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Montie Ho Sing, of Sydney, Australia built some rollerguns independently of me. Like me, he used a Rob Allen aluminium rail barrel. He used the original Rob Allen handle and modified the Rob Allen muzzle. I love his muzzle, it is a very neat solution for catching the rubber. He has some other neat tricks too! Below, Montie describes his rollergun design:
- Niko
The pulleys allow for the bridle to stroke along the entire length
of the barrel, thereby making almost the entire length of the gun
active.
I have found that when speargun rubbers are stretched taught, the force that they exert is very non linear. If the rubber is "overstretched" even by a small amount, the final few centimeters of that stretch becomes very stiff. If the gun is set up this way, then this "stiff portion of the stroke cause excessive recoil and the gun shoots high.
The pulley arrangement and the point at which the rubbers are secured along the barrel allow for very easy set up to get a good "Linear Stroke". I believe that a further possible point of development is to have an easily adjustable anchor point, that slides. In this way set up can be both for linearity to accommodate variances in rubber quality, but also to vary power.
The bores in the muzzle for the rubber are smaller than the
unstretched diameter of the rubber. In this case, 16mm bore, 20mm
rubber.
When fully stretched, the rubber is much thinner, travels through the
bores
and around the pulleys for at least 90 % of the stroke. Towards the end
of
the stroke, the bores restrict the rubbers and slow them down, the
slightly
larger diameter of the rubber at the bridle ties, prevent the bridle
assembly and ends of the rubbers from pulling completely through the
muzzle.
- I would like to find the optimum bore sizes for both 16mm and 20 mm
rubbers - at present, I cannot experiment too much because I do not
have
unbored muzzle castings, nor can I bore the muzzles out to more than
16mm.
Furthermore, if I can make the bore adjustable, then this is another
way to
have a variable power gun.
I have attached lead ball sinkers to torpedo swivels to
make up a "Quick Change" bridle. This allows us to use 2mm Kevlar
reinforced braid which sells for 50c a metre from yacht shops. It is as
strong as cable, much less abrasive to the plating on the spear,
friendlier
to the hands and only takes seconds to change when worn.
These pictures show a way that I have found which uses a small hook.
It is made of 4mm stainless rod, is approx only 7 cm long and only
weighs a
few grams. The looped end of the hook is threaded through the float
line and is
prevented from slipping along the entire line by a knot in the line.
The
hook is free to slide on that section of the float line between the
shark
clip on the grip and the knot.- approx 140 cm between knot and shark
clip.
A small rope cleat is attached to the side of the barrel about 30cm
from the grip. (For
more on cleats, see ClamCleat - Niko) The hook is placed
over the bridle, the one end of the line
is already attached to the grip via the shark clip and the other end is
now
free to be pulled through the cleat. The hook's looped end acts as a
pulley
and reduces the effort to stretch the rubber by half, the cleat
prevents the
rope from sliding back.
This arrangement allows for the ability to hold the rubber to be
held in a "half cocked " position while the butt of the gun can be
comfortably positioned on the chest pad. Both arms and hands are free
to
grab the rubber and fully load the bridle into the spear notch.
I have forgotten to mention that I fill the barrels of the
guns with
polyurethane foam. I have had to do so since with all this mucking
about the
barrels have been drilled full of holes! In doing so I have found that
the
foam does contribute to sound deadening.
The other end use a sawn off end of a conventional flopper spear. The end that fits into the tube engages into the tube for 40mm and is machined to have 3 bearing surfaces. The sections between the bearing surfaces have been recessed to 6mm diameter to prevent jamming from corrosion. The bearing surface are a slide fit and the quality of fit is augmented by incorporating two small o rings into the bearing surfaces. Two flats are machined onto the end of the drop tip to allow for a 1.6 mm stainless cable to be attached. I have found that the quality of fit of this arrangement is very good and does not sacrifice accuracy, the tri cut front end is very good for penetration.
- Montie Ho Sing
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