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C:B Ratio's Tell me what works for you

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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C:B Ratio's Tell me what works for you

Unread postAuthor: shiftyguy » Thu Sep 27, 2007 6:46 am

I have herd allot about C:B Ratios and every body uses something different, (probably due to what they found for free) I would like to hear about everyones c/b ratio so we can sort out what works and what barely works.

To calculate C:B (chamber-Barrel) volume
Stuff a PEI Potato down the barrel Pour water in chamber until full and measure, To measure the barrel do the same thing, however you should all the volume of the potato you are using as a plug.

Please if you wish to correct or use a more mathmaticl approach feel free to post your system.

Everyone's distance will vary based upon their configeration, accelerant, spark type and fan used.

****************************************
Barrel Length vs. Feet per Second

The following contains some general guidelines to determine the effect of barrel length on range and bullet speed. Ammunition, especially surplus military, varies widely in performance. We based the following on a 700 grain US military round.

To maintain accuracy the bullet must stay supersonic. Every inch of barrel adds more weight and makes the rifle more unwieldy. So pick the right barrel length for your needs.

Here is the breakdown:

17" - 29" Barrel Length

FPS

Inches Min* Max*
17" 2236 2236
18" 2261 2271
19" 2286 2306
20" 2311 2341
21" 2336 2376
22" 2361 2411
23" 2386 2446
24" 2411 2481
25" 2436 2516
26" 2461 2551
27" 2486 2586
28" 2511 2621
29" 2536 2656

17" barrel - 2236 FPS (feet per second), accurate to approximately 1600 yards.
You generally gain approximately 25-35 FPS per inch of barrel over 17", then drops lower at 29".

29" - 39" Barrel Length


FPS

Inches Min* Max*
29" 2656 2656
30" 2674 2677
31" 2692 2698
32" 2710 2719
33" 2728 2740
34" 2746 2761
35" 2764 2782
36" 2782 2803
37" 2800 2824
38" 2818 2845
39" 2836 2866

29"-32" barrel - Is the ideal in terms of long range, weight, and powder usage. 2656 FPS, accurate to approximately 2200 yards.
You generally gain approximately . 18-21 FPS every inch over 29", then drops lower at 40".


40" Barrel Length

40" barrel - 2876 FPS, accurate to approximately 2500 yards
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Unread postAuthor: shiftyguy » Thu Sep 27, 2007 7:00 am

My current C:B ratio in inches is
120:80 or better known as 3:2
Accelerant - air freshener
Chamber - 3" x 16
Barrel - 1.5 x 46
Fan - Yes 2"
Spark - Camera/coil
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Unread postAuthor: jackssmirkingrevenge » Thu Sep 27, 2007 7:11 am

We based the following on a 700 grain US military round


Sounds like 50 bmg.
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Unread postAuthor: psycix » Thu Sep 27, 2007 8:08 am

shiftyguy wrote:My current C:B ratio in inches is
120:80 or better known as 3:2
Accelerant - air freshener
Chamber - 3" x 16
Barrel - 1.5 x 46
Fan - Yes 2"
Spark - Camera/coil


You still havent got below 1.5:1 C:B ratio!
Take a smaller chamber or a longer barrel coz the results we are looking for are 0.5:1 - 1.5:1 ratios, not 1.5:1 and higher.

You can cleary see the speed isnt at max with your 40" barrel.
Its still rising.
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Unread postAuthor: shiftyguy » Thu Sep 27, 2007 9:51 am

First of all 3:2 is the same as 1.5:1.

Also the point of this topic is to see what other people are using and to see their results

Inorder for me to achieve a 0.5:1 ratio (1:2) with the same sized chamber i would roughly need a 138" barrel wich is not practicle or use a 5.5" chamber with a 46" barrel.

Come to think of it. a range of 0.5:1 ~ 1.5:1 (1:2 ~ 3:1) is one big range to have ......
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Unread postAuthor: jimmy101 » Thu Sep 27, 2007 10:03 am

Of all the questions about what makes a combustion spud gun work well, CB ratio is probably the best understood.
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See the Spudwiki, and the references cited therein, for more info (http://www.spudfiles.com/spud_wiki/inde ... rrel_ratio).
Latke, and a few others, have done this enough times, and with enough different gun configurations, for the "CB ratio" question to be considered a solved problem.

The "best understood" does not mean it is completely understood but compared to many of the other design characteristics of a combustiong gun (number of sparks, position of sparks, +/- fan, +/- inner bevel on the cutter, etc.) CB it pretty well understood.

I would say that the only real questions remaining about CB ratio is how the optimal changes as a function of other design characteristics. Is the optimal CB the same with 1 spark versus 3? If the friction between ammo and barrel changes does that change the optimal CB? What is the optimal CB for a burst disk or hybrid?

BTW, at a CB of 1.5:1 a standard gun's performance usually drops off a lot compared to a CB of 0.6~1.0.
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Unread postAuthor: psycix » Fri Sep 28, 2007 1:23 am

See, your chamber is way too large :)
If you would fit that 138"barrel (ok make it a bit shorter for friction etc)
You would maybe go like supersonic :)

Combustion cannons just happen to work better in the 0.5 - 1.5 : 1 ratio
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Unread postAuthor: jimmy101 » Fri Sep 28, 2007 10:06 am

psycix wrote:Combustion cannons just happen to work better in the 0.5 - 1.5 : 1 ratio

Look at the Latke graph I posted above. Estimate the muzzle velocity at a CB of 1.5 CB. Would you call that "working better"?
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Unread postAuthor: williamfeldmann » Fri Sep 28, 2007 10:32 am

Uh, is the correct answer "no" :wink:

I agree this is basically a dead arguement. There has been more than enough studying done on C:B ratios.

There are plenty of other things that can be studied. They were mentioned earlier. If you really want to further your knowledge and ability of your cannons, research with the wiki.

I only wish Latke had used golf balls or another uniform, consistent ammo rather than spuds. But the proof is plain.
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