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Ladder Test Question ???

 
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  #15  
Old 01-28-2006, 07:46 PM
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Join Date: Feb 2005
Posts: 564
Re: Ladder Test Question ???

Bart, thanks for taking the time to check out my website.

As you know, I value your opinion.

I don't know a whole lot about the physics involved with the barrel--only what Chris has shared with me in our emails and conversations. You mention the garden hose and the whip wave... Chris said that he was working on a very tall radio tower (around 1400 feet high, as I recall) and he was passing time sitting at the bottom of the tower, next to one of the guy wires. He noticed that when he whacked that cable that the vibration wave would move up to the point on the tower where it was tied off, and then it would return. He said that the cantilever beam (which is what a barrel is) would work in a similar way, with the shock wave originating at the chamber, moving to the muzzle, reversing back to the chamber, then back to the muzzle, etc., at the speed of sound in steel (~18,000 fps). That would be around 4 trips before the average bullet was able to exit.

Of course the idea would be to get the bullet to leave the muzzle during the time that this wave was back at the chamber end of the barrel. We can usually find two accuracy nodes for any given bullet and powder (a low one and a high one) and Chris' model seems to explain this.

He uses some kind of engineer's program called Matlab, which allows computer simulation of different physical occurrences. Of course the data input to the Matlab program would have to be accurate for good results.

Chris has mentioned seeing other ideas of barrel behavior published, but none of them can explain why the Federal Gold Medal (and other such recipes) can shoot so well in so many different barrels. Basically, the other models he has reviewed would seem to preclude any such "universally good" load recipe (like the Federal GMM) from existing. But as we know, those recipes do exist.

Anyway, as mentioned, the engineering end of things is beyond me. I just knew that there must have been something in effect that would allow Federal Gold Medal match ammo to shoot so well from so many different .308's--and I'm pretty convinced that Chris has figured it out. Chris wrote some of the software for RSI ballistics re optimum barrel times and such (I don't own this software, so I don't know much about it).

I'm more of a "bare bones" kind of reloader and shooter. I stick with the basics, and rarely even use a chronograph. But I get by pretty well... [img]/ubbthreads/images/graemlins/smile.gif[/img]

Dan
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  #16  
Old 01-29-2006, 09:20 AM
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Join Date: Dec 2005
Posts: 2,114
Re: Ladder Test Question ???

Here’s some data I got using a barrel vibrating software program that calculates the resonant (fundamental) frequency of a barrel. It was written by a mechanical engineer who own a business that deals in vibrating frequencies of materials for civil engineers and architects. He guaranteed accuracy to 2 decimal places based on the rigidity characteristics of both type 416 stainless and 4140 chrome moly which are typical of barrel steels used today.

I entered dimensional data for a stainless steel 30 caliber barrel 26 inches long with a 1.2-inch diameter reinforce 3 inches long at the rear then tapering to a muzzle diameter of both .700- and .900-inch. Also, I used a stand alone barrel with both breech and muzzle ends free and a barrel fixed at the breech end with the muzzle free. The results in each barrel’s fundamental (natural resonant) frequency plus its weight are as follows:

.700-inch muzzle, 5.114 pounds; both ends free = 278.9 Hz, breech fixed = 70.7 Hz

.900-inch muzzle, 6.064 pounds; both ends free = 295.3 Hz, breech fixed = 60.6 Hz

Interesting stuff indeed.
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