FIELD OF THE INVENTION 
     This invention relates generally to air guns, and, more particularly, to an adapter to reposition a compressed air tank of a pre-charged pneumatic rifle and facilitate pressure measurement and refilling. 
     BACKGROUND 
     A pre-charged pneumatic (PCP) rifle releases compressed air to drive a projectile. A tank stores the compressed air. The tank has a threaded neck for connecting to a rifle. Cocking the rifle exposes a breech for manually loading a projectile. During firing, a hammer is released to strike a valve. Prior to being struck by the hammer, the valve is held closed by a spring and the pressure of the compressed air in the tank. When struck by the hammer, the valve opens briefly to release a burst of compressed air. The compressed air drives the projectile from the breach and through the barrel of the rifle. 
     Unfortunately, the tank must be removed from the gun for refilling after a limited number of firings (e.g., 25 to 40). The tank is usually filled from a high pressure air supply, such as a diving cylinder, or by recharging with a hand pump. Refilling requires use of special adapters to couple the tank to the diving cylinder or pump. Frequent removal and refilling is not only time consuming and laborious, but also risks damage to the threaded neck. 
     Determining when a refill is necessary can be difficult, particularly for rifles not equipped with pressure gauges. While a user may sense a lack of sufficient pressure as the tank is depleted, this method of pressure management is highly imprecise. Counting the number of shots is inconvenient, conducive to error and ineffective for a rifle with an undetermined number of prior shots from a tank. Additionally, neither of these methods enables a user to determine the pressure before the rifle is fired. 
     In some air rifles, the centerline of the tank is aligned with the centerline of the barrel. In such rifles, the tank doubles as a butt stock. However, because the tank is bulbous, it can interfere with aiming. A scope is typically mounted to such a rifle using high scope mounts. As the distance between the barrel and a high scope is substantial, aiming is compromised, resulting in frustration and fewer target hits. Lowering the height of the scope to bring the line of sight closer to the projectile path will result in discomfort. A shooter may have to cram his head onto the tank and crank his head at an extreme angle in order to aim through a low scope. Conversely, if the scope is mounted too high, not only will aiming be compromised but the shooter may find himself hovering over the tank, without his head touching it. Either case will induce unsteadiness as a shooter fights to acquire a sight. 
     The invention is directed to overcoming one or more of the problems and solving one or more of the needs as set forth above. 
     SUMMARY OF THE INVENTION 
     To solve one or more of the problems set forth above, in an exemplary implementation of the invention, a pre-charged pneumatic air rifle air tank adapter lowers an air tank relative to the pre-charged pneumatic air rifle. The lowered air tank facilitates aiming. The adapter also provides a substrate for mounting accessories, including a quick disconnect refill plug and a pressure gauge. These accessories facilitate refilling and monitoring the pressurized state of the tank. 
     An exemplary adapter includes a female rifle fitting that has a threaded central conduit. The threaded central conduit has internal threads, a length and an inner diameter configured to threadedly receive a tank mounting fitting. The adapter also has a male air tank fitting with a central conduit, external threads, a length and an outer diameter configured to threadedly engage a neck of an air tank for the pre-charged pneumatic air rifle. The centerline of the central conduit of the female rifle fitting and the centerline of the central conduit of the male tank fitting are parallel, coplanar (i.e., aligned in a plane) and about 0.75 to 1.5 inches apart in elevation, preferably about 1 inch apart in elevation, measured vertically from centerline to centerline. This difference in elevation enables lowering the tank by the distance. A body of the adapter has an internal conduit extending from the female rifle fitting to the male air tank fitting. The internal conduit of the body fluidly couples the threaded central conduit of the female rifle fitting to the central conduit of the male air tank fitting. The internal conduit of the body, which fluidly couples the threaded central conduit of the female rifle fitting to the central conduit of the male air tank fitting, is oriented at an acute angle (about between 30 to 75 degrees) relative to the central conduit of the male tank fitting. 
     Optionally, the adapter has one or more accessories. The body has a first side and an opposite second side. A first threaded aperture is provided in the first side. The threaded aperture is in fluid communication with the internal conduit of the body. A pressure gauge with an analog display and threaded fitting opposite the analog display is threadedly received in the first threaded aperture in the first side of the body. The analog display of the pressure gauge responds to gas pressure within the internal conduit of the body. The pressure gauge is configured to measure pressure up to about at least 3000 psi. 
     The body has a second threaded aperture in the second side. The second threaded aperture is in fluid communication with the internal conduit of the body. A quick disconnect plug with a coupling end and a threaded fitting opposite the coupling end is threadedly received in the second threaded aperture in the second side of the body. The quick disconnect plug provides a “make or break” fluid-tight seal, until engaged by a female quick disconnect coupling. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other aspects, objects, features and advantages of the invention will become better understood with reference to the following description, appended claims, and accompanying drawings, where: 
         FIG. 1  is a side view of an exemplary prior art air rifle with an air tank; and 
         FIG. 2  is a side view of the exemplary prior art air rifle with the air tank removed; and 
         FIG. 3  is a side view of the exemplary prior art air rifle with an exemplary adapter according to principles of the invention; and 
         FIG. 4  is a side view of the exemplary prior art air rifle with an installed exemplary adapter according to principles of the invention; and 
         FIG. 5  is a first side view of the exemplary adapter according to principles of the invention; and 
         FIG. 6  is a second side view of the exemplary adapter according to principles of the invention; and 
         FIG. 7  is a plan side view of the exemplary adapter according to principles of the invention; and 
         FIG. 8  is a rifle end view of the exemplary adapter according to principles of the invention; and 
         FIG. 9  is a tank end view of the exemplary adapter according to principles of the invention; and 
         FIG. 10  is a first perspective view of the exemplary adapter according to principles of the invention; and 
         FIG. 11  is a second perspective view of the exemplary adapter according to principles of the invention; and 
         FIG. 12  is a third perspective view of the exemplary adapter according to principles of the invention; and 
         FIG. 13  is a fourth perspective view of the exemplary adapter according to principles of the invention; and 
         FIG. 14  is a section view of the exemplary adapter according to principles of the invention; and 
         FIG. 15  is a dimensioned view of the exemplary adapter according to principles of the invention. 
     
    
    
     Those skilled in the art will appreciate that the figures are not intended to be drawn to any particular scale; nor are the figures intended to illustrate every embodiment of the invention. The invention is not limited to the exemplary embodiments depicted in the figures or the specific components, configurations, shapes, relative sizes, ornamental aspects or proportions as shown in the figures. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an exemplary prior art rifle  100  includes a barrel  105  with an open end, an upper accessory rail  110 , a power adjust wheel  115 , a forearm  120 , a cocking handle  125 , a safety  130 , a scope mount  135 , a trigger  140 , a bolt  150 , a pistol grip  155 , a compressed air tank  160  with a neck  165 , and a scope  170 . The exemplary air rifle is intended to represent a range of air rifles having a tank  160  aligned with the barrel  105 . Such air rifles may have additional and or different features. One exemplary prior art rifle for which an adapter according to principles of the invention is well suited is an Air Force TalonP® air rifle by Auto-Numatic Corporation, a Texas corporation, d/b/a AirForce Airguns. 
     In  FIG. 2 , the tank  160  is removed from the air rifle  100  by unscrewing, exposing a threaded male fitting  175  (i.e., tank mounting fitting) for threadedly receiving the tank  160 . In  FIG. 3 , an adapter  200  according to principles of the invention is positioned between the rifle  100  and air tank  160  for installation. As shown in  FIGS. 3 and 4 , the adapter  200  includes a body  205  with a conduit extending from a rifle fitting  210  to an air tank fitting  215 . The adapter  200  lowers the air tank  160  a determined distance and moves it away from the rifle  100 . In doing so, the adapter  200  facilities aiming through the scope  170 . The downward adjustment allows a shooter to comfortably position his head on the tank  160  while taking aim through the scope  170 . The adapter  100 , including benefits of the adapter  100 , is discussed in more detail below. 
     As shown in  FIGS. 5 through 13 , the adapter  200  includes a body  205  with a conduit extending from a rifle fitting  210  to an air tank fitting  215 . The rifle fitting  210  comprises a female threaded aperture  230  (i.e., a conduit) configured to threadedly engage the threaded male fitting  175  extending from the back of the rifle  100 . For different rifles, the rifle fitting  210  diameter and threads may vary. The air tank fitting  215 , which is opposite the rifle fitting  210 , is a male threaded fitting with a central aperture  235  (i.e., a conduit). The air tank fitting  215  is configured to threadedly engage the neck  165  of the tank  160 . For different air tanks, the air tank fitting  215  diameter and threads may vary. The aperture  230  of the rifle fitting  210  and the aperture  235  of the tank fitting  215  are fluidly coupled by a conduit in the body  205 . Thus, compressed air from an air tank may flow into the aperture  235  of the tank fitting  215 , through the body  205  and out of the aperture  230  of the rifle fitting  210  into the rifle  100 . 
     The exemplary adapter includes a pressure gauge  220 . The pressure gauge  220  is attached to a side of the body  205 . The gauge  220  may be attached by threading into a threaded hole in the side of the body  205 . The sensing end of the pressure gauge  220  is within the hole and in fluid communication with the conduit in the body  205 . The reading end of the pressure gauge  220  faces outwardly from the side of the body  205 . Thus, in the unlikely event of failure of the gauge, any components of the gauge are not propelled sideways, not towards the users face. Any pressure gauge that is small enough to attach to the side of the body  205 , without interfering with use of the rifle and capable of gauging pressure up to at least about 3000 psi, may be used. However, in an exemplary embodiment, the pressure gauge is a Bourdon gauge that contains narrow coiled or C-shaped tubes which tend to straighten or deform into a larger diameter coil as pressure increases. As pressure increases, the motion of the tube is converted by gears into rotation of a needle. Such gauges measure pressure, relative to ambient atmospheric pressure, as opposed to absolute pressure with sufficient accuracy for determining the pressurized state of the tank  160 , determining when a refill may be necessary, determining how many more shots may be fired before the pressure drops to a level below which performance is compromised, and detecting leaks. 
     The exemplary adapter includes a quick connect plug  225 , also known as a quick disconnect or quick release coupling. The plug  225  is attached to the side of the body  205  opposite the pressure gauge  220 . The plug  225  may be attached by threading into a threaded hole in the side of the body  205 . A threaded end of the plug  225  is within the hole and in fluid communication with the conduit in the body  205 . The other end of the plug  225  faces outwardly from the side of the body  205 . The quick connect plug  225  provides a fast, make-or-break connection for fluid (i.e., compressed gas) transfer. A preferred quick connect plug  225  contains a poppet valve used to stop fluid (e.g., compressed air) from flowing when the two mating parts of a quick disconnect are separated. The poppet valve is a spring biased valve that is urged into an open position when connected to a female quick disconnect coupler. Thus, the quick connect plug  225  enables refilling the tank  160  through the adapter  200  without disconnecting the tank  160  from the adapter  200 . 
     To refill the tank  160 , a source of pressurized gas (e.g., a compressed air source) may be coupled to the plug  225  using a mating quick connect female coupler and a hose leading to the source. The plug  225  contains a self-sealing valve, which, upon disconnection, will automatically contain compressed gas in the adapter  200 . When coupled to a mating female quick disconnect fitting, the sealing valve in the plug  225  is urged open. So long as the pressure of the source exceeds the pressure of the tank  160 , and the volume of pressurized gas in the source is sufficient to refill the tank  160 , the pressurized gas should flow from the source to the tank and refill the tank  160 . When the pressure in the tank  160  is restored to the full pressure, as determined by monitoring the pressure gauge  220 , the source of pressurized gas may be disconnected. Upon disconnection, the self-sealing valve in the plug  225  closes and automatically contains compressed gas in the adapter  200  and the connected tank  160 . 
     Referring now to  FIG. 14 , a section view of the exemplary adapter  200  is provided. The section view shows the internal threads  232  in the aperture  230  of the rifle fitting  210 , the external threads  216  and aperture  235  of the tank fitting  215 , and the conduit  240  disposed in the body  205  between, and fluidly coupling, the apertures  230  and  235 . Also shown is a threaded aperture  245  into which an accessory, such as a pressure gauge or a quick connect fitting, may be threadedly connected. An exemplary pressure gauge  220  and quick connect plug  225  are described above. 
     An important aspect of the exemplary adapter  200  is the configuration of the conduit  240  disposed in the body  205  between, and fluidly coupling, the apertures  230  and  235 . To mill the adapter from a single piece of aluminum, such that the adapter  200 , excluding its accessories, is integrally formed, the conduit  240  must be angled to be formed by inserting a bit straight through the aperture  230  of the rifle fitting  210 . A conduit  240  as shown in  FIGS. 14 and 15  may be milled using a straight bit. Because the aperture  230  of the rifle fitting  210  has a larger diameter than the aperture  235  of the tank fitting  215 , a bit is inserted at an angle through the aperture  230  of the rifle fitting  210  to form the conduit  240 . The conduit  240  is not formed by inserting a bit through the much narrower aperture  235  of the tank fitting  215 , which would interfere with the necessary angling. 
     With reference to  FIG. 15 , the adapter  200  lowers the tank  160  by a determined distance h, preferably about 0.75 to 1.5 inches, and more preferably about 1.0 inches. In a preferred embodiment, the angle, φ, of the centerline of the body  205  is acute. The exterior surfaces of the body, which may be planar, non-planar or curved, generally follow the angle, φ. Thus, the surfaces may be parallel or nearly parallel to the centerline of the body  205 . This angled configuration ensures adequate clearance, so that the body  205  does not interfere with structure of the rifle. The lengths, l 1 , l 2 , and l 3 , are approximately 1 to 1½ inches, and, more preferably, about 1 inch, about 1 to 1.25 inches and about 1.25 inches, respectively. Variations of about ±15% are within the scope of the aforementioned measurements. 
     Using an adapter  200  according to principles of the invention confers several benefits. The adapter  200  facilitates aiming by lowering the tank  160  relative to air rifle  100 . The adapter  200  provides a pressure gauge for accurately determining the pressurized condition of the tank  160 . Thus a user may quickly and accurately determine if and when a refill is needed and spot a leak. The adapter  200  also provides a refill plug  225 , enabling a user to refill the tank without disconnection from the air rifle. 
     The adapter  200  may be comprised of a wide range of materials, including metals, composites and plastics. However, in a preferred embodiment, the adapter  200  is comprised of anodized aluminum, which offers acceptable strength, durability and corrosion resistance. Additionally, in a preferred embodiment, the body, female rifle fitting and male tank fitting of the adapter (i.e., the entire adapter excluding accessories) is integrally formed. Integrally forming the adapter avoids seams that may be susceptible to leaking, especially when subjected to the high pressure compressed gas that drives the air rifle. The risk of leakage from seams increases further, as the rifle is used in various temperatures causing expansion or contraction, and as the rifle is used in humid and wet conditions which can saturate, dry, corrode and degrade gaskets and sealants, and as the rifle is manhandled during ordinary use. 
     While an exemplary embodiment of the invention has been described, it should be apparent that modifications and variations thereto are possible, all of which fall within the true spirit and scope of the invention. With respect to the above description then, it is to be realized that the optimum relationships for the components and steps of the invention, including variations in order, form, content, function and manner of operation, are deemed readily apparent and obvious to one skilled in the art, and all equivalent relationships to those illustrated in the drawings and described in the specification are intended to be encompassed by the present invention. The above description and drawings are illustrative of modifications that can be made without departing from the present invention, the scope of which is to be limited only by the following claims. Therefore, the foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents are intended to fall within the scope of the invention as claimed.
A pre-charged pneumatic air rifle air tank adapter lowers an air tank relative to the pre-charged pneumatic air rifle. The lowered air tank facilitates aiming. The adapter also provides a substrate for mounting accessories including a quick disconnect refill plug and a pressure gauge. The adapter includes a female rifle fitting with a threaded central conduit, a male air tank fitting with a central conduit and external threads parallel to, coplanar with and about 0.75 to 1.5 inches apart in elevation from the central conduit of the female rifle fitting, measured vertically from centerline to centerline. A body of the adapter has an internal conduit extending from the female rifle fitting to the male air tank fitting.