Frank Clarke - twist-grip Air Pistol (reproduction and patent)
Frank Clarke - twist-grip Air Pistol (reproduction and patent).
Reproduction Webley Mercury pistol
Thunderbolt 'Senior' (1947 patent)
This pistol was never made commercially. Here is a pic of a reproduction by Mac Evans from the original patent drawings (patent below):
Frank Clarke - Twist-grip reproduction - John Griffiths.
This amazing reproduction by the author of The Encyclopedia of Spring Air Pistols.
<b>"Unlike Mac Evans' version the grip has a flattened cross section, which makes cocking a bit easier.
Although steel balls gave excellent results for cocking, they did seem to put a lot of wear and tear on contacting surfaces, so I finally opted for rotating pegs. These gave quite a smooth action with much reduced damage to the piston rim. I did take the belt-and-bracers approach though, and case hardened the piston to completely eliminate the risk of damage.
After polishing the gun 'in the white' I had to decide on any lettering to add, if any. I decided to define the gun with an appropriate description on the top flat, and in a less obvious place, on the foot of grip, to put my name, place and date of making. Finally I hot blued it, to a modern jet black rather than a vintage black, as the gun is not pretending to be anything other than a modern reproduction of a patent design
The first thing I realised on using the gun was that even though the cocking “pegs” could rotate freely, they could not just roll around the rim of the piston during the twist cocking action, and in fact they were forced to roll in the opposite direction to the twist action. This meant that either the piston itself did not rotate and the pegs skidded over the piston rim surface, or the piston was dragged around with the twist action. In the event, the latter happened. This was obviously a source of inefficiency and if the spring bit into the cylinder and piston end it would resist such a rotation, which would make cocking harder.
At first this did happen and you could hear twanging and grating noises from the main spring during cocking. By carefully flattening and smoothing the ends of the main spring and applying grease this was overcome. A short steel rod was also inserted into the centre of the spring and acted as a spring guide, which seemed to stop some “cockling” of the spring as it entered the piston, and this contributed further to a smoother cocking stroke.
The power of this design was never going to be high as the calculated swept volume is only 9 cc, which compares with 12.9 cc for the Gat, 10.9 cc for the Whiting, and just exceeds the 8.5 cc of the Tell 2. As I could not easily get the Tell 2 spring into the twist-grip pistol (it was stronger and also needed too much compressing) I had to use a shorter spring, cut down from a Gat. So I expected the power of the twist grip to be significantly lower than that of the Tell 2, but in the event I was pleasantly surprised to find that it gave consistently higher muzzle velocities than the Whiting for a similar cocking effort, and matched the velocity of an optimally performing Tell 2 very closely.
These are the figures, obtained with Milbro cup slugs (carefully sized for consistency), and using a new Gat and a near mint Tell 2 with checked seals.
Average muzzle velocity over 10 or more shots:
Gat : ~ 300 fps
Whiting: ~ 175 fps
Tell 2: ~ 200 fps
Twist-grip: ~ 200 fps (max. 211, min. 180)
The average weight of the slugs was 5.74 grains.
Using pre-sized waisted pellets (average weight 7.56 grains) an average velocity of 178 fps was obtained. The calculated muzzle energies for the two types of ammunition turned out to be identical, at 0.57 ft lbs.
Accuracy tests were difficult as I could not find a way to clamp the pistol and cock it in situ (which I could do with the Whiting), so I had to shoot hand-held from a bench rest.
One of my pictures shows a target with 10 successive shots from 6 yards using the waisted pellets – quite a spread and not as good as the Whiting.
After putting at least a hundred shots through the gun, I can vouch for the pleasantness and convenience of the twist grip action (within the constraints of spring strength). However, this is offset by the inconvenience of the breech screw loading method.
Without some sort of ratchet action, only a 3/4 twist is achievable by a normal human being, and to get a mechanical advantage of about 2 the pitch of the twist has to be such that a piston stroke of only about 2 inches is possible. This means that the swept volume can never be high. A low swept volume cannot be increased by a longer cylinder and increased pitch as you lose mechanical advantage. Similarly the low swept volume cannot be compensated for with a more powerful spring because of the restricted mechanical advantage. So, in my opinion, the twist grip approach was never going to be a viable proposition for reasonably powerful pistols or air canes, and could only ever be useful for low power plinking guns. This is presumably why such designs never made it to the market place."</b>