Thermal management is also a significant problem when using continuous fiber composite (CFC) material for the outer shells because the resin content within the CFC is a relatively poor conductor (i.e., insulator of the heat generated by hot gasses within the liner). Additional layers of CFC windings exacerbate the heat removal problem. During operation, the barrel will heat up. In the case where the matrix phase is an organic polymer, if the cured resin within the CFC reaches its glass transition temperature, Tg, the CFC softens significantly and the mechanical integrity of the composite barrel is compromised. As the barrel is heated to even higher temperatures, irreversible thermal decomposition of the cured matrix occurs and barrel structural integrity is further compromised.
A further problem relates to stresses within the barrel arising from thermal expansion differences between the composite outer liner and the inner liner of the composite barrel. As the inner steel liner heats during operation, it expands both radially and longitudinally. Composite structures in the prior art have a substantially lower average effective coefficient of thermal expansion (CTE) in the longitudinal direction than steel and so when heated, the CFC outer shell expands substantially less than the steel liner. This may increase or decrease thermal stresses in the barrel depending on the state of thermal residual stress from processing. For example, when a stainless steel liner and a typical CFC are subjected to heating during operation, uneven expansion can produce thermal stresses on the liner-CFC interface, possibly even causing separation of the CFC from portions of the liner or fractures within the CFC shell. Even if no separation occurs, minor variations in the CFC or metal liner properties, or geometric variation, may promote uneven thermal stresses at the interface between the barrel and CFC that may result in nonlinear deformation or displacement of the barrel from its original axis. Even a very slight displacement can significantly degrade accuracy. Furthermore, even if the barrel and liner remain perfectly true, the various layers of windings within the CFC can have different CTEs, especially longitudinally. When subjected to elevated operating temperatures, differences in the thermal expansion of adjacent winding layers within the CFC can result in high level of interlaminar shear stress and even delamination.
From the above, it can be seen that there is a need to produce an improved composite barrel that uses materials that provide superior axial and torsional strength and stiffness while minimizing weight, radial bulk and interlaminar stress, resists deformation when heated and reduces high amplitude vibrations at the muzzle end of the barrel.