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# Future H.A.T B.C. Test shoot

#64
02-06-2010, 11:48 AM
 Silver Member Join Date: Mar 2009 Posts: 214
Re: Future H.A.T B.C. Test shoot

Mark,

"... Not sure if that remains the same going to a different bore size?"... I missed your appendix last night, and it does bring up some interesting issues.

Bore size, and twist-rate are essentially scalable. In 5.5+ caliber projectiles there is another factor that comes into play which has an effect that must be taken into account.

The ZA relies heavily on tail geometry to maintain high velocity stability. In order for the tail design to be effective hovever, laminar flow needs to be preserved across it's surface. The Reynold's effect states, in essence, that an object of "A" diameter, at "B" velocity, will produce the same laminar/turbulence flow properties as an identical object of (1/2) "A" diameter, and (2) "B" velocity.

To simulate a 50 caliber projectile at 2,500 fps, I would need to run a 25 caliber model at 5,000 fps. Bryan might want to weigh in here, but the practical result is that a projectile needs to change tail geometry, across caliber transistions, in order for the scaled twist-rate to work.

Best,
Noel
#65
02-06-2010, 11:56 AM
 Platinum Member Join Date: May 2008 Location: South of Canada and North of Wyoming Posts: 6,068
Re: Future H.A.T B.C. Test shoot

Noel,

I guess I'm a little confused why you reffenced Bryan's formula for calculating BC to Jon? In any case, I would get bullet manufacturer's twist recommendation bfore having a barrel made. The provider of that calculator did make this statement....

Quote:
 The basic twist rate calculator above uses Bowman's equation modified with the SG correction quoted by Howell. For flat base bullets, the calculator should give a good estimate for the twist needed. The estimates should work for some boattail designs, but VLD and ULD style bullets may well need more twist.
I assume you'll provide the required twist for your bullets when you are ready to sell them.

The twist calc seems to be fairly accurate for some other common bullets, but your's would definitely be in a different category.

Am interested in seeing the results of the shooting event and getting more info on your bullets when the time comes.

Cheers,

Mark
#66
02-06-2010, 12:09 PM
 Silver Member Join Date: Mar 2009 Posts: 214
Re: Future H.A.T B.C. Test shoot

Mark,

Jon was asking how I arrived at twist requirements. While there are some mathematical means of generating minimum parameters, such as Bryan's referenced formula, the bottom line is that actual testing is necessary.

- Noel
#67
02-06-2010, 12:27 PM
 Platinum Member Join Date: May 2008 Location: South of Canada and North of Wyoming Posts: 6,068
Re: Future H.A.T B.C. Test shoot

Thanks Noel, understood.
#68
02-06-2010, 12:46 PM
 Platinum Member Join Date: May 2008 Location: South of Canada and North of Wyoming Posts: 6,068
Re: Future H.A.T B.C. Test shoot

Quote:
Quote:
 Originally Posted by noel carlson Mark, "... Not sure if that remains the same going to a different bore size?"... I missed your appendix last night, and it does bring up some interesting issues. Bore size, and twist-rate are essentially scalable. In 5.5+ caliber projectiles there is another factor that comes into play which has an effect that must be taken into account. The ZA relies heavily on tail geometry to maintain high velocity stability. In order for the tail design to be effective hovever, laminar flow needs to be preserved across it's surface. The Reynold's effect states, in essence, that an object of "A" diameter, at "B" velocity, will produce the same laminar/turbulence flow properties as an identical object of (1/2) "A" diameter, and (2) "B" velocity. To simulate a 50 caliber projectile at 2,500 fps, I would need to run a 25 caliber model at 5,000 fps. Bryan might want to weigh in here, but the practical result is that a projectile needs to change tail geometry, across caliber transistions, in order for the scaled twist-rate to work. Best, Noel
Noel,

I missed this post until just now. Thanks for the reply. Something else I wonder is how much bearing surface comes into play. We lengthen tails and noses to reduce drag but I would think that at some point we get dimishing returns in terms of the bullets stabilitiy if the bearing surface gets short enough the bullet may begin to "rock" going through the tube. Interesting stuff and beyond me.

Mark
#69
02-06-2010, 01:01 PM
 Silver Member Join Date: Mar 2009 Posts: 214
Re: Future H.A.T B.C. Test shoot

Mark,

Your point is well taken. In-bore cant is the single greatest cause of dispersion, and you do not need a short bearing surface to have the problem.

The longer the column of lead, present in jacketed VLD projectiles, has a tendency to distort upon acceleration. As a result, a "cant" is induced in the projectile itself.
#70
02-06-2010, 01:08 PM
 SPONSOR Join Date: Mar 2003 Location: N.GA Posts: 139
Re: Future H.A.T B.C. Test shoot

Noel
Shall we stick the torch back in to the gunpowder.
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