An Improved Pellet Trap (part 2): Design
The design of the pellet trap I'm building is fairly simple, using a combination of friction and impact to stop pellets. First, a replaceable layer of 3mm PVC plastic will use friction to decelerate the pellets, and then two layers of 12mm polyurethane (PUR) sheet will use impact to stop the pellets.
I'm fairly happy with this design, as it meets most of my design goals:
- It's low maintenance. The PVC sheet will need replacing from time to time, but not too often. The PUR sheet should last a very very long time, especially when using wad cutters at no more than 12J.
- It's quiet. The impact with the PVC sheet will be audible, but much less so than most other materials like wood, hard plastic, or steel.
- It's cheap. Only small amounts of PVC and PUR sheet is required, and the cost per m2 of these materials is low.
The one property of the design that I'm not too happy about is, that PVC and PUR sheet is fairly difficult to come by in the small quantities required - and whole reel of PVC does cost a lot of money. Patches of conveyer belt can work just as well as PUR, so depending on your geographical location, this may be easier to come by. I have yet to find an alternative to PVC.
The main advantages of PVC is that it heals (closes) really well and that no material is "removed" on pellet strike. No other materials I've looked at close anywhere near as good and they leave a big mess. Common materials used such as patches of carpet, news paper, phonebooks, and cloth, they don't "close" the whole after a pellet strikes (carpet and cloth does to a limited extent) and over time they make a mess from the trace materials they leave on pellet strike.
Bullet traps in general are based on one of three principles:
Impact and friction designs are much simpler than a deceleration design, and maybe as a consequence, no commercially sold pellet trap is based on this design. A pellet carry much less energy than a bullet fired from a firearm, and is thus much easier to stop, which also detracts from using a deceleration design.
Because of the inherent "klang" from a pellet trap based on an impact design, I decided the design also needed to incorporate a friction element, to reduce the energy of the final impact strike. With a deceleration design, there will still be a pellet strike, and probably also a "klang".
Many people are reporting that the "klang" from most pellet traps can effectively be suppressed using some form of moderator (cloth, foam, insulation, etc.), however there is still the risk of lead dust to consider.
Friction style bullet traps, in general, are typically based on one of the following designs:
- Sand berm
- Rubber berm (same as sand berm, but using rubber granule)
- Sheets of material
The sand berm is a fantastic engineering solution. Cheap, very effective, and "self-reorganizing" so it overcomes the inherent problem of concentrated high energy impacts. Use of rubber granule was inspired by the sand berm. A material was needed that had the same properties as sand, but made separating material from bullets easier.
The US Army also did an extensive study on commercially sold bullet traps, and it's an excellent walkthrough of different design principles and the associated pros and cons. Here's the (original link through Google) as well as local copy.
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