mortar chambers

[dominick]

 An individual who wants me to build a 2.75" bore competition mortar, asked this question.  Does anyone have input on this matter,  Thanks..Dom

"The powder chamber design should be in keeping with the latest consensus of those who are using their mortars for competition, and large enough to get max distance possible with an assortment of projectiles."

[Rickk]

I have a 2.25 inch mortar. It has no sub-chamber.  Firing blanks to make noise with 4 ounces of 1F is a lot of fun, but for competition shooting it would probably not do well.

Firing a 2# solid lead ball at ranges as short as 100 yards require charges in the order of 150-160 grains of Cannon grade powder. Without a subchamber, loads that small are a bit tricky. I am working out a technique that makes 90-100 yard shooting doable, but with a sub-chamber I am pretty sure that the accuracy would improve substantially.

I assume your intended projectile would be concrete (about 18 ounces) or ice-filled (12 ounces) beer cans. They will be quite a bit lighter than my 2# balls, and go alot further, so your powder charges will probably be even less than what I am using.

So, while I'm not sure what size sub-chamber you will need, you will need a sub-chamber. My next mortar will have one for sure, so I will be following this post to see what suggestions get posted.

[GGaskill]

My bronze 2 pounder (lead) Coehorn, which is only 2 5/32" diameter by 4 7/8" length bore, has a 1" by 1" chamber and will throw a 2 lb. lead rock cod sinker 600 yds. on a full chamber. 

Since 2.75" diameter sounds a lot like beer cans, the charge limitation will be the strength of the concrete, if that is what is used as projectiles.  Ordinary concrete breaks up in the bore when I load a hot charge.  If he will be using metallic shot, then the material strength of the mortar and its design will be the limiting factor. 

"Max distance" is kind of an absurd requirement as one can make the mortar and projectiles strong enough to shoot them out of sight.  Most competitions for small caliber mortars are relatively short range.

[Cat Whisperer]

...
"Max distance" is kind of an absurd requirement as one can make the mortar and projectiles strong enough to shoot them out of sight.  Most competitions for small caliber mortars are relatively short range.

Agreed.  Unless you have a LOOOOOONG range and plan on using a forward observer. 

[Tropico]

I second GGaskill . I have a Bronze Beer Can Mortar ., built by Powder Keg .

This last July we fired Ice and Concrete ..,varying charges.
The  sub-chamber is 1" diameter I believe ? And 1.5 depth.

A full charge dissenigrates a beer can full of ice., a tremendous boom and a Whooof ., a large smoke ring and small pieces of aluminum floating down.

At half a chamber load and concrete in the can.,we managed a louder boom a bit less smoke and a projo that stayed "Intact"

On the third test (we believe we found our sweet spot here) we used about 3/4 of a chamber load...,a can of concrete ., recieved a great report from the muzzle .., plenty of smoke .., the ringing sustain of bronze for a bout 4 seconds..,and then 4 seconds later (8 seconds of air time) we could hear the sound of branches breaking as our projo was returning to earth.

I love my mortar.

[Cannonmaker]

In the 24pd Coehorn which I have built the powder chaimber was slighty too small, what would not fit into powder chaimber will make a dome.  The dome of powder fits nicley in the bottom of the soda can.  Shooting 360 gr of ffg Goex, the Ice stays intact, will travel 750' to 900'. have shot a pop can of ice up to three times.

Rick

[Double D]

Here is an excert from Gibbons on Chambers:?

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 .-- 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.

With pieces which are fired nearly horizontally, and loaded with a rammer, the cylindrical
chamber is the best, and being connected with the bore by a conical surface, the charge
slides up into the chamber without any difficulty. In the 8 -in. siege howitzer, which is short
enough to be loaded by hand, this connection is not necessary, and it is made spherical for
the purpose of using the shell without a sabot. The advantages of which will be mentioned
hereafter.

For small charges a chamber is always advantageous, no matter what the length of the piece;
but the advantages are more marked the shorter the piece is made. In proportion as the piece
is lengthened and the charge is increased, the influence of the chamber becomes less
marked, until with a bore of 10 or 12 calibres long and a charge 1 -7th the weight of the
projectile, it ceases to be appreciable. In guns where the charges are large a chamber would
diminish the velocity by lengthening and conning the charge, thus retarding its
inflammation.

In all our guns and howitzers, the bottom of the bore is a plane surface perpendicular to the
axis of the piece, and joined on to the sides of the bore by an annular surface whose
cross-section is part of the circumference of a circle described with a radius of 1 -4th the
diameter of the bore or chamber at that point. This construction, which facilitates the boring
and cleaning of the gun, adds somewhat to the solidity at the breech.

[dominick]

Thanks for the info everyone..Dom