Help with GB mortar design...

[MikeR C]

I would like to enter the "Walk the walk" mortar shoot, but it will mean putting together a Golf Ball Mortar in order to do so.

So this is my quandary, I have gone back through all of the back pages of posts, all 68 or so, and looked through the reference sections at the beginning of the forum. I pretty much read everything that seemed to be about GB mortars, including the posts on the last mortar contest.
What I would like to build is a GB mortar of 2 to 2 1/2 bore diameter length specifically to shoot golf balls, possibly blank loads, and no solid balls. I am looking for a maximum range of aprox 100 yds and obviously to ability to accurately shoot to 30 yards for the contest. I would use 35 to 45 degrees of elevation for the barrel.

What I found was mortars with bore diameters ranging from about 1.68 to 1.75 bore diameter, powder chambers ranging from bore diameter down to 1/2" by 1", powder charges ranging from 10 grs. to 100s of grs., and max distances ranging from a couple hundred yards to Earth orbit. I am afraid I don't have the time to put together several test designs before the contest deadline.

The following is an excerpt from this post:

http://www.gboreloaded.com/forums/index.php/topic,97389.0.html

It is posted by Double D:

"Reason to fire a golf ball mortar.
1. Smoke...yep it will do that (I like smoke!)
2. Noise...yep it will do that although not real load (I don't need a real loud noise to deafen spectators)
3. To watch the ball fly, yep you can do that. (I like that)
4. To watch the ball the ball land (Or to see how close you can get to a target}. (I don't see too well, so short ranges are good)"

"In this case don't think max charge think how far do you want to shoot.
With a 1/2 chamber you can accurately lob a golf ball 30 yards with 24 Grs of FG. Start with that load and work up one grain at a time and see how far the ball will go for specific charge until you determine the maximum range you want to lob the golf ball. You don't need a chamber much longer the 3/4 inch for launch golf balls. Anything longer is just unneed and a waste of powder if used."

This sounds like exactly what I am looking for. The last contest entrys remarked on the fact that it takes a very small amount of powder to go 100yds, and the smaller charges get erratic trying to keep the range to 30yds, If everyone pretty much agrees with Double D's assessment (no offense intended Double D) then I will use his chamber size and just go away. If not I would appreciate very much to hear from anyone who has a GB mortar. I would like to know the bore diameter and length, powder chamber size, powder charge, and some kind  of idea what distances you shoot and what changes you would make to what you have. I am hoping with this information I can figure out a configuration that will fit my requirements for the contest and some fun shooting.
Thank you very much for your consideration,

MikeR C
PS I am assuming at some point Double D will look at this post, (and if he doesn't take vast offense to my impugning his GB mortar designing skills and deletes my account and has my mouse confiscated), I would like some clarification on the "Walk the Walk" rules. Are we allowed to use sabots on the golf balls, this would help with bouncing after impact, failing that is it cheating to set the pin in a plowed field?

[Cat Whisperer]

DD's was a good summary assessment.

One of the first one's DD & I shot a few years back was my hand-held 4140 steel round/octagon 1.70 bore with about 5/8 round/deep powder chamber.  Total unknown until after we lit the fuse.  Used a 45 acp case or two filled with fffg.  Range was about 100 to just under 200 yards with those two loads.  Being hand-held it was ABOUT 45 degree elevation.  1.70 is a tight fit.  That MAY be good for consistency if it is cleaned between shots.  When we shoot the next series, I will have several mortars there to make comparisons with - to determine what works best empirically.

I have thought of firing one at 45 and another at 75 degrees elevation - the variables would be different - one would have more air time and susceptible to the wind where the other would tend to roll the golf balls more (except for the one's hitting the meadow muffins.  Perhaps putting the stake in a sand-trap would be better?

Another thing I'll do with this one is to record the landing position of each round fired (much easier to do at 30 yards than at 100) to determine if sequence of firing has any notable effect on POI.

Good questions!  It's much better to  have done the research BEFORE cutting chips!

[MikeR C]

So, if I understand you correctly the bore should be a bit bigger than 1.700, IF I don't want to have to clean it after every shot. I have seen references to 1.720 to 1.730, with 1.75 being a little large.
I just measured a fired .45 Auto case and it is aprox .45 (surprise!) by about .710 deep and appears to hold about 26 grs of FFFg. According to your tests that would send the ball about 100 yds. (I realize the cartridge case is tapered inside and you have very little windage)
So why not a chamber .5 dia by .75 deep, which would hold about 30 grs and I'd be set. What do you think about narrower but longer sub chamber, say 3/8 by 1? Any guesses on how it would behave? (besides fouling problems)  I'm trying to avoid too small a charge. Maybe the ticket would be a 1/2 by 3/4 chamber but use FFg.
Higher elevation might be a good compromise, 1/2 by 3/4 chamber with say half charge of 13 grs FFFg and then just raise the elevation to bring the impact point closer. On a still day in a plowed field the ball should hit and stick. The only problem I see is the tendency for the balls to hook, the farther it would travel the worse the hook would be.
Let me know what you think.

"Good questions!  It's much better to  have done the research BEFORE cutting chips!"
I respectfull disagree, I don't really mind wasting the stock, It's much better to have done the research BEFORE lighting the fuse!

Thank you,
MikeR C

[Double D]

MikeR C.

First let me say, there ain't no right way or wrong way...okay might be some wrong ways. There just isn't one best way.

For a first time GB mortar, just copy what some one else has done. Take a look at the four plans posted under Safe Loads and Cannon plans

Learn to shot the mortar first.   Either of the designs by Cu Cannon or GGaskill will make a nice first time Mortar.  Then you can move on to more advanced techniques.

We are applying some basic rules of thumb for the building of these little cannons and mortar.  Breech wall thickness should be one caliber.  Length of projectile bore(not including powder chamber) for mortar should be 2 to 2 1/2 calibers. windage  should be 1/40th Bore diameter. 

A golf ball gun with proper 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
http://www.learnaboutgolf.com/beginner/balltypes.html

Rules of thumb: "A rule of thumb is a principle with broad application that is not intended to be strictly accurate or reliable for every situation. It is an easily learned and easily applied procedure for approximately calculating or recalling some value, or for making some determination.  http://en.wikipedia.org/wiki/Rule_of_thumb.

If you look at my plans you will see that my bore is even bigger.  Read my description and you will see why I did it.

Now to the powder chamber dimensions.  Keep in mind that original mortars adjusted range by increasing and decreasing the powder charge. If you are going to  fire your mortar you might want the ability to adjust your load also.  I think I would be more inclined to make a large dimeter powder chamber than a smaller deeper.

I can't remember why I choose the powder charge I did. It may have come form comments made by others. The was a reason. I do remember that the targeted load was 24 to 25 grains.  I used my adjustable powder measure to take my measurements from. I adjusted the measure for 25 grains then measure the diameter and depth with a ruler.

By the way I used Cannon grade not FFG.  But this little GB mortar might be just teh tool to test and see if you canget extra range justby changing granual size.,

I certiainly wouldn't fire my match in a parking lot.  Fire your match in any safe place you want.

I hope this makes sense.  I just got off the plane after spending 24 hours traveling from RSA to USA and my speech and typing may be slurred.

[Cat Whisperer]

...
I just got off the plane after spending 24 hours traveling from RSA to USA and my speech and typing may be slurred.

I can't speak for your speech being slurred, but you typing is better than average!

Build it dude!  Launch a few.

A 1/2 x 3/4 deep will contain the powder better at an angle than one that is 3/4 dia x 1/2" deep but a smaller hole to get the powder into.  The narrower the powder chamber the more focused the pressure on the projo - IF that makes a difference.

Mine would occasionally hook or slice.  Less so on the one's with more clearance - but that's just an impression, not an emperical measurement.

[lance]

hey MikeRC, different powders and sizes of powder will throw the ball different lenghts. yesterday while playing, 15 grains of fffg goex was too much for 30yds. while 30 grains of ffg Elephant was not enough. hey i got to do something to use up that Elephant powder. i know what i'll do, pour it in goex cans and give it to Tim.

[m223]

Good advice by all Mike, just build it. There is no shame in giving it a go, I have doubts that I'll do to well as my powder chamber is a touch on the large size. Who knows, you can always build another one. You may want to start with the smaller powder chamber see how it performs, you can always drill it out larger.  Tracy

[MikeR C]

Well, I have every intention of building one. Half the fun of putting one together, or most anything else for that matter, is doing the research beforehand. I want to figure out what kind of performance I will get , and whether it will perform to the requirements that I have determined before I start on it.

CW,
Thanks for reminding me about the focused pressure, read about that in the article posted about sub chamber shapes. They had problems with the hollow shells being punctured by the blast out of the chamber.

Lance,
Now you tell me! My FFFg is Gearhart-Owens, better than 30 years old. I will just get my load figured out and then have to switch to that "newfangled GOEX stuff .

M223,
I'm seriously thinking about making a test mortar with interchangeable chambers so I can test a couple of things. An offset sub chamber to force the ball to spin was brought up once and sounds intriguing...

Thank you all for your responses, this is a very informative and well run Forum. The moderators are to be congratulated.

MikeR C

[m223]

Your right Mike, This forum is wonderful, and the people are great. I wish I would of found this place before I made my first mortar, even though it turned out to be a real shooter it would of been nice to research it a bit first. I have thought about making a reduced size powder chamber that I could sleeve my existing mortar with. I'm thinking if you drilled and tapped a couple of holes in the face of the sleeve you could make a puller to remove it for cleaning, which would have to be done after it was used or corrosion would set in pretty quickly. Your test mortar sounds like a neat idea. If you can post pictures, Please do. Have fun!, be safe and good luck, Tracy

[lance]

hey MikeRC, 30 year old powder works too. you'll just have to find what works for your mortar. everyone's mortar will shoot different with the same powder and same load.