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BC / muzzle velocity trade off


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Hi Chaps,

 

One question I have been pondering for a while is:

 

Is there any way to work out what the best trade off between MV of a bullet and it's BC with respect to windage effects or is it just always that the highest BC bullet for a particular calibre is always best.

 

In the GB F Class TR results there seem to be people using bullets from 155 grains up to 200+ with nothing seemingly obvious in the results to indicate which are best in the .308. Obviously the heavier bullets have a higher BC and so are less affected by a cross wind but the lighter bullets get to target much quicker and so the wind has less time to act upon them. ( I would have also thought that vertical might be more difficult to get consistent in the heavies)

 

Specifically I have been looking at .284 loads as Baldy is building one for me at the moment. I am going to start with the 162 a max which has a bc of (according to my Iphone ballistics calculator 0.625) most people seem to be using a say a Berger hybrid or something with a bc of 0.674 or thereabouts. My crux question I suppose is how much faster would my bullet have to go to have less or the same wind effect than the berger doing 2800fps MV at 1000 yards for example or is that simply impossible to work out.

 

I hope this is not too stupid a question!

 

All the best

 

Rich

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Hi Chaps,

 

One question I have been pondering for a while is:

 

Is there any way to work out what the best trade off between MV of a bullet and it's BC with respect to windage effects or is it just always that the highest BC bullet for a particular calibre is always best.

 

In the GB F Class TR results there seem to be people using bullets from 155 grains up to 200+ with nothing seemingly obvious in the results to indicate which are best in the .308. Obviously the heavier bullets have a higher BC and so are less affected by a cross wind but the lighter bullets get to target much quicker and so the wind has less time to act upon them. ( I would have also thought that vertical might be more difficult to get consistent in the heavies)

 

Specifically I have been looking at .284 loads as Baldy is building one for me at the moment. I am going to start with the 162 a max which has a bc of (according to my Iphone ballistics calculator 0.625) most people seem to be using a say a Berger hybrid or something with a bc of 0.674 or thereabouts. My crux question I suppose is how much faster would my bullet have to go to have less or the same wind effect than the berger doing 2800fps MV at 1000 yards for example or is that simply impossible to work out.

 

I hope this is not too stupid a question!

 

All the best

 

Rich

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You open a potentially extended (but interesting!) debate covering form factor and bullet weight etc in calculating resultant BC values and then downrange performance differences.

 

I think that you are also considering MV against a G1 BC value in comparing bullets which is only valid at the one maximum speed and will vary considerably downrange as it is calculated against a 'standard projectile' shape (old artillery shell style) rather than a modern boat-tailed and possibly long-nosed VLD bullet whose drag is very different.

 

Try to input either velocity-banded G1 BCs to your ballistics program if you can find these values and your program will accept these OR (better still) get a more representative G7 value to run your comparisons.

 

If you are into these issues then I REALLY recommend Brian Litz ballistics book.... its a goldmine of info.

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Using jbm and the litz figures from the bullet library to match the windage of a 180vld berger at 2800 you need to push the 162 amax at 3020. that's ok in theory, however as you probably know real world results depend on other factors. Can you get those speeds without hitting pressure? What's the es like? Accuracy? Terminal performance( if Varminting/hunting) etc etc.

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Interesting indeed and very revealing issue,though the mathis complex.DaveT is right.If at all possible use a G& based set of data,not G.Quoted BCs can vary-more than one way of calculating BC,and you really need to compare like with like.Vertical isn't much of an issue-the laws of gravity will not change at all over the same rifle range.Here is comparative data for a few calibres-all based on the sierra match king bullet as it is pretty much the same shape in them all-so BCs are directly comparable,as below all are sierra published ones for their bullet.To get the same wind deflection at 1000 yards,in this example 70 inches for all,

6mm 115 g bullet (BC .585)Mv 3065

6.5 142 (BC .565)Mv 2950

7mm 175 (BC.596) Mv 2800

308 220 (BC.627) MV 2650

 

Remember these are for calibre-it does not matter what the cartridge/chamber is.There are of course consequences for recoil and barrel life,and 'shootability',but that is what is needed for that performance.To a very good approximation higher BC is better- remember we are only considering external factors,so the general trend to be easier to get the velocities in heavy bullets might need to be modified by (unmeasured) case efficiency etc,but the above remains true wrt velocities.

 

You ask about weight/BC trade off:OK,you will be very close using this: you can go 500 fps slower for every +.100 gain in BC,(or 50 fps slower for every +.01 BC gain,and so on, pro rata),and still match the wind deflection. 50fps slower for every .01 gain in BC,50 fps faster for every .01 loss in BC). So go play,the above will keep reality in focus.

 

Any complaints,contact the God of Ballistics,or his consultant,if different,B.Litz

 

george

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Thanks chaps, have just ordered Mr Litz' book. I was hoping to get just over 3000 fps out of the amax so looks like it might work subject to all mentioned above. It will be interesting doing the load development.

 

Cheers

 

Rich

OK-remember the Litz figures are what is needed for specific performance,not neccessarly what it is prudent to try to achieve.The 284 win has seen some revival,in contemporary bolt actions, via F class etc,so there is some information around,so ask-as ever work up carefully,but the properly chronograhed experience of others is always helpful,especially in comparable actions-as will be Baldie's informed knowledge.

george

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Thanks chaps, have just ordered Mr Litz' book. I was hoping to get just over 3000 fps out of the amax so looks like it might work subject to all mentioned above. It will be interesting doing the load development.

 

Cheers

 

Rich

OK-remember the Litz figures are what is needed for specific performance,not neccessarly what it is prudent to try to achieve.The 284 win has seen some revival,in contemporary bolt actions, via F class etc,so there is some information around,so ask-as ever work up carefully,but the properly chronograhed experience of others is always helpful,especially in comparable actions-as will be Baldie's informed knowledge.

george

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OK-remember the Litz figures are what is needed for specific performance,not neccessarly what it is prudent to try to achieve.The 284 win has seen some revival,in contemporary bolt actions, via F class etc,so there is some information around,so ask-as ever work up carefully,but the properly chronograhed experience of others is always helpful,especially in comparable actions-as will be Baldie's informed knowledge.

george

 

I agree .... Trust the Bald one (AKA Dave!).... his knowledge is amazing ... as are his rifles!

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Here you go Rich. I barrelled your rifle today, and got it ready to go to proof. Then my mate rang and said he was going to the range....... :lol:

 

I can never resist....so i slipped it back in the interceptor stock, and bolted one of my own nightforces onto it. Grabbed a box of new ammo loaded for my own gun and set off.

 

We bore sighted it and then shot and cleaned it for three rounds as i walked them onto the dots.

 

The right hand 5 shot group is the first group out of the gun. I let it cool, then shot the left hand group of 5.

 

IMG_1343.jpg

 

Crap them border barrels aren,t they ? :lol::lol:

 

Load developement done for me...it obviously likes the 180 grain smk.

 

I never forget a favour buddy. ;)

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