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In this blog post I will consider some ways we can re-shape the traditional leather pouch so that it shoots faster and lasts longer.

We all want to shoot reasonably fast. Speed gives flatter trajectories and higher impact energy. Ideally, we want to do that without having to pull a lot harder. Some people who may already be shooting tiny pellets from thin tapered bands stretched to the elastic limit will have just about run out of design parameters to tweak to get the super speeds they seek.

The weight and shape of the pouch is actually pretty critical to performance. Along with the pellet, the pouch travels the greatest distance and must be accelerated the most out of any part of the bandset. If the pellet weighs only one or two grams, but the pouch weighs 5 or six grams, it's as if the pellet's dragging a huge ball and chain down the flightpath. Have a look at this:

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The red line shows the velocity of the projectile at different weights. Say you were shooting a 2g projectile. Increasing the pouch weight by 5g could cost you over 10m/s (30-odd fps). Decreasing the pouch weight from the approx 1g I was using by half might give you 5 or more m/s. At the same time, having a pouch that is too big can be inaccurate and is like dragging a small parachute a short distance. I don't thing it's as material as the inertial effects, but it makes sense to keep it small.

In both cases, it's less critical if you are shooting large heavy projectiles, but a big heavy pouch will you lose you some speed nonetheless. That's why Hunter Catapults has great success with their bigger and heavier pouches. It's a big heavy band optimised for big heavy projectiles and a bit of extra weight makes not a lot of difference. At the other end of the scale, Baumstamm shoots small steel balls with thin tapered butterfly bands and he has rightly picked the thinnest leather to make very light pouches.

With all this in mind, I set out to make some of the lightest pouches possible, using things like nylon mesh, linen thread and Dyneema.

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Scale Postal scale Gauge Gas Measuring instrument


These pouches can be made down to below 0.1g but they come with some disadvantages. They take longer to make, they may not last as long and not everyone would (or should) trust them. They're great for record attempts, but leather still has a lot more potential if we look at it like engineers. We cut away what's not needed (dead weight) and beef up the parts that tend to break first (weak points).

I make the Fastband pouches out of tanned cowhide. Its tested specifications are given below:

Name: Upholstry leather
Animal: Bovine/ Cowhide
Split: top grain
Part of hide: believed to be bend
Tanning: believed to be vegetable tan, chrome re-tanned, aniline
Post treatment: None
Weight: 0.10g/cm²
Thickness: 1.25mm
Tensile strength: 22kg/cm width

I started out making the bands out of simple 7cm x 1.5cm strips with a round hole punched in each end equidistant to the sides and a third to centre, the shot dead centre in the pouch. These pouches are smaller than most, but are ideal for the size of ball I like most at between 7-10g. They have shown they can last many thousands of shots, but they are still the shortest lived part of the Fastband bandset. If your bands don't last as long, you might be able to get away with narrower bands still.

The main limiting factor on the narrowness is the tensile strength of the leather and the width of the band tie hole. If the hole is wider, the leather at the side of these holes is narrower and weaker and this is inevitably where it will fail due to wear and tear. For the Fastbands, there's about 0.5cm between the hole and the side. The breaking strain is about 11kg (or 108Newtons or 24lbs) and theoretically 430N in total. The draw strength of the bands as tested by member shot in the foot was probably around 76N based on his description of how they were tied and drawn, so a breaking strain of 6x the typical draw force seems to give a sufficiently robust pouch.

The only enhancement I have made recently is to cut the redundant corners off the pouch. Not much weight is saved, but it makes the bandset more tidy looking when stretched and worn in.

Below I have drawn my design and some possible enhancements under consideration.

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Forum member Nico has recently proposed cutting a 'stress relieving' slot at the side of the tie hole opposite the elastic. Actually, I've seen it before on other bands and previously copied it myself when I was cutting conventional tapers in my bands. It's a great idea, but I don't think it actually relieves stress on the pouch. Instead, it allows you to make a narrower hole and still get your bands through, which in turn allows the leather at the side of the hole to be wider and that's how it works. The shape at the central side of the tie hole is not critical, and I have sometimes used oval or pear-shaped punches.

The other ideas drawn explore Jörg's idea of having double centering holes and possible ways of cutting away leather that isn't needed. Notice that it's important to consider the shape of the pouch once it's been stretched. The leather I'm using stretches more than the leathers used for boots or sports balls, but is lighter and some leathers such as the deerskin I tested stretch even more.

Another idea would be to make the tie holes very small indeed and slip the bands in with some of that friction modifying fabric, but I don't want to waste that stuff and the long slot idea could sidestep the problem entirely.

You may wonder why I don't skive the leather thinner between the tie holes. Indeed that would save weight, but I'm not good at skiving soft chromium tanned leather, it could introduce cuts that would become stress-risers and initiate a tear and it would be hard to make pouches consistently every time. As it is, it's hard to cut accurately shapes that are not straight without a custom made punch.



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bunnymansp
Oct 26 2010 01:43 AM

how about a circle with pear shaped holes for the bands then three holes in the center forming a triangle then tow on each side of the pear shaped holes
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stretch into a sort of long circle


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ZDP-189
Jan 23 2011 03:32 AM

how about a circle with pear shaped holes for the bands then three holes in the center forming a triangle then tow on each side of the pear shaped holes stretch into a sort of long circle
I foresee a problem there in that the pouch will end up tall and not very long. The leather doesn't stretch so much. Sometimes, people even complain that my 7cm x 1.5cm pouches aren't long enough. Also, with an elliptical pouch, there's a lot of pouch not in tension and not contributing anything but inertia.


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ZDP-189
Jan 23 2011 03:38 AM

One of the best collections of pouch designs that I have seen are the various body shape variations of Sellaphora Pupula. These are microscopic diatom algae, but if you scaled their profiles up to pouch size, you'd end up with very efficient and strong pouches.

Source: http://rbg-web2.rbge...

Source: (http://rbg-web2.rbge....html)


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mr.joel
Aug 07 2011 12:47 PM

I haven't tried them, but I think you are really on to something with the mosquito net pouch. If they are accurate I'd pursue those, particularly for small bullets. The combination of those and TBB in a semi butterfly or butterfly draw with say, a .31 lead ball I bet would be fantastic and compliment your A frames nicely.


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Steinschleuder
Aug 08 2011 11:23 AM

These algae have copied my pouch design - does anyone know a lawyer with a microscope?

You net material pouch is for shure a good idea. The advantages in weight and air resistance are obvious. Your net pouches compare well to the principles of a draft shield: Here a net/hedge (structure with holes in it) gives much less turbulences than a wall. Apart from woven nets perforated foils might be used, like the cutting blade of an electric razor (slightly too sharp!). The holes could be punched, lasered or water-cut. Material-wise leather is still very competitive. Does anyone have an idea for an easily laser/water-cut, tough material? The gain could be a little speed and maybe a lot of precision when all other variables are sorted.


Carnivore Fawn Dog breed Grass Terrestrial animal

ZDP-189
Aug 15 2011 04:34 AM

Thanks for your thoughtful comment. If it is to be punch cut, then a homogenous material rather than a woven one would be best. So far, I haven't found better than leather for this application.


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GreyOwl
Aug 16 2011 05:50 PM

Hello Dan,
your comparison study with sketches is most than interesting. I love very thin leather for the pouches I use for 8 or 9mm steel balls.
Have you tested a "losange" shape with half rounded cuts under and above the main center hole?
Mine are 5.5cm x 2cm and weight 0.5gramm.
Thanks for sharing,
Phil



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Tex-Shooter
May 14 2012 09:54 AM

I use top grain cowhide (not split with no top grain) non-waterproof boot leather and is a thickness of 1.7 to 2 MM thickness. Both my formed and flat pouches are 25 MM wide by 69 MM long. -- Tex


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horst
Dec 10 2012 06:49 PM

One of the best collections of pouch designs that I have seen are the various body shape variations of Sellaphora Pupula; These are microscopic diatom algae, but if you scaled their profiles up to pouch size, you'd end up with very efficient and strong pouches
 
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