Dual-friction barrels

[boilingleadbath]

I figured it'd be best that it get it's own thread so that:
1) We don't get splinters in jimmy's topic on combustion spudgun models
2) We can discuss it in further detail.

Anyway, the posts that exist in Jimmy's thread and relate to this topic are reprinted below:

<b>SPG</b>:
<i>I'm wondering looking at all of this (and I should admit I only understood about a third of it) is there wouldn't be some merit in what I'll call a dual-friction barrel.

Imagine that the first couple of inches of the breach end of the barrel is high-friction, then leading in to low friction. Surely this would allow a higher pressure to build in the chamber during combustion, and thus more gas to be combusted, before the spud makes any initial movement, but also allow the spud to move freely once this high rate of combustion has been acheived.

I'd be interested to see for instance if there was any difference in performance between barrels with internal and external chamferings of the spud knife.

Effectively what you're trying to reproduce is the "burst-disc" effect, but without a burst disc.

Secondary to this, what conclusions can we draw from this on hybrid performance in relation to disc burst pressures, can we model an optimal pressure (as a ratio of peak chamber pressure) for the disc to burst at.</i>

<b>Jimmy</b>:
<i>
A "dual friction" barrel is a great idea. Any ideas on how? Glue a sheet of sand paper on the inside of the barrel at the breach end? Ream out the breach end of the barrel a bit, for a breech loader, so that the spud in the breech is a bit wider than the barrel?

"I'd be interested to see for instance if there was any difference in performance between barrels with internal and external chamferings of the spud knife." I believe there is evidence that an internal bevel increases the muzzle velocity, presumably by increasing the static friction. Of course, playing with the geometry of the muzzle knife changes both the static and dynamic friction. As you suggest, you probably want high static and low dynamic friction.

The current adiabatic model makes a very simple prediction for the optimal burst pressure of a hybrid. The burst pressure should be as close to the peak pressure as possible. I suspect that that is a bit simplistic and probably is not the best answer to the question. To correctly answer that question I think the model will have to be non-adiabtic so that heat loss is taken into account. The higher the burst pressure the better, but at high pressure the temperature is high and heat is being lost quickly. Hard to say if there is an optimal burst pressure that maximizes pressure but minimizes heat loss.</i>

<b>FLONE</b>:
<i> In terms of the dual diameter barrel, use a 2" sched 80 barrel with SDR 21 or 26 1.5" inside for the first couple inches?</i>

*************

...to which I respond to this:
Perhaps a sub-caliper washer - like, say, .01" undersize - with a chisle grind (a taper on only one side) oreinted so that the ground side faced towards the muzzle would be good.

I'm thinking that you'd hold it in place with a coupler or union or something.

Seems it'd grind off a happy little bit, significantly reducing friction.

The only problem is that you'd have to control both the outside and inside diameter of the washer to several thousandths.

[spuzi14]

I don't know if this would squeeze the potato (assuming that's what we're firing) apart but...

Using some type pipe that will compress when pressure is applied to the outside but stay sturdy other wise, extend the barrel into the chamber until it's about 3 inchs away from the end cap. Load your spud into it (using a muzzle knife with the bevel on the inside) and then fire. In theory the pipe would compress causing increased friction.

The only things I see as problems are where to get this type of pipe and that any pipe ahead of the potato would probably compress causing your potato to go nowhere.

Just my guess, I really don't know but I think it's going to be tough to do this. I use an inside bevel on all my muzzle knives and it makes a difference.

[boilingleadbath]

I doubt that the squeeze applied by your standard pressures will be noticable. (and I don't want to do the math)

...but maybe one could have the barrel stick into the chamber a bit, and solvent-weld a coupler (or a custom ultra-tight coupler-like-object) onto that stub? I've noticed (in my co-axial pneumatic) that this squeeze increases friction significantly... though, in my case, it's at the muzzle. (a bit like a choke. I wonder if it makes it pattern better...)

[spuzi14]

Well I was just throwing an idea out. Perhaps some random member will read it and modify and be succesful.

[man_o_brass]

Oddly enough, I had the same idea about a week ago while sitting through a particularly boring materials class. I was planning on trying it out by using an engine-cylinder hone (which I already acquired for a different gun project)to bore out the majority of the barrel and leave the breach at a smaller ID.

There's alot of variables, though, mainly having to do with combustion in ways that I don't exactly understand. If the breach had a good enough grip on a spud, combustion would progress much closer to completion before the spud started moving. While this would be great for a short barreled gun, I don't know what effects that would have on propellant gases as the spud traveled farther down a longer barrel.

[spuzi14]

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote"> doubt that the squeeze applied by your standard pressures <hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">

So then in a pneumatic this could be plausible?

[boilingleadbath]

One generaly doesn't want any squeeze in a pneumatic, or at least that's what the GGDT indicates.

Anyway, I don't know quite what to make of these compressive strengths (because, honestly, I don't quite know what they mean) and thus I don't know exactly how to use them... but working the other way around (tension expansion) we note that a 2" pipe would expand something like .008"

...(edit) at 100 psi.

I don't know if that's significant or not.

[man_o_brass]

It certainly wouldn't be helpful in a pneumatic, where you only want a tight enough fit to create an air seal. As far as the compressive data on PVC, for it to really be significant from an analytical standpoint, we'd need to know some material properties about potatoes. Young's Modulus and Poisson's Ratio for starters. Rather than test for these values, it would be alot easier to just test the friction in barrels with muzzle knives of different pitches.

[spuzi14]

Now that I think about it, if you are shooting spuds then a friction barrel would cause problems wouldn't it? I'm imagining a few inch's of the barrel to be real rough instead of smooth. I think that a rough edge would chew your potato up to the point where once it got into the normal barrel...the amount of air going by would cancel out your friction.

Well, that's a thought anyway...right or wrong.

[SpudBlaster15]

I thought of this idea about 2 months ago. It works, and seems to add (at the very least) some velocity gains. I made the dual friction barrel by using a 2" male threaded adapter on the breech end of my 2" barrel. As you know, the ID of threaded adapters is much less than that of the pipe they are designed to join. I ground out the inside of the adapter until it's ID was only slightly less than that of the barrel. When the potato is cut, it fits tightly into the adapter, but given enough force, it will slide past, and will travel the length of the rest of the barrel with almost no effort.