14 replies to this topic

[Daniel Beaver]

I've always wondered what sort of dart fit air-powered automatics (Magstrike, PC, RF20) should have. Since they are air-powered, I assume that they should have a loose-ish dart fit? I see CPVC used a lot in mods, though.

[BritNerfMogul]

I would say loose as the air doesn't have enough time to build up behind the dart.

[aishihigeo]

The reason we tend to place air guns with loose barrels, and spring guns with tight barrels is because is the velocity of air. Automatics use a plunger based system yet they still move faster then spring guns, but still their not as fast as air guns. So I gave mine and semi tight barrel, as you would want the barrel in between loose and tight.

[Doom]

Optimal barrel friction/fit is a function (primarily, at least) of how quickly the pressure increases. Flow restrictions and the slow increase in pressure from the spring mean that the dart starts moving before the pressure has reached a certain level (if it ever reaches that level due to the movement increasing the barrel gas volume). A tight fit requires more force to move, so the pressure can build up higher and consequently performance can increase.

The downside is that high friction the entire length of the barrel will decrease performance. The friction obviously removes some energy from the dart. A tight fit initially for high static friction and low dynamic friction for the remainder of the barrel is ideal. Some people already know this.

I suppose the answer is "it depends." I honestly don't know enough about the mechanics of the MS/PC/RF20 class of blasters, but someone who does should be able to figure it out with my explanation of which fits are appropriate. For a more definitive answer, try some testing.

Edited by Doom, 30 May 2009 - 09:28 PM.

[Shrub]

What something Doom doesn't know how is this possible he always has some type of lengthy theory or diagram or formula or something. Anyways I say short barrels and and shrot darts due to a short plunger stroke.

[Wes7143]

Why don't we just take 2 MS or PC clips, re-barrel one with PETG, the other with CPVC, use the same darts, and see which one fires further?

[Zorns Lemma]

Optimal barrel friction/fit is a function (primarily, at least) of how quickly the pressure increases. Flow restrictions and the slow increase in pressure from the spring mean that the dart starts moving before the pressure has reached a certain level (if it ever reaches that level due to the movement increasing the barrel gas volume). A tight fit requires more force to move, so the pressure can build up higher and consequently performance can increase.

The downside is that high friction the entire length of the barrel will decrease performance. The friction obviously removes some energy from the dart. A tight fit initially for high static friction and low dynamic friction for the remainder of the barrel is ideal. Some people already know this.

I suppose the answer is "it depends." I honestly don't know enough about the mechanics of the MS/PC/RF20 class of blasters, but someone who does should be able to figure it out with my explanation of which fits are appropriate. For a more definitive answer, try some testing.

Doom, correct me if I'm wrong, but the practical application would be as follow:

Optimal barrel fit is tight enough on the dart (high enough friction) so that the dart is dislodged when the force from air pressure (F = p/A) is at maximum. Since an air tank has a (relatively low) set pressure for any given number of pumps at the moment the valve is opened, there is no "building up" of pressure, which means you want a looser fit (relatively low friction).

A spring gun on the other hand, builds up pressure until the force from the compressed air is equal to that supplied by the spring at the place the plunger head stops. You can easily demonstrate this to yourself by taking a well-sealed NF, blocking the plunger tube, and firing. The plunger rod flies forward until it hits a point (before the end of the plunger tube) where it stops dead. You can kind of gauge the air pressure by the force on your thumb blocking the barrel. If you put in a stronger spring, the plunger rod travels further before stopping. Put in a weak spring, and the plunger rod doesn't go as far.
Ideally, you want the dart fit to be tight enough such that the dart doesn't move until this point, and then is at 0 friction afterwards, for a volume equal to plunger tube volume.
Realistically, you want to use a nested barrel setup.


For the question at hand:
All automatics are powered by air. So go with a looser fit if most of the system is stock.
If you upgrade the piston spring, and run the system at 70 PSI, you'll want a tighter fit.

An automatic is like a hybrid springer/air gun. It is like an air gun, because air is released instantaneously at a given pressure. It is like a spring gun because the blowback piston valve has a spring that regulates the operation pressure of the valve.

[SchizophrenicMC]

The reason we tend to place air guns with loose barrels, and spring guns with tight barrels is because is the velocity of air. Automatics use a plunger based system yet they still move faster then spring guns, but still their not as fast as air guns. So I gave mine and semi tight barrel, as you would want the barrel in between loose and tight.

I'd just like to point out that the plunger is not a propulsion method, but a timing method.

Go with what Wes said, though. I couldn't tell you what would be best. CPVC and PETG are dirt-cheap, so testing this shouldn't be too bad.

I mean, unless you're using the N-Strike MS.

[Zorns Lemma]

Automatics use a plunger based system

I'd just like to point out that the plunger is not a propulsion method, but a timing method.

It is none of those. The spring serves as a pressure regulator for the blowback valve. Stronger spring = higher pressure needed to open the valve.

[Daniel Beaver]

As a follow-up:
Banding the bladder and banding the piston -- do both of these mods increase the working pressure of the blaster? And would optimal barrel fit change at all with either of these mods? My instincts tell me that the answer is no.

[Doom]

Optimal barrel fit is tight enough on the dart (high enough friction) so that the dart is dislodged when the force from air pressure (F = p/A) is at maximum.

Optimally the dart is dislodged at the time that would maximize energy transfer to the dart. This is not necessarily when the pressure is at a maximum, but I'd imagine in most optimal cases it'd be close.

Since an air tank has a (relatively low) set pressure for any given number of pumps at the moment the valve is opened, there is no "building up" of pressure, which means you want a looser fit (relatively low friction).

This doesn't seem quite right. What matters is how quickly the pressure gets to a certain level. High static friction (i.e. a tight fit) can be advantageous in pneumatics as well if the valve opens slowly or doesn't have very high flow such that the pressure takes a long time to build. As I don't know how quickly the valve opens here, I can't really make a good suggestion.

If the pressure is relatively low then that might be another solution because it won't take long to build to that level. But that's not necessarily what's happening in pneumatics.

A spring gun on the other hand, builds up pressure until the force from the compressed air is equal to that supplied by the spring at the place the plunger head stops. You can easily demonstrate this to yourself by taking a well-sealed NF, blocking the plunger tube, and firing. The plunger rod flies forward until it hits a point (before the end of the plunger tube) where it stops dead. You can kind of gauge the air pressure by the force on your thumb blocking the barrel. If you put in a stronger spring, the plunger rod travels further before stopping. Put in a weak spring, and the plunger rod doesn't go as far.

Makes sense.

Realistically, you want to use a nested barrel setup.

Right. This is true for essentially every gun using compressed gas in some way, but exaggerated on guns with slow pressure build-up like springers.

As a follow-up:
Banding the bladder and banding the piston -- do both of these mods increase the working pressure of the blaster? And would optimal barrel fit change at all with either of these mods? My instincts tell me that the answer is no.

Banding should increase the pressure of the bladder. Banding's a rather poor way to do it and I suggest using bicycle tubes, replacing the bladder with higher pressure latex tubing, layering a latex tube over the bladder, or using a higher pressure regulated fixed volume gas chamber if you want to increase the operating pressure.

As for the piston part, I know little about that. What Bob said makes sense given my limited understanding of how the system works.

[bpso86]

Banding the bladder decreases the volume occupied by the bladder at a given pressure, thereby (probably) increasing the capacity and maximum pressure. This will have no effect at all on individual operations of the piston, but will probably increase the number of shots per refill of the bladder.

Are you sure about that Bob? I've always thought that banding the bladder/shortening it (in terms of a powerclip) would cause more air to be released per shot, thereby decreasing the total number of shots in between pumps. If you band the bladder, you have more force on it, and when the air is allowed to escape, more gets released per trigger pull.

As a side note, my powerclip has a shortened bladder and about 2-3" of 17/32" brass (they are the length of the stock barrels so I can still use stock darts, not too sure how long exactly) and I can only get one clip off without repumping (20 pumps). I tried petg when I first rebarreled it, but it got much worse ranges. I'd recommend going with a barrel material that is slightly tight on your darts, as it has worked very well for me.

[CaptainSlug]

In addition to that, the energy stored in the spring is used to advance the clip or turret when it is released.

No, the spring in an automatic does the opposite. It works against the pressure. The spring regulates the pressure level at which the air is released at the end of the stroke, it then retracts the turret/clip advancing mechanism.

Banding the piston increases the pressure level reached before it fires.
Banding the bladder increases the output pressure and flow rate of the bladder.

Edited by CaptainSlug, 31 May 2009 - 11:08 AM.

[TantumBull]

I know imaseoulman has had luck with tighter barrels on the magstrike, even though the air release is fast. Maybe try making the barrels removable at first so you can test different fits. Or if you have a spare clip use half with one barrel material and half with another, then with differently marked darts run some tests.

[Zorns Lemma]

Part of the reason tighter barrels work on the magstrike and powerclip is because you can only make your barrels so long.

Various mods that allow use of 4-5" barrels (or using the RF20) means you'll want to drop to looser barrels.