optimum barrel length for distance?

[mortarnoobie]

I have read a few things on here saying that people acheive much better distances with longer barrels than shorter ones because the charge has more time to push the projectile.  If i were to construct (or have someone else construct for me) a golfball mortar, what would be a good barrel length to have in order to get the longest/highest shots?  would 10 inches be a good length (with an inch by inch chamber under it)?

I know some people have said that i should just purchase a mortar first, but i wouldn't feel right spending 300+ for something that i could get done to custom specs for less.

[Cat Whisperer]

What range are you going for?

Golf balls past 100 yards are a little difficult to track visually.

I have a Napoleon style cannon barrel 18" long that will chuck a lead ball 1.75" in diameter over a mile (measured).  At that range you have to have a wide open space in which to shoot and lots of clear space behind point of impact too.

One of my experiments with this contest we've got will be to try varying barrel lengths - from one to four or five calibers long with the same powder charge and angle to find out emperically what the differences are.

Again, 100 to 200 yards is cool with golf balls and mortars.

[Double D]

Mortar barrel bores are normally two calibers long, (the length of two golf balls) In mortars range is adjusted by increasing or decreasing the amount of powder used.  

$300 is a good price.  If you can find some one to hobby-lobby the gun for you or if you got access to machine tools and did it yourself you might be able to do it for less.

I don't think  based on the questions you are asking you quite grasp all the concepts yet to strike out on your own and build a gun.  You are doing he right thing however and that is asking questions.  the only dumb questions  is the one you don't ask.

We all have given you some good ideas here.  Several have posted drawing to work from.

Why don't you look those drawing over and pick one you like. then get that mortar built. don't look to have it built in one big project, but a part at a time.  Go to the scrap yard and find your self a piece steel this month.

Next month take the steel some where and have it bored out.  The following month get the profile turned, followed every month by each sucessive step until built.  You will have a nice decent safe mortar that you will get good results from and not just something that makes smoke and noise and flings balls out of sight. And, when you are done you will have some thing of value

[mortarnoobie]

when you refer to boring out the barrel, is this done by drill or by lathe (or both?).  I have a drill that according to the person i bought it from, will drill through metal.  would i be able to purchase my own large drill bit (carbide tipped?) and bore it myself?  if i could, this would probably reduce the cost.  

by the way, i'd rather build with atleast some future in mind than to build small and outgrow my mortar.  we don't have unlimited space for firing, but we do have a lot. i think it'd also be neat to fire at steep angles and watch the thing sail up and over the clouds  (-:

i do need to find time to go to the scrap yard for sure though.

[GGaskill]

The term "expansion ratio" (which is the ratio of the volumes of the chamber plus bore to the volume of the chamber alone) is very relevant to explaining why chambered pieces like mortars and howitzers do not need particularly long bores.  

I will use my beer can mortar as the example--it has a 2.65" bore of 10.62" length (4 calibers) and a chamber of 1" diameter and 1" length.  So the chamber volume is .785 cu in and the bore volume is 58.6 cu in (bore plus chamber is 59.4 cu in.)  The expansion ratio is therefore 59.4 divided by .785 which equals 74.6.  What this means is the initial pressure is reduced to 1/74.6th (1.34%) at the muzzle.  You can see that adding more barrel at this point will not add much velocity to the shot as the pressure will continue to fall as the expansion ratio increases.  

Since real military weapons have to balance a number of factors (transportability, cost, combat conditions, etc.), the slight ballistic gain of increasing barrel length does not compensate for increased weight and cost and loss of manueverability in the field.  

A long barrel becomes a postive attribute only when you are burning enough powder to have a useful pressure still in effect.

[Cat Whisperer]

Here are two pix of drilling (prior to boring) a 1" ID solid 1-1/2" OD of 1018 for a 'hand gonne'.

Note the solidity of how it's held.  (The lathe weighs about 1100 pounds.)  It is in a 3 jaw scroll chuck and has a steady-rest to allow it to turn.

You NEED something like this to drill/bore steel of this size.

The third pix is of the hand gonne (cannon on a stick), and two HAND-HELD hand gonnes' one in Fuji film can size and the other in golf-ball caliber (weighs about 8 pounds - 4140 steel).

Don't even think of using a hand drill.  If you're skilled you could 'get-away' with using a large drill press and good clamping.


DD gave you a good summary of advice.


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[GGaskill]

Since you are also a machining newbie, I recommend that you study this Army Training Circular (http://metalwebnews.com/machine-tools/fmt.html" target="_blank">TC 9-524) and get some understanding of machines and machining processes.  Then our comments will make more sense.

By the way, boring is the process of turning the inside of a hole with a single point tool; similar to turning the outside.  It uses a tool holder called a boring bar to reach down into the hole.  Generally, the setup is not as rigid as normal turning and you encounter chatter problems.  But not always.

[mortarnoobie]

thanks for the replies everyone.

i've been PMing with powderkeg about him turning a basic no-frills (straight edges, plain outer profile, etc.) golfball tube for me from 4" stock.  if he did the metal work for me (at a quote he gave which sounds like a bargain) i could do my own base and mount it at home (i have plenty of relatives that have more than enough experience in welding to assist me in mounting trunnions and anything else that would need to be done)

between my various ideas of fire extinguishers, drill press bores, and scrap yard pipe, i think this is my best option.

this is a great forum... plenty of expertise to keep noobs from hurtin anyone  :lol:

[Cat Whisperer]

Sounds lieke a plan!

We're looking forward to pictures now!

[CAV Trooper]

what would be a good barrel length to have in order to get the longest/highest shots? ...  i think it'd also be neat to fire at steep angles and watch the thing sail up and over the clouds

Keep a couple of things in mind. Mortars were not designed to be long range weapons. The idea was to lob an exploding shell over the enemy's walls and take out his troops and guns. For distance, they'd use a long barrel cannon. As CW said, much beyond 100 to 200 yards and you'll never see where a golf ball went. You can see a beer can a bit farther out but not a great deal. If you can't watch where your projectiles go, you might as well be shooting blanks.

Also, be extremely careful trying to fire at extremely high angles. It's very common for the wind a hunderd feet or more up to be coming from an entirely different direction and at a higher speed than what you feel on the ground. Guess wrong on wind speed and direction and there's a possibility that the projectile could be pushed right back at you. Not good. I'm sure that you have no desire to become a candidate for a Darwin Award.   :wink:

Light projectiles, like golf balls, will be affected the most but beer cans will be too. The safest thing would be to go no higher than 60 to 65 degrees on the angle of the tube and fire with the surface wind coming from behind you if at all possible. If you want to get the best range from any shot, 45 degrees is the optimum angle.

[GGaskill]

I also want to introduce you to new tooling and its cost.  I bought a new http://www.use-enco.com/CGI/INSRIT?PARTPG=INSRAR2&PMAKA=303-1610&PMPXNO=951431" target="_blank">Chinese import 2 5/32" drill to drill the bores of my 1/2 scale mountain howitzers for 2 lb fishing weights.  As you can see, they are not cheap even coming from China.  It comes with a MT5 shank which is bigger than any of my lathes.  This required buying an MT5 to MT4 adaptor, which turned out to be a good thing as it allowed me to drill a much deeper hole than the drill alone will.

You will find that getting a lot of tooling with a used lathe is about the best deal you will ever run into, as individually, drills and chucks and tool holders, etc., are pretty expensive and the total adds up quickly.

[Cat Whisperer]

I also want to introduce you to new tooling and its cost.  ...  are pretty expensive and the total adds up quickly.

George - I agree - good source for tooling (ENCO) and it adds up quickly!

I've found that I spend much time designing and making tooling (as well as the normal making the of fixturing).

The cost of the lathe or the milling machine is only part of the cost of tooling.

[mortarnoobie]

very good point on watching the varying wind up high... maybe i'll slap on the old full face BMX helmet from back in the day for high shootin.    


i'm going to whip up a CAD diagram of what i want and email it to him, and we should be on our way soon enough   :grin:

[Cat Whisperer]

 .... Light projectiles, like golf balls, will be affected the most but beer cans will be too. The safest thing would be to go no higher than 60 to 65 degrees on the angle of the tube and fire with the surface wind coming from behind you if at all possible. If you want to get the best range from any shot, 45 degrees is the optimum angle.

One year, it had been below -10F for 3 weeks, typically once a year and often worse.

We took the beer-can mortar out on the river (frozen solid) since the banks went up straight 15' and you could get out of the wind.

After a number of rounds I tried shooting one INTO the wind (does the wind blow in Iowa?).  The can went up and up got caught by the wind and sailed past us and landed 50 yards behind us!  Easy to track  since it was up high enough.

I blame it on 'cabin feaver'.

[Cannonmaker]

CW said the cost of the Lathe or Mill is only part of the cost.  How true this is.  I'am always buying tooling.
My customers ask me about the cost of metal working equipment and I tell them its like buying a tractor.  spend $100,000 on a tractor and its of no use till you spend $200,000 on implements to hook up to.
Not much time lately to build Cannons, been busy keeping the farmers and ranchers going.
Once in a while you can fine a Machine shop having a Auction and pick tooling up for a good price.

[CAV Trooper]

One year, it had been below -10F for 3 weeks, typically once a year and often worse.

We took the beer-can mortar out on the river (frozen solid) since the banks went up straight 15' and you could get out of the wind.

After a number of rounds I tried shooting one INTO the wind (does the wind blow in Iowa?).  The can went up and up got caught by the wind and sailed past us and landed 50 yards behind us!  Easy to track  since it was up high enough.

I blame it on 'cabin feaver'.

CW,

I know EXACTLY from whence you speak. I lived in Iowa for 14 years and 14 of those frigid, wind swept, below zero winters are why I now live in South Florida.    :-D

[mortarnoobie]

pics of his offer.  look like a good deal for 150?  i will be able to build the sled and everything for it at home and at a friends home shop.

[CAV Trooper]

Cool.   :grin:   You don't see a hex shaped mortar very often. Looks nice.

[mortarnoobie]

ya i wasn't expecting it either.  i think it will look nice once i give it some polishing.

now the end isn't rounded off as you can see... anybody have a picture of a good way to mount it on a sled so the angle can be changed freely without the back hitting the base?  angle iron brackets to lift it a few inches maybe?

thanks
-Jon

[GGaskill]

Use a sled mount instead of a block mount.  It allows elevation adjustments, too.

http://cwartillery.org/ve/mtr10sc40.jpg">

[C2ND]

Something like this should work.

[mortarnoobie]

yes something like that is just what i wanted. nice and simple too.  thanks for the picture.  is yours oak i assume?

[C2ND]

Yes, it's oak, started with a piece of full dimension 2 x 8 x 6' long.  
It still needs detail work, a couple tie-down rings, maybe a couple handles for lifting. It weighs about 100#.

[mortarnoobie]

shoot thats heavy... maybe a tow rope instead of lifting handles (=

what size barrel and bore and stuff do you have on there?  it's hard to judge from the picture.