Question about GB mortar

[wvdad]

I am kinda confused about the proper bore for the GB mortar. I want to use the same design I have used for my 1 inch mortars. Kinda a hybrid between a cannon and mortar. The bore will be about 5.5 inches deep and I dont want to use a powder chamber so I can vary my charges. I always package the powder in foil and pack it tightly in the back of the bore and usually pack a partial paper towel as wadding. Would this be considered patching, which I know is a no-no with that big of a bore.

[Double D]

Why not use a chamber? You don't need as much powder for a mortar as the charge is really a lofting charge as much as it ia propelling charge. 

You can use a package charge for a mortar, but loose powder is acceptable. And you certainly can very your charge in a chambered mortar.  They did it in the real thing to adjust range.

If you make a foil cartridge as described in the sticky in safe loads and cannon plans you have no need to add wadding.  Wadding, escpecially a paper towl just become a flaming projectile headed down range.

A golf ball gun with the rule of thumb windage would need a minimum bore of 1.723 inch. The diameter of a golf ball is 1.68 inch. .  (1.68/39)x40=1.723.  You will also find some good idea for mortars in that safe looads and cannon plans sticky.

Patching makes it difficult to seat the ball properly and can raise pressures  in these larger bores.

[RocklockI]

as far as bore dia. goes i'd say 1.750 " as it gives alot of windage . i dont own a gb mortar yet but i am a firm believer in chambers they are great . more so if you want to see the ball .....ymmv

lots of bang for the buck .

[wvdad]

 Well I notice alot of people use the chamber so they can go with alot thinner walls, which I understand. But I will be using 3.250 solid stock.No welds or bolts. My thinking is if the powder chamber will hold 200 grains for instance, if I pack in 100 there will be a headspace between the powder and ball. I realize this isnt a muzzleloader but any headspace between the two will rob it of pressure. On the flipside if I decide to put in 250 grain into a 200 grain chamber then I would have a mushroom shaped charge and I dont know how that would change the effect. Thats why I like the idea of a solid bore with 30 degree angle at the back.

[Double D]

Headspace is something that you deal with in cartridge firearms not muzzle loaders. It has to do with how brass cartridge cases fit to seal the chamber.

It does not matter in the mortar if you have airspace, you do not have a tight fitting projectile.  The windage between bore and ball will reduce the pressure

People are not using chambers so they can use thinner material, on the contrary.  They are using chambers because they don't need the greater capacity. 

If you make a golf ball bore size mortar with no chamber you need a barrel that is 5.169 inches in diameter to meet recommend safety limits. Walls need to be the same thickens as the bore.  3 x 1.723.   

The Maximum recommended safe load for a 2 inch bore is 2 oz per inch of bore. Switlik's chart for small cannon stops at 1.5 inches and 500 grains.  If you extend the track of Switliks charts you will find it come out to just at 2 ozs. or 875 grains at 2 inches.   Using the 2 ozs. per inch of bore rule the maximum safe recommend load for the 1.723 inch golf ball bore is 754 grains of Cannon grade powder.  If you load 754 grains of Cannon grade in a golf mortar and fire it, you will never see the golf ball leave the bore and you will never see the golf ball again.

This might be fine for flat trajectory guns like cannons, but in mortars it's a waste of powder.

If you really think you need a mortar without a chamber, what I suggest you do is get yourself a 4 in x 4 in post.  Drill a 1 3/4 inch hole in the end until the hole is deep enough to hold the max load of 754 grains of Cannon grade powder.  Now drill the hole 3 1/2 in deeper.  That is how deep your bore needs to be.   Now add another 1.7 for safe back wall thickness at the back and cut the post off and this is the length of your barrel.

The largest bore you can use without a chamber with your 3.250 stock is 1.083. Maximum safe load for 1.083 is 210 grains of Cannon grade.

200 grains is a lot of powder in golf ball mortar and probably far more than you need and after one or two shot far more than you will probably shoot again.  Read some of the post in the safe loads and cannon plans sticky at the top

In my golf ball mortar 94 grains of FG powder will send a golf ball just beyond 200 yards.  Let me qualify that.  The 200 yards was a measured 200 yards.  The golf balls were found on the ground just beyond the 200 yard mark; 30 or 40 estimated yards beyond the measured 200 yards. When fired only occasionally could the ball be seen in flight.   

To make the golf ball go 30 yards all I needed was 23 grs. FG.  To go 100 yards I needed 40 grains.

I suggest you use a one inch chamber.  The one inch chamber has a maximum load of 175 grains.  Using a block of wood drill a hole deep enough to hold the max charge.  Measure the depth of the hole and add 3.446 for the two diameter rule for the ball and you have you bore depth.

You can trust use here on the board when we say 175 grains of Cannon grade or for that matter FG is a lot of powder for a golf ball.   

[wvdad]

 I know the headspace in a chambered round will screw things up but even in a muzzleloader the ball must be seated all the way back against the powder to achieve the correct velocity. And thank you for pointing out the wall thickness rule. I guess I misunderstood the rule. I took it as the total outside diameter had to be 2x the bore. 1 inch bore meant 2 inch outside diameter. I also thought the powder chamber would elimate that particular rule because alot of the mortars (golfball and bowling ball) I see have a really thick, heavy chamber but then extend into pipe that comes nowhere near the thickness that rule would mandate. I will definitely use the 1 inch chamber you recommended. Thanks for the information. I am trying to learn all i can and will always err on the safety side of things.

[Cat Whisperer]

Powder chamber centers the charge.  Without one, the charge (with the tube at 45 degrees elevation) lies in the bottom corner.  Both work.

I've made a bunch of mortars at 1.75" and some at 1.72 or so.  Both work.  Tighter gives greater distance with less powder, as does longer barrel.  Shorter gives sharper bang.  Loooonnnnger gives the classic 'thunk', great range and very little smoke (more complete combustion inside the tube).

1 45acp case of 3F in a two caliber long tube will get you 100 yards or more.  Two will get upwards of 200 yards.

I have a number of them made of 4140.  4" diameter.  28 Lbs.  Cute on steroids.

[Double D]

   And thank you for pointing out the wall thickness rule. I guess I misunderstood the rule. I took it as the total outside diameter had to be 2x the bore. 1 inch bore meant 2 inch outside diameter. I also thought the powder chamber would elimate that particular rule because alot of the mortars (golfball and bowling ball) I see have a really thick, heavy chamber but then extend into pipe that comes nowhere near the thickness that rule would mandate.

Don't forget that the one wall thickness rule applies to the breech also, the wall thickness at the back of the chamber must also be the same thickness as the diameter as the chamber.

[Rickk]

If you do not go with a powder chamber and decide to try some short range shooting with lighter loads, the charges will be wide and short sitting way down there. Ignition will not be as consistant as it would be with more of a "square" length x width charge and your accuracy will suffer.

In my chamberless 2.25 inch mortar, ignition is noticably inconsistant below about 110 grains of cannon powder. The aluminum foil charges are only about 1/2 inch long. It is hard to even seat them flat... I wind up taping them to the ball like a sabot to get them to load straight. The gun wants about 100 grains when shooting at 100 yards at a 45 degree angle, so I wish it had a chamber down there.

Of couse, if I decide the people in the next state 10 miles away are a problem to be dealt with, I am probably all set