An Introduction to the Gunpower Stealth

© Bob Craske

The Stealth is an innovative and enormously efficient pre-charged pneumatic (PCP) air rifle. Due to a design which is free of all but the most basic valving and triggering mechanisms, and which possesses an air path which is free of bends, restrictions and convoluted metering systems the design is simple to maintain, rugged in use and capable of quite awesome performance..  



The Rifle is cocked by means of the cocking handle [A] which is screwed directly into the plastic Breech Cover. The Breech Cover, which incorporates 2 ‘O’ ring seals, is tubular in design and slides on a machined reduction in the barrel's circumference. As it is pushed forward it exposes the breech for pellet insertion and simultaneously pushes the Hammer Slide [D] against its spring [F] which is thus compressed against the Barrel Mounting Ring [G].

As the chamfered rear face of the hammer slide passes over the Trigger release Sear [E], the sear pops upward under spring pressure and the 3 trigger bars are aligned under spring pressure forming an 'equilibrium block' which balance spring forces in a lateral plane preventing the sear from moving and therefore holding the hammer in the cocked position under spring tension. At the same time a safety mechanism consisting of a baulk plate is drawn beneath a projection on the sear plate by a spring, preventing disengagement until the safety is manually released. The Cocking lever is manually drawn fully rearwards by the firer and rotated to lock it in position. The two ‘O’ rings shown at [C], then form an airtight seal between the breech end of the barrel and the Air Bottle Release Valve, usually known as the 'Top Hat' [B].

When the gun is fired, operation of the trigger causes the rear Trigger Bar to rotate about its pivot in a clockwise direction, tripping its adjoining bar and allowing the sear to rotate and fall. With nothing to prevent its movement except a small return spring, the sear is pushed downwards by the chamfered front face of the Hammer Slide which, propelled by its much stronger spring, travels rearwards to impact forcibly with the front face of the Breech Slide. This transfers the Hammer Spring's energy through the slide and directly onto the face of the Air Bottle Release Valve. The Valve is momentarily forced from its seat against the air pressure in the bottle and a charge of air passes through the opened valve inside the bottle and through a longitudinal hole drilled in the valve body before impinging directly on the rear of the pellet which has been pre-positioned in the barrel’s breech. The pellet is thus driven up the barrel by the air charge.

Once the energy from the Hammer Slide’s impact has been dissipated, the Air Bottle Release Valve, acting under the influence of the high-pressure air in the bottle and a small internal return spring, returns rapidly to its sealed position shutting off the air supply. This cycle is repeated for each shot.

Tips and Tweaks

Even with such a simple mechanism, there are factors which can effect the firing cycle of the rifle and cause erratic accuracy and power.

It should be understood at this point that accuracy and power in the rifle are very closely related, especially with the heavier ammunition this weapon prefers. The pellet relies entirely on the blast of high-pressure air generated during the ‘valve open’ phase of firing to not only expel the pellet from the barrel but do so at a known and consistent velocity. Any alteration in the pressure generated will effect not only the trajectory of the pellet but also its stability due to the spin imparted to the pellet by the barrel’s internal rifling. This will affect the Point of Impact (POI) of the pellet.

There are several crucial factors that can affect the delivery of air into the breech of the gun and, for the sake of brevity, they are simply listed below.

  1. Unlubricated hammer slide bearing surfaces

  2. Unlubricated breech seal surfaces and ‘O’ rings

  3. Worn Breech Seal ‘O’ ring seals

  4. Weak Hammer Spring

  5. Dirty or Damaged Air Bottle Valve Stem

  6. Loose Air Bottle Valve Stem

  7. Incorrect Air Bottle Pressure (Too low or too high)

It has been demonstrated in tests that a dry hammer slide can ‘chatter’ as it travels rearwards during the firing cycle. This chatter will dissipate a surprising amount of the hammer’s energy, all which is ideally required for opening the air valve. A dry slide will travel rearwards more slowly, thus the valve receives an insufficient impact to fully open and a ‘short charge’ results. This is often heard as a muffled report from the gun and a shot which passes wide of the target.

Unlubricated breech seal surfaces and ‘O’ rings will also absorb Hammer Impact Energy and can also allow air leakage at the ‘O’ rings. It is highly advisable to regularly and sparingly grease the assembly with a silicone-based lubricant to both prolong ‘O’ ring life and maximise efficiency by reducing wear.

The hammer spring is, in the opinion of the author, rather weak for the task it is required to do. Basically, its job is to knock the air valve fully open as quickly as possible after trigger release and it often fails to do this for the above-mentioned reasons. Increasing the preload on the spring has been shown to have a dramatic effect on the consistency of the valving system and is easily achieved by packing the spring it its foremost end where it abuts the rear Barrel Mounting Ring. Steel or Brass washers of a suitable internal and external diameter are the most suitable items for use as shims and usually packing to a depth of approximately .5” is sufficient to achieve a marked improvement. This will normally increase the muzzle velocity of the weapon and care should be taken in readjusting the gun back down to legal energy limits if applicable by means of the Top Hat.

Note: On American guns there is a ‘Power Adjuster’ which simply preloads the hammer spring by means of a movable spring packing device on a thread.

A slightly more complex improvement is to have an exact copy of the hammer made up in a heavier material, brass being the obvious choice because of its weight and corrosion resistance. Many users have reported excellent results using replacement aluminium hammers. A heavier hammer will suffer less from countering forces and, with its greater mass, will more effectively deliver energy from a correctly tensioned spring to the valve during impact. It will also cause the valve to remain open for fractionally longer increasing power delivery.

The Air Bottle Release Valve

The Air Bottle Release Valve neck is threaded to accept a small ‘Top Hat’ adapter which engages with the Breech Slide ‘O’ ring seals to form an airtight join during firing. The Top Hat assembly may be screwed in and out on its stem and, by so doing, affect the amount by which the valve is struck open during hammer impact. The further out the Top Hat is screwed the further the valve may be struck open and vice versa. Screwing in the Top Hat will reduce the rifle’s Muzzle Energy while screwing it out will increase it. In order to prevent the adjustment being lost during the rigours of hammer impact, a small recessed securing screw is set into the outer collar of the Top Hat and tightened by means of an Allen Key (Hexagon Wrench). The default setting from which to start calibrating for the UK's 12ft/lb power limit is approximately 1.5 turns out from the fully screwed-in position.

One of the most common causes of erratic firing with this rifle is a loosening of the Top Hat itself. Once the Top Hat has loosened, the valve is no longer presented to the Breech Slide in identical manner every time leading to variations in hammer strike. Additionally, the loose Top Hat will tend to dissipate hammer energy unpredictably. The grub screw used for securing purposes is rather inadequate for the job and, after setting-up the gun in conjunction with the other adjustments described here, the application of a small quantity of thread sealing compound is highly beneficial. For higher powered rifles with heavier hammers and uprated springs I have successfully used Araldite Epoxy Adhesive to permanently lock the Top hat once set. This is achieved by setting-up the Top hat to its optimal position and then measuring the distance between the base of the Top Hat and the valve body with feeler gauges. The Top Hat is then removed, a light film of epoxy applied to the valve stem threads and the Top hat refitted to its previous position and the adhesive allowed to cure. The difference this simple precedure makes in terms of consistency and reliability has to be experienced to be fully appreciated. Moderate heat will soften the epoxy sufficiently to allow removal if needed.

The Barrel

The removable barrel plays a crucial part in the delivery of the chosen projectile and a worn, bent or damaged barrel will produce at best, indifferent results. Sharp, hard and abrasive objects should never be introduced into a Stealth barrel as this could damage the rifling and adversely affect projectile control.

Early guns had a proprietary barrel, which gave less than ideal results. However, later Stealth’s (Stealth 2000 and later) have a Walther match grade barrel which produces excellent accuracy, especially in conjunction with the optional bipod.

The single most common problem with the Stealth barrel is mounting. I’ve found loose barrels, barrels mounted at an angle due to foreign objects jammed inside the gun body and even barrel support rings missing or damaged. Put simply, a clean, solidly (and accurately) mounted and firmly secured barrel in good condition WILL deliver accurately. A nice touch which may (marginally) improve accuracy is to get the plastic barrel mounts remade in brass or steel at your local machine shop.  


There have been threads on various BBS’s, which have recommended drilling out the valve stem for more power. DON’T DO IT UNLESS YOU’RE TUNING FOR SERIOUSLY HIGH POWER AND HAVE A FIREARMS CERTIFICATE (FAC)! Firstly it’s irrevocable, Secondly it can weaken the valve stem if overdone and Thirdly it’s not necessary for a UK 12 foot pound rifle. Finally, unless you possess a FAC, carrying out dramatic alterations to your Stealth can turn it into an illegal firearm with no easy way of returning it to a legal condition. The penalties for this are extremely severe with huge fines and prison sentences, even for first offences. Remember too that, if you do take your Stealth up to FAC power levels you MUST get it included on your FAC by a Registered Firearms Dealer. In my expereience they will charge in the region of £20 or so for completing the paperwork for you.