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Mod 401 - Darts And Barrels

by Cxwq - February 09, 2002

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#1 cxwq



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Posted 23 February 2004 - 05:01 PM

Dart and barrel selection is one of the most difficult and confusing tasks for a mod newbie and is often overlooked by veterans. Every Nerfer can stand to learn more about this important subject. Put simply, the eventual success or failure of any mod is predicated on the correct dart and barrel choice.

When planning a mod, it's important to adopt a holistic point of view. Think of the gun as a system where each part must be in balance with every other part. That 12" brass barrel works great on your SM5k but would be a poor choice for your Lock 'n Load.

The first thing you should look at is the air delivery characteristics of the gun you're working with. The metrics here are speed and volume. A SM1500 delivers a medium volume of air very quickly. A crossbow delivers a large amount of air fairly slowly. An additional characteristic to look for in spring guns is force magnification or reduction. If the piston tube has a 1.5" radius and your barrel has a .25" radius, the cross-section ratio is 36:1. That means that the force acting on the dart is 1/36 of the force required to compress the spring. On the other hand, it also means that the air delivery is going to be very fast compared to a typical spring gun.

Each of the 5 commonly used dart types has its advocates but choosing from them need not be difficult if you know the advantages and disadvantages of each. Nerf megas are the largest commonly used darts with a 3/4" tip. Nerf micros, Zero darts, and Stefan megas all measure 5/8" at their widest point. For the Nerf micro this is the rubber tip, for Zero darts, it's the craft foam collar. Stefan micros are the smallest at 1/2". For this discussion I will assume that each dart type is weighted appropriately for the power of the gun it will be used in.

Aerodynamic drag on a long (large L/D) cylinder is directly proportional to the cross-sectional area of the cylinder. Applied to Nerf darts this means that compared to a 1/2" dart, there is 56% more drag on a 5/8" dart and 125% more drag on a 3/4" dart. Aerodynamically speaking, Stefan micros come out way ahead of the competition and should be a serious consideration when maximum distance is required.

The other effect dart diameter has on the system is barrel volume. A 12"x5/8" barrel has 56% greater volume than a 12"x1/2" barrel does. Therefore, neglecting barrel friction, a given gun will get a micro Stefan down a 1/2" barrel 64% faster than a mega Stefan down a 5/8" barrel. Dart/barrel friction increases as the square of velocity and tends to be a limiting factor here. Still, expect at least a 20-30% increase in muzzle velocity when using a 1/2" barrel instead of 5/8".

Nerf micros are very successful for a reason that appears to be contradictory at first. They have a higher dart/barrel friction coefficient than other dart types. The rubber tip causes the dart to stick in a snug fitting barrel. This negates one disadvantage of all spring guns - slow air delivery. As the plunger moves forward, pressure builds until it reaches the 'breaking point' and the dart begins moving. This delay allows spring guns to fire at higher pressures than they would normally be able to achieve. Beware the down side to this phenomenon however, the increased friction continues down the length of the barrel causing Nerf micros to be friction limited at comparably lower velocities than other darts. If you're using an air pressure gun this can be avoided by using silicon lubricant in the barrel, or better yet, by using a different type of dart.

Zero darts are Stefan micros with a collar made from a thin foam sheet. These darts have the lowest dart/barrel friction and work extremely well in air pressure guns. The plunger inertia issue makes Zeros a less than ideal choice in spring guns. Zero darts should be considered especially effective in large volume pump guns like the SM5k where a larger barrel diameter won't hurt as much and the lower dart/barrel friction will be critical.

Barrel choices are a fairly recent phenomenon in Nerf modification. Until recently most barrel replacements were done with 1/2" SCH40 PVC. Now several other barrel materials are being tried and it is useful to compare the strengths and weaknesses of each.

1/2" SCH40 PVC is still a versatile and viable choice for barrel replacement. It fits Nerf micros, Stefan megas, and Zero darts perfectly with its 5/8" ID. Dart/barrel friction ranges from moderate to high in PVC with Nerf micros having the highest. As mentioned previously this can be useful in spring guns but is less than desirable in air pressure guns as the darts become friction limited sooner than they would with other dart/barrel combinations.

Crayola pen barrels are used most frequently as a barrel plug mod combined with 1/2" SCH40 PVC. They offer the benefit of a reduced size sleeve to fit the body of Nerf micros and Zero darts, increasing the efficiency of pump gun air volume. They also allow Stefan micros to be used in PVC, though the relatively short Crayola section may not allow enough time to reach maximum velocity before transferring to the oversized, 5/8" section.

1/2" SCH80 PVC is a great alternative for using Stefan micros in guns with relatively fast air delivery and large air volume. With a 17/32" ID, they are somewhat wasteful of air, but allow a higher velocity due to decreased dart/barrel resistance. Avoid using this type of barrel in low air volume guns.

9/16" (O.D.) brass tubing is the ideal barrel for using micro Stefans in any pump gun. It offers fairly low dart/barrel resistance and fits micro Stefans perfectly for highly efficient air usage. Avoid using this combination in most spring guns since the dart will be gone before any significant amount of pressure could develop.

As a barrel recap I'd like to discuss the main problem with barrel advances to date, and suggest a solution. All of the new materials being used are best applied to pump gun modification. The best solution for spring guns is still Stefan megas or Nerf micros in PVC. Both of these are imperfect pairings. The Stefan mega leaves the barrel too easily and the Nerf micro becomes friction limited at too low a velocity.

The telescoping brass barrel aims to avoid these problems by offering a graduated amount of dart/barrel resistance. High at the start to overcome plunger inertia, then a moderate amount to provide air efficiency and fast acceleration, and low at the end to allow the dart to reach maximum velocity before leaving the barrel. This can be accomplished with brass due to the nesting nature of the tubing. Brass tubing is sold in 1/32" increments and has 1/64" wall thickness meaning that each successively smaller size fits perfectly in the previous size.

[07-11-04 Edited the sizes in the following paragraph to more closely match the evolution of this concept -C-]

Take a 19/32" brass tube and cut it to 10". Drop a 6" piece of 9/16" tubing inside it. Now put a 2" piece of 17/32" tubing inside that. They're all lined up at one end so that you have a 10" barrel that starts off with a 2" section that's slightly tight, a 4" section that fits perfectly, and a 4" section that's slightly loose. Solder them together at the end where they line up and epoxy the whole thing inside a 10" piece of PVC. Glue a PVC coupler on the end of your crossbow or BBB air chamber to complete the breech loader. Stuff a micro Stefan into the 17/32" end of the barrel and stick it in the coupler. You've got the rate of fire of a breech loading spring gun and the distance and accuracy of the best pump guns.

2002, 2003 by Cxwq - Hail Eris! All Hail Discordia!
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