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An Introduction to Tuning the Gunpower Stealth
By Bob Craske
As the rear edge of the hammer slide passes over the Trigger release Sear E, the sear pops upward under spring pressure locking the hammer in the cocked position. The Cocking lever is manually drawn backwards by the firer to the rearmost position when the two ‘O’ rings shown at C, form an airtight seal between the breech end of the barrel and the Air Bottle Release Valve B.
When the gun is fired, operation of the trigger causes the Trigger Release Sear to rotate about its pivot in a clockwise direction causing the release of the Hammer Slide. The Hammer Slide, propelled by its spring impacts with the front face of the Breech Slide transferring its 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. A charge of air passes through the opened valve and through a longitudinal hole drilled in the valve body and impinges directly on the rear of the pellet which is pre-positioned in the rear of the barrel’s breech.
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 spring, returns
to its sealed position shutting off the air supply. The cycle is repeated
for each shot.
Tips, Tweaks and Sneaky Snippets
It should be understood at this point that accuracy and power in the gun are very closely related, especially with the heavier ammunition this gun 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 by the barrel’s internal rifling.
There are several crucial factors which can affect the delivery of air into the breech of the gun and, for the sake of brevity, they are listed below.
1. Unlubricated hammer slide bearing surfaces
It has been demonstrated that a dry hammer slide will ‘chatter’ as it travels rearwards during the firing cycle. This chatter will dissipate a surprising amount of the hammer’s energy which is required for opening the air valve. A dry slide will also 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 advisable to regularly 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 .25” 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.
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 slight 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. A heavier hammer
will suffer less from countering forces and 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
One of the most common causes of erratic firing with
this gun is a loosening of the Top Hat Collar. 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.
There have been threads on the various BBS’s which
have recommended drilling out the valve stem for more power. DON’T DO IT!
Firstly it’s irrevocable, Secondly it weakens the stem and Thirdly it’s
not necessary. On the bench and in test conditions, I’ve shot 32 Foot Pounds
over my chrony using just the tweaks described above.
Early guns had a proprietary barrel which gave less than ideal results. However, later Stealths (Stealth 2000 for example) have a Walther match grade barrel which produces almost frightening 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 inside the gun body and even barrel support rings missing
or damaged. Put simply, a clean, solidly mounted and firmly secured barrel
in good condition WILL deliver accurately. A nice touch which will (marginally)
improve accuracy is to get the plastic barrel mounts remade in brass or
steel at your local machine shop.
Removing the Barrel
Depending on the weapon you have you may find several small plastic
spacer ringsapproximately .5” thick. These are Hammer Spring preload spacers
and are factory fitted to bring Hammer Spring tension within tuneable limits.
As previously discussed, adding to these will improve the situation further.
These spacers, and any further packing, should be fitted at point A on
the above diagram unless you wish to increase the Hammer’s effective mass
when additional spacers may be fitted adjacent to the hammer’s front face.
Be aware that increasing the hammer’s mass and spring tension will dramatically
alter the performance of the gun and may make it illegal.
To reassemble the barrel
2. Fit the required spacers against the rear barrel locating ring followed by the Hammer Spring
3. Now, after greasing the Hammer slide liberally inside and out, slide it onto the barrel and introduce the barrel assembly intro the gun body aligning the screw holes in the barrel locating rings with the holes in the bottom of the barrel housing. Gently locate the barrel’s breech into the Breech Cover against the Hammer Spring’s pressure and, whilst holding the barrel assembly in with the finger against the muzzle, align the rear mounting screw hole with its mounting ring and secure the barrel ring with a grub screw. Do not tighten fully at this point.
4. Now insert the second barrel ring screw finger tight only. Do not tighten fully at this point
5. Take the end cap/silencer threaded insert and, aligning the securing holes, push it gently into the front of the barrel housing. Secure firmly with its grub screw.
6. Now tighten both barrel mounting screws equally and in stages until firmly finger tight with a 3” Allen key.
7. Check the cocking and safety action for correct operation and work 20 or so times. When satisfied, remove excess grease from the weapon with a clean lint-free rag.
Test fire the gun in a safe manner for 20 or shots until the valve mechanism has ‘seated in’ and check the safety and trigger sear are operating correctly with no ‘slip’ or unintended release. Adjust Muzzle Velocity using a Chronograph as detailed earlier.