Touch hole liners??????????

[davec2]

Pletch,

You can pick my brains at any time (although I must warn you that the pickin's are often very slim). 

Couple of thoughts:  First, Acer is right about a radiused hole lasting longer.  Any sharp edge will heat and erode much faster than a rounded edge.  Second, as Dan pointed out, and I tried to indicate by saying that this was a pyrotechnic issue, radiant heat and hot particulate from the pan flash are driving the speed with which the main charge goes off.  It is not the flow through an orifice, of whatever shape, as, initially, there is no pressure driving the flow.  The pan side combustion is occurring at one atmosphere and the pressure inside the barrel is one atmosphere.  While there is some very minor local pressure increase as the pan goes off, it is very, very small and cannot result in much gas flow.  Acre is also right in that flow through a counter sunk / coned liner is flow through a venturi, but that comes into play from the inside to the outside as the main charge goes off and there is about 10,000 psi inside the barrel driving gas flow out of the vent.  The throat (i.e smallest cross section of the vent) will go sonic (the gas velocity will be the local speed of sound in that gas at that density and temperature).  If the vent is coned on the outside, the gas will increase in velocity to super sonic.  It is this very high velocity, very hot gas that causes gas cutting and local erosion at the vent.

[bluenoser]

This is a fascinating thread and my little brain is struggling to take it all in.

Davec2

I have read your last post several times.  Am I correct in understanding you to say a radius at the inside of the vent where it meets the cone would reduce gas cutting, but a radius at the outside (which I see as somewhat akin to coning) will actually accelerate vent deterioration?

Laurie

[Joe S]

Mean straight hole = 0.044 seconds
Mean White Lightening liner = 0.036 seconds.

0.044 - 0.036  =  0.008

[davec2]

Laurie,

No.  What I meant was that any sharp edge in a hot gas flow will heat and erode faster than a smooth radiused edge.  The more rounded the contour the less heat will be transfered from the hot gas to the metal in the same amount of time.  Imagine a hot torch flame impinging on a square steel bar and on a round bar of equivalent size.  The sharp corners of the square bar will come up in temperature much faster than the round bar.  A knife edge would be even worse.

[Daryl]

The problem with external cones, in my experience, is that they are more prone to flashes in the pan if they have any fouling the the external cone or even not. I have had unexplainable flashes with external cone vents. I have never had an unexplainable flash  with the WL/English type.

The only thing that will produce a flash is if a flake of fouling gets into the liner and blocks the vent, the rifle will flash and then its necessary to break the flake of fouling with a pick or grass stem (I seldom carry a pick). This is the only thing that has caused the rare flash in several hundred rounds with this rifle.


 I KNOW that SS liners are not HC. But I don't care. The rifle has to WORK. Along with some others here I hunt areas where there are large carnivores with bad attitudes at times. You just don't knwo what time.
Then there are places where the bears come to gunshots.
 Dan

Absolutely - and - the only time I get flashes in the pan with my old Siler on the .40 rifle, is when using 2F powder.  With 2F, I seem to get the odd 'plugged vent' - it is always a hard piece of fouling. Seems to happen willy/nilly - maybe once in 12 shots.  If I pricked the vent every shot, that wouldn't happen at all, but I'm lazy and do that only when it fails to fire.  If I hunted with this rifle, I'd prick the vent before priming, every time.

[Cody Tetachuk]

You guys are WAY over thinking this whole touch hole thing.  .................... The longer, skinnier the hole to burn through, the longer the pyrotechnic train burn time required to get the main charge lit.  I really don't think it is any more complicated than that.

So simple and yet, to quote Sonny Bono "the beat goes on"

[jwh1947]

Well, 3000 or so comments ago, I started a discussion on touch hole liners.  I've read it all and have tried to absorb all the comments, both the physical science and the comments from the field and range.  Admittedly, there's much to it.

I have three up on the bench, if we include the one that John and our two new apprentices, Linda and Dragana, are finishing off.  Two of the three are simple and are intended to look original and kill deer, that's all.

I will be staying with a #50 drill and a hole as near the breech plug as possible.  John and the girls can do as they wish with theirs. 

On the other hand, I intend on doing my next ones with a liner.  From this thread I have reduced it to the "White Lightning."  With a White Lightning and ample but not excessive Rakija, I hope to approximate the ignition time of my M1.   Wayne