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Muzzle Knife: inner or outer bevel?

A place to ask general spud cannon related questions.
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Unread postAuthor: Pete Zaria » Thu Aug 09, 2007 3:13 pm

I've always filed my muzzles to a point roughly dead center in the pipe wall, beveled in from each side. I also try to make it a long, gradual bevel, which makes it much easier to ram potatoes in. I've done this on at least 2 ABS and 1 PVC cannons, combustion and pneumatic, with no related problems. Usually with 1.5" or 2" barrels.

I seriously doubt the added friction of a slightly oversized organic projectile could cause pipe failure.

Although it's already been explained mathematically, let me put it into layman's terms:
The hot, expanding gases of a combustion, or the compressed air of a pneumatic, want to escape their chamber, and will follow the path of least resistance to do so. As long as pushing that potato down the barrel requires less energy than ripping the wall of the pipe, the gases will do exactly that. This would probably change in a hybrid environment where there's a rapid pressure spike, though.

Peace,
Pete Zaria.
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Unread postAuthor: MisterSteve124 » Thu Aug 09, 2007 3:52 pm

i mostly bevel the outside but I do bevel some of the inside. As long as you don't do it too much it won't matter.
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Unread postAuthor: boilingleadbath » Thu Aug 09, 2007 5:15 pm

Folks, let's presume that combustion has not completed by the time the potato starts to move (at all). We should all be able to agree on this.

Further, let's generalize and say that the combustion procces takes a fixed amount of time.

And, as jimmy said, a larger amount of static friction will tend to delay the start of the projectile's movement.

And I think it is reasonable to assume that higher amounts of friction will hinder the progress of the spud down the barrel.

Thusly, we find that at any given point in time after the ignition of the fuel-air mixture, the high-friction tuber is closer to the breach than the low friction tuber - but the same amount of combustion has taken place, which means that the pressure in the chamber will be higher.

The increase in the maximum chamber pressure is not necessarily proportional to the increase in static or dynamic frictions.
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A heavier projectile will result in roughly the same happenings and outcome.
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Now, as I said, this is not really an issue in properly constructed launchers, because the pressure cannot exceed 120psi or so under any - even "perfect" (if a scenario like this can be called perfect)- situations.
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And I never said that a spudgun knife should have an inner bevel, although it probably <i>is</i> a good idea. Rather, I expressed my surprise that it was possible to use a forcing cone that <i>big</i> (personally, my knives are rather lacking in forcing cones... probably comes from being built when I only had pneumatics in my possession. The slugs don't leak pre-firing, though.)
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Unread postAuthor: elstevedore » Thu Aug 09, 2007 11:28 pm

We'll, in the interest of safety, I've switched to an outside bevel. It will be interesting to see how this change affects performance.
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Unread postAuthor: Pilgrimman » Fri Aug 10, 2007 2:08 am

@MrCrowley

The reason I believe this to be true is that my father is in law enforcement, and his friend who is in charge of firearm-related laws/issues/police-crap-that-I-don't-know told him explicitly that it is dangerous to get a tight seal with your combustion gun ammunition for the reason of possible failure. While I do agree that if a cannon explosion could happen that it would be rare, I think that,given certain conditions, it is highly possible. Maybe if the seal was too tight, and the gas combusted, and the resulting pressure spike couldn't be relieved by the spud moving in a few milliseconds, the shock of the spike may cause failure. I should have mentioned that this is the belief of somebody else before I posted, though... :oops:
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Unread postAuthor: MrCrowley » Fri Aug 10, 2007 2:58 am

If you use pressure rated PVC exluding pipe failure the barrel/chamber will not explode. As BoilingLeadBath stated with his figures of max peak pressure of a combustion at 120psi in perfect conditions, most hair spray combustions only generate around 50psi, even with propane at 80-ish psi a pressure rated spudgun will not explode with a potato as the projectile.
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Unread postAuthor: Insomniac » Fri Aug 10, 2007 4:31 am

A properly made launcher will NOT explode from a stuck projectile. Even if it doesn't move an inch, a good launcher will be able to contain the combustion pressure.

@Paaiyan: The reason for cutting the inside of the barrel is to give the spud a bit of friction. It works a bit like a burst disk, not allowing the projectile to move until a certain pressure has built up. Once that pressure is reached and the spud starts moving, the friction is vastly reduced. To see what I mean, rest your arm on the computer desk. Let it go limp, then gently and VERY SLOWLY drag it back towards you. It will grip the desk and won't move, until the force increases enough that the grip 'lets go' and your arm will move several cm in a little burst of speed. This is like what happens in the gun, the spud grips the barrel until the combustion almost reaches its peak pressure, then releases and the spud goes flying.
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Unread postAuthor: PinHead » Sat Aug 18, 2007 12:24 pm

I put a slight inside bevel on my combustion gun along with an outer bevel. It definitely seals better, but I didn't go very far because I didn't want it to be a pain in the ass to get the ammo down.

As a side note, what does everybody use to bevel their barrels with? I did mine by hand with a sanding block, going around and around, which kind of sucked. On the other hand, I think that if you took a funnel and lined it with some sandpaper, you could just rotate it and get a perfect bevel.
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