Planning a sodacan mortar seeking comments on my tube design

[Cannoneer]

Thanks, Cat.

[Victor3]

 Dang, Boom. Talk about "necro-posting." This is a four year old thread.

 Are you planning on making a new mortar?

[Cannoneer]

Victor,
 I'm glad DD put those stickies to the top of the page, because I swear I was thinking about this thread where George & Tim were commenting about the powder chamber size of a beer can mortar.
I wish I had the the stuff to build the mortar myself, (maybe someday) but for now I'm going to have to rely on one of the sponsors to build it. Do you have any suggestions for features on a b-can mortar?

[Victor3]

 Boom - Oops. I didn't realize it was a sticky.

 One thing I can say about can mortars is I like the Dictator Dom made for me better than any other I've seen. Easy elevation adjustment, unblowupable with the thick-walled barrel and best of all, looks way cool. 

[Zulu]

Boom - Oops. I didn't realize it was a sticky.

 One thing I can say about can mortars is I like the Dictator Dom made for me better than any other I've seen. Easy elevation adjustment, unblowupable with the thick-walled barrel and best of all, looks way cool. 

Show us a picture of it.
Zulu

[Victor3]

 

[Zulu]

Sweet!!
Zulu

[Cannoneer]

Boom - Oops. I didn't realize it was a sticky.

 One thing I can say about can mortars is I like the Dictator Dom made for me better than any other I've seen. Easy elevation adjustment, unblowupable with the thick-walled barrel and best of all, looks way cool. 

Victor,
While I definitely agree with you, I'm pretty well set on getting a CSA type coehorn first, which I'm sure Dom would also do a fine job on.

One other aspect of the mortar I've been thinking about is the front of the powder chamber. Should the surface be concave to match the convex surface of a round (zinc/steel) ball when seated there, or would this be a relatively insignificant design feature?

[Double D]

My Dom dictator is concave ball seat cylinder powder chamber.

My scrap yard Mortar has a flat ball seat.  I am going to make a tapered Gomer style chamber for it.

[dan610324]

I guess the concave ball seat will deliver a slightly higher velocity as the total volume to fill behind the ball by the expanding gases is less and the pressure will thereby be a little higher

[GGaskill]

Should the surface be concave to match the convex surface of a round (zinc/steel) ball when seated there, or would this be a relatively insignificant design feature?

A concave surface's major value would be the smooth transition from the base to the wall rather than a corner.  Gas flow issues would be minor compared with a drill point or even a flat base.

[dan610324]

yeah , and not to forgot to mention the strength of the tube will be much higher

[Cannoneer]

Thanks everyone, for this latest round of answers; I'm making a list of things I want to have on the mortar, and soon I'll be bugging the sponsors with details.

[dan610324]

the following here is just some theoretical thinking from me , I dont have any scientific evidence for it

a golf ball mortar with 2-3 cal bore depth and using a light weight golfball instead of a zink, iron or lead ball
lets say you got a 3" diameter bar to start from
most people would make a 1"x1" chamber I think or maybe 3/4"x1 1/4", then load it with fg powder and get an satisfying result  

but with that very very short barrel length wouldnt it be much better to make an an powder chamber with 1/2" diameter and maybe 2" deep insted , and use fffg powder ??

I guess that would give the powder a much larger posibility to burn inside the bore then after the ball left the bore as the fffg is much faster then the fg powder and the amount is smaller.

it will also give you a 2,25/1 chamber/wall ratio instead of a 1/1 ratio with a 1" chamber in a 3" bar
so the higher pressure from the faster burning powder wouldnt be any problem

anyone else who have been thinking the same ?? or maybe even tested this idea .
let me hear as many opinions as possible about this

or maybe go so far as to have a 3/8" diameter chamber and use ffffg powder
the lighter ball and the shorter the bore is the more difference I think it will make
so if its a 2 caliber bore depth and you use a golf ball as a projectile I think it could make a hughe difference
the first idea for this came when I read that austrian guys website about gonnes

I think this thinking could be modified to suit a beercan mortar also

[Double D]

Dan,

I think you are correct in theory, except for one thing.  Directly in front of the powder chamber is a big expansion chamber.  I think, I f I remember right from my ready the better results were gain by modifing chamber shape.  The cylinder concentrates the effects of firing on a small area of the projectile, the diameter of the of the chamber.  Where Gomer style chamber is frustum of a cone with the projectile sitting on the larger diameter.  This fans the force of the of powder more uniformly across the bottom surface of the projectile giving a more uniform boost.

The  idea of getting the all the powder to burn inside the bore is sound, just the expansion chamber ball bore seems to me to negate it.  I really believe the chamber shape is the key.

[jeeper1]

DD is right about spreading the pressure over as large an area of the projectile as possible. Too much pressure in one spot will damage or destroy the projectile. I've been there and done that. I've had a half a dozen or so golf balls exit a launcher in pieces in the past.

[dan610324]

well then the question is ,
what is the ultimate chamber design for a more fast burning powder type fffg for example
as the bore is so extremely short we can all agree that its better to use a faster powder I suppose
if the rest of the design is secure enough for that

how about this design ?
would it hold enough fffg for a golfball ??

[jeeper1]

The volume of the pictured powder chamber is less than one half of one cubic inch.   

[dan610324]

do you need more if you use fffg and a golf ball ??
or maybe ffffg

[dan610324]

isnt it anyone who have tested what amount need of a finer granulated powder ??
I guess there must be someone who actually have tested it just by curiosity

[MikeR C]

Dan,
I have used fine powder in my golfball mortar. My mortar's barrel length is aprox 2 times the bore diameter with fairly thick walls, a 30 degree taper down to a .5" by 1.25" chamber. The barrel is fixed at 45 degrees (see pict below) It was was built along the lines of DDs mortar bore/chamber:

http://www.gboreloaded.com/forums//index.php/topic,70859.0.html

But turned from solid. The chamber holds about 50 grains of FFFG powder. I fired it with a full chamber at the Montana shoot with an old golfball, using FFFG and DD said he found the ball just over 400yds from the line (I put my name on my golfballs before I shot them).
I fired a dozen balls or so at the thirty yd stake and determined that 2 - .22 long rifle cases of FFFFG, yes 4f, got me pretty close to the stake. Courser powder gave me erratic ignition until I got about 15-20 grains of it in there, then I shot way over the stake. Balls fired with the barrel close to horizontal and a full to half load seemed to hook and slice and generally go in everywhichaway.
If I was going to do another mortar/shoot, assuming the standard 30 yd stake for golfball mortars, I would do the following: I would put a smaller chamber in the mortar, or an arrangement whereby I could insert a smaller chamber and hold it in with a threaded vent so I would end up with a full chamber to prevent erratic ignition. (Maybe 1/4" X 1/2") I would definately screw sabots to the balls to keep them from spinning, once the ball is spinning the dimples do their job, they make the ball fly and if you can't get the spin in exactly the same place each time the balls will not go straight.
I started a test mortar with an offset chamber to try to spin the ball on purpose, in a predictable way, but never finished it.

If you search out the threads on the golfball cannon contest a couple of years back, larger chambers gave erratic ignition, but shooting at short ranges, the powder charges were very small and I expect FG or FFG. I suspect that the ignition would be fine with a full chamber.

[Victor3]

but with that very very short barrel length wouldnt it be much better to make an an powder chamber with 1/2" diameter and maybe 2" deep insted , and use fffg powder ??

I guess that would give the powder a much larger posibility to burn inside the bore then after the ball left the bore as the fffg is much faster then the fg powder and the amount is smaller.

anyone else who have been thinking the same ?? or maybe even tested this idea .
let me hear as many opinions as possible about this

or maybe go so far as to have a 3/8" diameter chamber and use ffffg powder
the lighter ball and the shorter the bore is the more difference I think it will make
so if its a 2 caliber bore depth and you use a golf ball as a projectile I think it could make a hughe difference

 I don't know if a long skinny chamber would be better or worse with a golf ball.

 One concern might be that with such a light projectile in a short bore, some of the powder at the front of the chamber might be blown out ahead of the flame front unburned (assuming a vent at the rear of the chamber), thus wasting it on muzzle blast after the ball was already on its way.

 But that's just a guess...

[Double D]

Somewhere, was it Matt Switliks book, the more complete cannoneer, the is some scholarly discussion on the this powder size issue in small cannons.

I seem to remember the discussion said that the grade of powder used in smaller bores should be similar to that used in equivalent caliber rifles and pistols.  The charge should be substantially reduce however as the cannons we shot are free recoiling where rifles and pistols are hand held and recoil controlled.

The warning about not using finer grades is in larger volume, large bore cannons. 

[dan610324]

the whole idea here was to try to find out the best possible style for a "match grade" chamber

the problem I got here in sweden is that it actually is illegal to fire projectiles so I cant do the experimenting , doing so would put me behind bars and Im not so interested in that kind of vacation 

how many grains of ffffg would be needed for a golfball and one of the new steel balls to achive 30 yards ??
2 full 22 cases should be for the golfball , but if you change to a steel ball , how much would be needed then ??
Im not interested to reach 400 yards     just the 30 yard pole

[Cannoneer]

I can only think of a few more questions to be asked on this topic; here's the first.
Gomer or cylinder shaped powder chamber; is one superior to the other?

My Dom dictator is concave ball seat cylinder powder chamber.

My scrap yard Mortar has a flat ball seat.  I am going to make a tapered Gomer style chamber for it.

"Gomer chamber. A chamber in the bore of a howitzer, or mortar named after the French Officer who first used it. It was at the extremity of the bore, cone shaped, rounded at the end so that the charge could be as compact as possible. The shell rested on the mouth of this chamber and the charge acted directly upon it."


"Gibbon's Artillery Manual
Chamber.-- When a light piece is intended for throwing large and heavy projectiles, the effect of recoil may be lessened by employing small charges of powder; but as they would be difficult to manage, and would form but a small mass retained with difficulty in its proper place in the gun, a cavity called a chamber is made in the bottom of the bore, designed to keep the powder together in its proper place, and by keeping it more confined, increasing its efficiency. There are three kinds of chambers: the cylindrical (A), Fig. 13, conical (B), and spherical (C).
CYLINDRICAL

GOMER

SPHERICAL

Cylindrical .-- In the first, the bottom of the bore at the mouth of the chamber is formed of a portion of a sphere, so that the projectile closes the mouth of the chamber. This is the chamber used in our different howitzers, and in the eprouvette mortar. It is, however, joined to the bore in the way above described only in the eprouvette, and in one of the howitzers (the 8 -in. siege).    
In the other howitzers it is connected by means of a conical surface, the junctions being rounded off to prevent being worn away by the action of the powder. Cylindrical chambers, when narrow and deep, give greater ranges than shallow wide ones, which do not confine the powder so much, but as in the former the gas acts on but a small segment of the projectile (usually hollow), it sometimes breaks it; and, for this reason, too great a depth in cylindrical chambers is avoided.
The Gomer chamber (after its inventor) consists of the frustrum of a cone connected with the bore by a portion of the surface of a sphere. This kind of chamber is considered the most .advantageous for mortars, and is used in most of ours. Being large at the mouth, it allows the powder to act on an entire hemisphere of the projectile, and no risk is run of breaking it.  It, however, gives a less range than either of the others, but its capacity is greater.
The spherical chamber consists of a sphere, joined to the bore of the piece by means of a small cylinder which serves as a channel to the gases. As this cylinder decreases in diameter, the gas fines more difficulty in escaping, and greater force is developed.
This chamber, however, is even more objectionable than the cylindrical one, from its liability to break the projectiles, although it gives the greatest range of the three chambers. This chamber is in the first place hard to make, and, in. use, soon becomes foul, and is very difficult to clean out. It is now entirely given up."


"Maj. Joseph Robert’s Hand-Book of Artillery, for the Service of the U.S. (Army and Militia.) 1863
6.  Why is a mortar constructed with a chamber?
In consequence of employing various charges, some very small, it becomes necessary to use a chamber to concentrate the charge as much as possible, so that the shell may be acted on by the entire expansive force of the powder.
7.  What form of chamber is given to mortars?
That of a frustum of a cone. The bottom is hemispherical in the sea-coast mortar. In siege mortars it is a plane surface, the angles of intersection being rounded in profile by arcs of circles.
8.  What is this form of chamber called?
Gomer Chamber.
9.  What is the advantage of the conical over the cylindrical chamber?
Cylindrical chambers are objectionable, as the projectile is frequently broken in consequence of the small surface exposed to the action of the charge. This defect is obviated by large chambers, and particularly by those that are conical, in which the charge is expended upon nearly a hemisphere.
10.  How are mortars designated ?
Usually by the diameter of the bore in inches.
11.  How are mortars mounted?
On beds of wood or iron. Those for the new model mortars are made of wrought iron."


Most period writings state that the cylindrical chamber fired the projectile a further distance, and that the Gomer chamber charge was less likely to rupture the shell when fired. The asset (if true) attributed to the Gomer chamber doesn't really apply to our purposes, since we're going to be firing solid projectiles. I can't find any literature that claims either chamber type was more accurate than the other.

Douglas; are you planning on giving your mortar a Gomer chamber just for historical accuracy?

From a machinist's viewpoint: Would it be a much more difficult task to fashion a Gomer chamber?

[GGaskill]

From a machinist's viewpoint: Would it be a much more difficult task to fashion a Gomer chamber?

The conical part would not be too difficult but on a manual machine, the segments of circles between the bore and chamber, and the conical walls and the rear face would be a problem.

[Double D]

After some recent shooting test I have decide to stay with cylinder and not a Gomer.  Two reasons.  One, 300 grains of Cannon grade powder tosses a 6 lb zinc ball just under 385 yards with the current chamber shape. Second of the 4 rounds I have fired that far, I have only found one.  We can see them hit in the stubble, but we can't find them.  South paw and I think the balls are burying themselves in the soft dirt of the stubble field.

[Cannoneer]

From a machinist's viewpoint: Would it be a much more difficult task to fashion a Gomer chamber?

The conical part would not be too difficult but on a manual machine, the segments of circles between the bore and chamber, and the conical walls and the rear face would be a problem.

GG, I'm not sure if I understand your answer correctly. I get the fact that the whole process of producing a conical chamber would be a lot easier with a CNC lathe, but I don't know what you mean by "the segments of circles between the bore and chamber" posing a problem. I take it that you're speaking about the concavity that's cut from the side walls of the bore down to the opening of the chamber, so that the ball is more stably seated above the chamber, but Dominic is already making some of his mortars with that feature; are you saying that it would be a more difficult propositon to do this on a mortar machined from solid stock as opposed to a mortar made with a breech plug?
"........, and the conical walls and the rear face would be a problem." I thought forming the hemispherical bottom of the Gomer chamber might be achieved by using a regular driil bit of the proper diameter to match the narrowest part of the cone where the transition was made from the chamber wall to the hemispherical shape that formed the bottom of the powder chamber.

[dan610324]

I think he mean the radius between the conical chamber and the bottom radius of the bore

[Cannoneer]

That's what it seems like he meant to me, Dan, we'll have to wait and see what he has to say about it.

After some recent shooting test I have decide to stay with cylinder and not a Gomer.  Two reasons.  One, 300 grains of Cannon grade powder tosses a 6 lb zinc ball just under 385 yards with the current chamber shape. Second of the 4 rounds I have fired that far, I have only found one.  We can see them hit in the stubble, but we can't find them.  South paw and I think the balls are burying themselves in the soft dirt of the stubble field.

DD, I'm probably also going to go with a cylindrical chamber. I don't really see any strong practical advantages that would make one of these two designs undeniably superior over the other. Another minus on the Franklin sheet concerning the conical chamber would be (at least in my case) the added cost that the maker would probably add to the bill, because of the increased difficulty in manufacturing it. To tell  the truth, one of the main reasons that I originally wanted a Gomer chamber was because it is historically correct: Hey, Charlie, looky here, this is called a Gomer chamber, and that's just the way the originals were shaped.   As a matter of fact, I would still like to have the danged Gomer chamber!