Li'l Brudder (¼-scale Coehorn)

[Terry C.]

Okay, here's the deal.

I'd decided that I wasn't going to post about any new projects until I had made real and verifiable progress on something.

Lately I've fallen into a habit of getting all worked up with ideas and drawings of new projects that just never seem to get off the ground. The Marsilly carriage was the latest but every time I thought I would get started, the weather turned bad (I have to do my woodworking outside). Then I got busy with other things. It will get done, one day.

So I've kept my mouth shut about this project until now. Even though it isn't finished, I think I've reached the point where it's time to break the silence.

This is Li'l Brudder, a smaller version of my GB Coehorn mortar. It still needs a trunnion and a vent, but the tube itself is finished.








Externally, it's based on ¼-scale 24-pounder dimensions. But internally, it's a ¼-scale 12-pounder.

It's designed to shoot the same balls I cast for my ¼-scale 12-pounder 1857 Napoleon. This makes it underbored, which adds strength and weight. Having the external dimensions of a 24-pounder Coehorn will make it a nice companion to the Napoleon.

The stock is the same type of chrome-plated rod that I used for the GB mortar, just a smaller diameter.

The bore ended up about .006" over my spec of 1.156" because, well, stuff happens when you're in a hurry. That means that the fit of the ball is a little looser than the textbook 39/40, but not much.

The bore depth is two calibers and the chamber is 9/16" dia. x 3/4"deep with a hemispherical bottom. This is smaller than the chamber was originally when the drawing was scaled down, I reduced the chamber size based on my experience with the GB version. I filled it with FFFg and it held 42.5 grains. I hope this wasn't a mistake.

I'll be building a Union-style bed base for this mortar.

[lance]

 I like it Terry C!!! That's going to be a nice mortar for your growing collection. if you have to do your woodworking outside, you'd better hurry, winters coming.......OH, that's right, you live way down south. guess i need to be the one dreading winter in these Virginia mountains. No wait!!!, you said i could come down and borrow your Harley, or something like that

[Victor3]

 I could be wrong of course, but isn't the powder chamber a little small for a lead ball of that size considering the windage? I suppose a charge of FFFG might make it pop fairly well though.

[Terry C.]

I could be wrong of course, but isn't the powder chamber a little small for a lead ball of that size considering the windage? I suppose a charge of FFFG might make it pop fairly well though.

The problem that I was having is that the chamber in the GB version is a little on the large size. After scaling down the drawing, I reduced the chamber proportions. It looked fine on-screen, but after it was done, it did look a little undersized.

It's very hard to tell these things with a drawing, you need to have it in your hand. Hence my comment "I hope I didn't make a mistake." The original ¼-scale drawing had a chamber of 3/4" x 7/8" and I'm beginning to think I should have left it this size.

Up until the trunnion is fitted, I still have the option of boring out the chamber. I didn't want to drill the vent until after the chamber is done, because if I go any deeper then the vent will be in the wrong place. That's the main reason it isn't drilled already.

[Evil Dog]

I used to fire lead ball out of my golfball coehorn all the time... seldom went above 90 grains FFg.  Doesn't take all that large of a powder chamber for that.

[Terry C.]

That's where my lack of mortar experience comes into play. Bear with me, please.

E.D., does it take more or less powder to propel a lead ball in comparison to golfballs? What range were you getting?

Yes, the windage is over spec. The lead balls usually come out a little undersized as well, there's a significant shrinkage factor. The balls are usually 1.118" - 1.120" instead of the the 1.125" mold size. I never worried about this in the cannon because of the long bore that is dead to spec, and it's never been a problem.

With the oversized bore and my undersized ball, total windage of the little mortar measured at .042". Calculated windage for a 1.125" ball would be .029".

It was either live with it or scrap the project. Well, I wasn't scrapping it, but then again I don't want a "display" mortar either.

I would like for the little mortar to have a max range of about 100 yards, and be reasonably consistent at 50 yards. Is this too much to expect?

Should I increase the chamber capacity to compensate for what will probably be significant blowby, or is it big enough to get the job done as-is?

[Double D]

Terry,

Shoot the Mortar first before you decide there is a problem.  If you come up with a problem address the problem.

I would think the 100 yards would be an extreme for this small a mortar.

[Terry C.]

I've been looking at the small fuse that I have. I may be able to flatten the fuse enough to get it to pass between the ball and bore, allowing me to test fire the mortar without drilling the vent first.

If I need to go deeper, I want to do that before I drill the vent.

What about 75 yards, is this more realistic? I would like some idea what I can reasonably expect before I start making changes.

I don't want to try to fix something that ain't broke.

[Double D]

Maximum load for a 9/16 bore is about 60 grains.  Get your powder scale out, yes scale and weigh out 60 grs of GOEX Fg by weight not FFFg.  Now drill a 9/16 inch hole in a block of wood deep enough for the 60 grs. ofFg  to fill that hole to the top, level.  Measure the depth of that hole and drill the 9/16 powder chamber in the mortar to that depth.  There you have it. 

Fill that powder chamber with Fg and you have a max charge of 60 grains. That will give you your max range with Fg.  Need a little more range try a different brand of Fg or FFg or even FFFG.  Just keep in mind that your charge with a differnt brand  or grade may no longer be 60  grains, and most likly will be more.

[lance]

 Terry C, i don't know if this will help you? you know those little mortars i make in .58 cal, anyway, 25 grains fffg will shoot a .577 460g Enfield bullet to the tree line 200yds away. i can hear it smack the trees, with more powder {they hold more} its a pretty good delay before ya get to hear the bullet smack. light weight .570 round balls are gone, but if it's quiet outside i can hear them smack way back in the woods. I don't know the weight of what you will be shooting,but maybe with my numbers, you can figure something out. hope this helps.

[Terry C.]

Well, I had a lengthy post typed, went to submit it and it went somewhere. Not here, apparently, but somewhere.

It's late and I'm too tired to do it all over again, so I'll just post the photo and get back to the details after I've had some sleep.

[lance]

 Hey, that's coming along great!!!! ya know, if you get it all done before Dec 1st, you can play MORTAR BINGO with it

[GGaskill]

Regarding range with lead shot, I have a bronze "half" scale Coehorn that is bored 2 5/32" for 2 lb rock cod sinkers.  It will throw them 600 yards with about 200 grains of Fg. 

My limited experience comparing golf balls and lead shot is that the low weight of the golf balls prevents building much pressure in the barrel.  The high density of lead shot gives them a higher ballistic coefficient, therefore longer range for the same initial velocity.  We really need to conduct some experiments and see what actually happens.

[Terry C.]

Lance, the balls weigh 4¾ ounces cast from wheelweights, or 5 ounces cast from pure lead. I use the wheelweights, which are cheaper and easier to get in bulk, and save my pure lead for rifle balls.

DD, your idea sounds good, except that the drill I used to cut the chamber is a foot long and has a Morse taper shank. Not gonna fit in my 3/8" Black & Decker.

Instead, I used this calculator:

VOLUME OF CYLINDER WITH HEMISPHERE

Calculating the volume in CCs and weighing powder with my Lee Dippers, I concluded that I could not drill the chamber deep enough to get the full 60 grains of Fg at 9/16" diameter without drilling considerably deeper. Too deep to maintain my minimum one caliber between the end of the chamber and the cutout for the trunnion. It would have been close, but close doesn't cut it.

I had originally drawn this mortar with a 5/8" diameter chamber, and already had a drill ground before I changed the plan. I saw that I could open up the chamber to 5/8" diameter and get a capacity of 60 grains by drilling only 1/16" deeper. This left me with my minimum thickness and some to spare.

So that's what I did. The chamber came out beautifully (near pefect hemisphere at bottom), and it holds exactly 60 grains of Fg. Max charge for a 5/8" bore is about 70 grains.

Is 60 grains "right?" I don't know, but I'm more comfortable with the 60 grain chamber than I was with the 40 grain chamber. I looks right and feels right.

And even if it's still too small, this is as big as I'm going. At this point the mortar is still fairly well overbuilt and I want to keep it that way. But I have a feeling that I won't need any more capacity.

Now it's time for bed, uh, I mean it's time to make the bed. The mortar bed, it's time to make the ... Oh, nevermind!

I gotta start gettin'  more sleep.

[lance]

Regarding range with lead shot, I have a bronze "half" scale Coehorn that is bored 2 5/32" for 2 lb rock cod sinkers.  It will throw them 600 yards with about 200 grains of Fg. 

My limited experience comparing golf balls and lead shot is that the low weight of the golf balls prevents building much pressure in the barrel.  The high density of lead shot gives them a higher ballistic coefficient, therefore longer range for the same initial velocity.  We really need to conduct some experiments and see what actually happens.

GG, i'll help with experiments, what's on your mind about how to do it?

[Double D]

Terry,

Yeah, I see the problem with the drill---I have been known to just chuck a piece wood up as best i can in the three jaw and drill...I got my machine tool training in college 20 years ago and have been on my own ever since.  But to qualify my credentials. The College shop was situated under two large oak tree---you know shade trees---so my techniques are immpecable.  You  might have noticed I seldom discuss machining techniques. I too am soaking up the knowledge shared here.

When you get to the range you will find out if you did things right...I think you did.

CW you might want to add that calculator to your reference page.  There is a link at the bottom that also leads to other conversions

[GGaskill]

GG, I'll help with experiments, what's on your mind about how to do it?

Nothing complicated.  You will need a supply of golf balls and 1 lb lead shot, and a mortar bored to fit.  The basic idea is to just record the interior details of the mortar (exact bore size, chamber dimensions, etc.) and of the shot (diameter and weight) and then, starting with small powder charges, fire a golf ball and a lead shot with the same charge and measure the distance, recording both charge weight and shot range for each type.  Multiples of each are even better for statistical validity.

Once all the firing is complete, report it on a new thread here.  Anyone who can keep good records is welcome to post their results in the thread, but be sure to fire BOTH golf balls and lead shot from the same mortar.

[lance]

GG, I'll help with experiments, what's on your mind about how to do it?

Nothing complicated.  You will need a supply of golf balls and 1 lb lead shot, and a mortar bored to fit.  The basic idea is to just record the interior details of the mortar (exact bore size, chamber dimensions, etc.) and of the shot (diameter and weight) and then, starting with small powder charges, fire a golf ball and a lead shot with the same charge and measure the distance, recording both charge weight and shot range for each type.  Multiples of each are even better for statistical validity.

Once all the firing is complete, report it on a new thread here.  Anyone who can keep good records is welcome to post their results in the thread, but be sure to fire BOTH golf balls and lead shot from the same mortar.

GG, i can set up a range, but there aught to be some cut off distance.  how far are you planning to test? 100yds, 2, 3 ?

[Artilleryman]

I look forward to the results of you experiment.  I could find nothing in my range tables that would give me definitive information for ranges using different weight projectiles.  Shells from guns were fired with lighter loads than those used for solid shot. 

Something else to think about.  It seems that when we elevate our siege mortar to an angle greater than 45 degrees that we sometimes get a greater range with the same charge.  Theoretically this should not happen.  The greatest range should be at 45 degrees.

Suggestion on picking a range.  A shorter range may give you a more reliable result by limiting the variable of wind that may affect the outcome at a longer range.

[GGaskill]

GG, i can set up a range, but there ought to be some cut off distance.  how far are you planning to test? 100yds, 2, 3 ?

I would say that you should figure out how much is a full chamber of the powder you normally use in the mortar, then use charges of 25%, 50%, 75% and 100%.  Or go in finer graduations if you like.  Use the same elevation for every shot.  What we are trying to establish is what range do we get with the differing shot for each powder charge.

Theoretically this should not happen.  The greatest range should be at 45 degrees.

45° is only the theoretical maximum range elevation if you disregard air resistance (doing so makes for easy calculus or algebra problems that plot out a parabola but since the air resistance force is many times {80 or so for high veloctiy bullets} the force of gravity, ignoring it leaves the simple equations poor approximations of reality.) 

[lance]

 GG, i'll have to pass on this experiment. the way the fields are, with tree lines and hills, 300yds is the most i can shoot and find something to measure. I don't think i could walk a straight line it the woods with a Rolatape, going around trees would add up to the measurement. i've shot tons of stuff way back in the woods, never knew how far either, never could find any.you've been shooting 600yds, so i guess trees are not a problem.so i'll have to sit this one out and watch what you other guys do........TERRY C, sorry to drift on your topic, how's things coming with the mortar bed?

[Terry C.]

Just to let everyone know that I haven't abandoned this project, just been busy with other stuff.

I did get some rough (very) inletting done on the wood for the bed this past weekend. It's the same oak I used for the GB mortar sled, and is a little over an inch thick. The four blocks total about 4⅛". My desired width is 3¾", so some wood will be need to be removed from the sides after the pieces are laminated to bring the dimensions to my specs.

There's a slight gap at each end of the trunnion (the slot is too wide, another result of the pieces being thicker than the ones in the plans) so I may also shave a little off the inside of the outer two blocks before gluing them up.

The two inner blocks will be left full thickness.






Next up will be the drilling for the tiebolts.

[Terry C.]

Li'l Brudder is comin' togedder...

Okay, that was bad.


I've got the bed glued up, cross-drilled, and trimmed to its final dimensions. The corner beading turned out pretty good I think, given my methods. I printed out the left and right profile on label paper and applied these to the sides. I used the table saw to remove as much excess wood as possible, then shaped the radius with a blender pad on an angle grinder.

It's not perfect, but after it's finish sanded and oiled I think it will look passable.

I was going to buy a bit and use a friend's router table, but I couldn't find a bit this size (¾" radius) that would fit a standard ¼" router collet.

The inletting still needs a tweak or two (the trunnion slot needs to be a hair deeper), and the entire bed needs a good finish sanding.

I need to get started on the hardware. I won't drill for the trunnion straps until I have them made.

[lance]

 looking good Terry C, you know i like woodworking and the base looks great!!!!

[Terry C.]

Thanks.

I cut the stainless steel ¼-20 threaded rod for the tiebolts over the weekend. I picked up the stainless steel acorn nuts and washers after work yesterday, from the industrial bolt supplier we use at work. So it has rods in the holes now.

[lance]

 Terry C, I would like to know how far it will shoot when you get it all done. i've been doing some figuring with a pencil and want to see if i figured close enough. do you plan to range test it? i know the balls will be hard to find, just wondering if paint might help?

[Terry C.]

I haven't quite figured out all the details to the 'range test' yet.

Yes, the balls will definitely be painted. I also considered adding streamers, if that's possible, to help track the balls in flight.

I fired a few 5 oz. cannonball sinkers, tearing out the eyelets first, in the Napoleon before I got my ball mold. In the short mortar I could (theoretically) turn the eyelet toward the muzzle and attach a streamer. But I'm afraid any material light enough not to impede flight would get cooked by the fireball.

I'll most likely just paint the balls for visibility and start with a low charge, adding powder until I can't find the point of impact anymore. If I can get several shots in maybe I can extrapolate the approximate max range from my known charge/range samples.



Or I could do my range test in a large parking lot and find the POI by listening for car alarms.

[lance]

 I like the streamer ideal, that will give me something to experiment on.  i use the different tree lines to help with small projectiles that i can't see, but can hear hit. i can only do 200 to 300 yds that way though. i'll add enough powder to hit the trees, then back off till i don't hear wood getting a smack. crude i know, but kinda hard to find or see the little ones. i thought about using the neighbor's pond, to see the impact, but don't want to add lead to the water.

[Cat Whisperer]

I've done yellow CRIME SCENE (or CAUTION) tape streamers on the 4.5" before (about 3' long).

Most get ripped off.  Those that stay don't really help visiblity that much.

But, what about something that whistles?

[GGaskill]

Maybe try Christmas ribbon as a streamer.