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Recently I made some new Larp crossbow bolts and at the same time gave my old crossbow an overhaul, tweaking the retaining spring and remounting the arms. I noticed with the additional distance I was getting that I had very poor flight characteristics beyond short range on my old bolts.

As I had to test my new bolts anyway I retested my old ones and both performed very badly. What did strike me however was that running through this might be of help to people who might not know how to check directional stability of a projectile.

This article is written about bolts but applies to all projectiles, just Larp bolts have pretty bad flight characteristics so most other items have fewer issues.

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After some experimenting:

Knocked the data out of at the lower ends of the range but the rest seems to fit nicely. I experimented with an artificial 0 intersect but it skewed the numbers out. I suspect there are some errorous effects from the multi strand shock cord/bungee taking tension up. Data here is for a 8mm and 10mm shock cord.

Now I can calculate a more accurate modulus of elasticity.

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I've been tinkering with designs for more effective LRP crossbows for while now for use in modern/steampunk settings. Objective is turn a fairly basic, and never brilliant LRP staple into a perfected bit of kit.

Objectives:

  • Range - greater range for greater threat
  • Accuracy - only counts if you hit something
  • Speed - speed is pretty driving for accuracy
  • Capacity - reload speed, magazines
  • Utility - size, weight, ease of operation, functionality in cover

I'll look at the details of these in detail over separate pieces.

Limits:

  • 30lb draw limit, common for UK LRPs
  • LRP Safe
  • Won't get me arrested
  • Unpowered (for now)

I've used holo-sights on crossbows to good effect but I'd like to build an easy ironsights for my bows. In the long run I'm looking for two designs, a CQB version and a long-range rifle type.

For a start, let's rub some science on it and see what sticks. On a classic LRP bungie crossbow the string runs at an angle back from a fixed arm. At max stroke they normally pull up to 133.4 N (25-30 lb) - however as the string moves forward the angle decreases and the string slackens, therefore the forward pull reduces to zero. The power stroke is therefore 130N to 0 N over about 200mm.

F = ma

So as the force of the stroke drops off so does the acceleration, ideally the design wants a consistent power stroke applying a constant Force for the entire stroke distance. To do this I'm planning to use a configuration similar to a diving speargun to propel the bolt forward. For maximise the power stroke I want the string to be under tension for the entire stroke and running parallel to the stroke so no force is lost as the string angle drops.

To keep the length to a sensible limit I expect that I'll have to run the string over a sheave and back along the bow. 

String length prediction:

Based on these rough numbers I'd be looking for a base string of 810mm, but to make this formula useful I'll need some modulus of elasticity values for prospective bungee (to the lab).

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