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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).