Dart advancement mechanisms

Posted on 2010-06-29 10:45:52

Filed under fang4

Possibly the biggest choice I can make in FANG 4 is which dart advancement mechanism to use. There are many options available today, and the problems of my past FANG guns all centered around the dart advancement mechanism.

Advancement mechanisms considered

Barrel mover - The barrel pulls back over a dart in a magazine. Example: APB5K
Projectile pusher - A dart is pushed from a magazine into a firing area. Example: FANG 1
Breech closer - A breech closes so that the captured dart can fire.
Turret - A turret of barrels (or alternatively, a single barrel and a revolver mechanism) is rotated for each shot. Example: RevoSemi
Shells + Barrel mover - Like the barrel mover design, except that shells are used in addition. Example: RSP-Skeleton
Hopper - Darts fall from a hopper off a wye fitting. Example: Hopper guide

A few other mechanisms like RSCB clips and inline clips were not considered. RSCB clips are not true semi-auto and inline clips would be too difficult to load, have too much dead space, and might potentially double fire too often.

Goals

Each mechanism was given a rating from 1 to 5 in its ability to satisfy certain goals. The goals were also given weights that described my priorities for this Nerf gun.

Ease of loading - FANG 4 should be as easy to load as possible.
High capacity - Perhaps this should be appropriate capacity rather than high capacity. If the mechanism can carry 10 to 15 darts, that would be fantastic.
Low mass - I want FANG 4 to be as light as possible.
Difficult to make - Obviously, as this is a DIY project, my ability to construct a mechanism is very important.
Reliability - Reliability of the mechanism was a huge problem with FANG 1. Some have said reliability is the most important thing for Nerf guns. FANG 4 is useless if it is not reliable.
Dart limitations - Some designs place limitations on dart length. This is undesirable for versatility.
Durability - Durability is a big goal of the entire Nerf gun project; this reflects its inclusion at this level.
High rate of fire - The design must be able to support a rate of fire of of dart every second or so.
Efficiency (dead space, flow, etc.) - A high efficiency gun is desired, so anything that reduces efficiency is very undesirable.

Using the goals above, each of the mechanisms mentioned above was put in a design matrix to get an idea of how well each performs in comparison with each other. This is not an exact method, but it can give me a rough idea of how successful each mechanism could be.

The design matrix

You can download the spreadsheet I used here in Excel format. Alternate formats are available.

Please note that the weights I used are completely subjective. They reflect my desire for a Nerf gun that primarily has a high capacity, is reliable, and is lightweight.

The rankings are somewhat subjective too, and I am certain fans of some mechanisms will take issue with them. They reflect how well I think each mechanism might serve this project, and as I used a spreadsheet, they can be easily changed.

The advancement mechanisms ranked

Turret (85%)
Barrel mover (79%)
Breech closer (76%)
Projectile pusher (69%)
Shells + barrel mover (66.5%)
Hopper (65.6%)

The quantitative rankings and compatibilities above are helpful, but some of the numbers are close, so I'll qualitatively analyze each mechanism below to help me determine my final choice for FANG 4.

Turrets

Turrets seem like a fairly obvious choice now, and they are what I intend to use in FANG 4. Their main disadvantages are weight, size, and the difficult to to make the automatic advancement mechanism. The weight and size problems can be avoided by using shorter barrels and a lighter barrel material (such as PETG). As I can use fused deposition modeling (a rapid prototyping technique), I believe I can make a semi-automatic turret without much difficulty.

Barrel mover, breech closer, and projectile pusher

These three mechanisms are fairly similar and they consequentially ranked similarly. Their use of magazines (which can be difficult to load) and their potential for unreliability prevent me from using them.Shells + barrel mover

This mechanism is very reliable as done by PVC Arsenal 17, however, loading is the worst of all the mechanisms due to the use of shells. That is what keeps me from using this mechanism.

Hoppers

Hoppers are the easiest mechanism to make, but what prevents me from using them is that they almost certainly won't work. Hoppers have problems with low volume Nerf guns. Split at NerfHaven told me he couldn't get a hopper to work with a "small" 4 in^3 firing chamber. My firing chambers are probably going to be between 0.5 and 1 in^3 in volume.

Additionally, the layout of a hoppered gun I find to be unappealing, and loading could be problematic. The limitations put on darts also are a convincing reason not to use hoppers for FANG 4.