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Restoring a spring piston 'strike pump' airgun  

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Garvin
(@garvin)
Curator in Chief Admin
Joined: 2 years ago
Posts: 4423
26th March 2018 16:25  

Restoring a spring piston 'strike pump' airgun 

With thanks to John G. 

John posted on the airgunbbs.com:

 

I had always wanted a real antique airgun but could never afford the sort of prices asked. Then a few years ago on a visit to Henry Kranks gun shop I spotted a 200+ year old air rifle which was on offer at a very affordable price, thanks to the fact that it didn’t work and it had had some poor restoration work done to it. The dealer said that it was probably a bellows air rifle, and as they had a problem getting it apart in the shop I had to take a gamble and buy it as seen. It might have had no mechanism inside , so it was a bit of a risk, but at £900 it seemed to be worth a punt, especially as externally everything seemed to be original, except for the brass cover plate on the stock.

When I got the gun home I managed to get the brass plate off, after the application of some penetrating oil and localised heating from a soldering iron to the rusted-in screws. When the mechanism was exposed, I was surprised to find that the gun wasn’t a bellows rifle after all, but was what is generally referred to as a “strike-pump” air rifle, which are in fact rarer. They have a spring piston power unit in the stock, which is usually cocked by turning an axle, which in turn pulls back the piston by what closely resembles a bicycle chain. 
When I removed the power unit it was clear that the reason why the gun would not cock was that the piston had seized in the cylinder. This was eventually freed and the disassembled unit is shown here. Everything about the unit is huge: the piston head is an inch thick cylinder of leather, the massive spring is made of roughly quarter inch square section steel, and the very long air transfer port has a diameter of about a centimetre.

When I got the gun home I managed to get the brass plate off, after the application of some penetrating oil and localised heating from a soldering iron to the rusted-in screws. When the mechanism was exposed, I was surprised to find that the gun wasn’t a bellows rifle after all, but was what is generally referred to as a “strike-pump” air rifle, which are in fact rarer. They have a spring piston power unit in the stock, which is usually cocked by turning an axle, which in turn pulls back the piston by what closely resembles a bicycle chain. 
When I removed the power unit it was clear that the reason why the gun would not cock was that the piston had seized in the cylinder. This was eventually freed and the disassembled unit is shown here. Everything about the unit is huge: the piston head is an inch thick cylinder of leather, the massive spring is made of roughly quarter inch square section steel, and the very long air transfer port has a diameter of about a centimetre.

After several attempts to cock the gun, it soon became apparent that the forces involved in compressing and holding back the mainspring were causing the stock to disintegrate where the sear and cocking unit was bedded into the wood. The damage can be clearly seen with the brass cover plate and heel plate removed:

I decided that if I was ever going to get the gun into a workable condition I would have do some serious reinforcement work on the stock. I always hesitate doing any restoration on really old guns, but in this case there had evidently already been bodged attempts at shoring up the woodwork in the past, so I felt that I would only be improving things cosmetically, and would also be helping to make the gun useable again. 
Old crumbling wood was scraped away, and the cavity was built up using shaped pieces of walnut glued in place with epoxy resin. Small gaps were then filled with walnut stained plastic wood. This was the result: 

The brass plate that covered the mechanism aperture was clearly a later addition and had been made much larger than the original plate to give more support to the crumbling wood underneath. In fact the original plate was still in evidence , but had been brazed to the underside of the new plate, presumably to provide more rigidity and to help locate the screw holes. The rectangular shape of the new plate was out of keeping with the general character of the gun, the original having scalloped edges similar to the plate on the other side of the stock. So the plate was reshaped with a fine saw and a file, giving something much closer to how the original would have looked:

The wood was then inlet to accept the newly shaped brass plate: 

This how the left and the right side of the stock now look:

 

 

 

The gun has some nice features. There is a built in option of open sights or a peep sight. The peep sight is adjustable for height and windage:

Alternatively, the peep sight can be dropped to its lowest position so that the open sights can be seen. The rear sight blade has a nice touch in that it is hinged and so has vertical adjustment, as well as the usual dovetail windage adjustment. 

For loading, the barrel is spring-lifted by pushing forward the lever in front of the trigger guard. This system differs from most bellows air rifles of the period, which generally relied on the natural flexing of the wood frame to lift the barrel rather than using a leaf spring.

The bore is a massive 9 mm, so in order to try out the gun I had to make a suitable dart (these guns were only ever intended to fire darts, or possibly wax balls). The large bore size can be best appreciated by comparing my home-made dart with more conventional commercial darts. 


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