Load Tuning for temperature

[mks]

Keith,

So, if I'm reading this correctly, far and away the largest component of "tune" is barrel temperature? The rate of reaction of powder is in play, but isn't really significant?

Changing subtropics...

What is your thought on barrel profiles and tuners/muzzle weights? Am I going to see you down in KY toting a 26" LV with a bigazz barrel weight?

Greg,
I am not so sure about "far and away the largest." That depends on the temperature/pressure sensitivity of the powder. I don't have burning rate curves for modern powders, but if older military research papers are any indication, good powders have a plateau of reduced sensitivity to temperature/pressure. That plateau pressure seems to be pretty high for 133 (best accuracy at really high pressure), but lower for 8208XBR (best accuracy at lower pressure). So my guess is heat losses are the largest factor, but reaction rate is not insignificant.

The long barrels are needed at long range to get the muzzle to flip fast enough for tuning. Tim in tx has done a lot of good work on this. At short range, I think we need barrels and/or stocks that are a bit less stiff in the vertical plane, and as stiff as possible in the horizontal plane. I haven't made an asymmetrical (wider than tall) barrel yet, but think this holds some promise, particularly for the new NBRSA sporter class where barrel contour is not limited. My new stock is much stiffer in the horizontal plane, and it shot OK last summer. I will be playing with a stock more flexible in the vertical plane this summer.

Cheers,
Keith

 

[tim in tx]

Keith that makes perfect sense.

Thanks for the complements Keith, You have a really good way of explaining things.I think the shockwave forming in the barrel can have some aditional pressure that would be hard to calculate as a constantor not?The extra drop and tune change was not the only reason I think it affects in bore time but when I am tuned for 100% compensation with higher baro at 1 mile there is little to no poi change as a matter of fact the drop is completely negated and the bullets are automaticly aimed higher remaining at the same poi and the only way I can see it aimed higher is that the muzzle is in a differing position at launch because the bullet is slowed in the bore enough to be aditionally compensated in a vertical respect with just a baro change.So the drop that is normally seen with regular muzzled rifles with higher baro is way more than the calculated drop and with 100% vertical compensation the poi is the exact same or even higher is some cases but not all. I have never seen a poi remain a constant on a standard muzzled rifle with baro change.I have some pictures of the air being compressed in front of the bullet and will try to find them,but by the time the bullet gets to the end of the barrel it is at higher pressure in front of the bullet and a lower pressure behind the bullet.I can certainly be wrong but some of this vertical compensation starts to show some things I have never seen before.That is why I am thinking what I am.

Tim in Tx

 

[mks]

Thanks for the complements Keith, You have a really good way of explaining things.I think the shockwave forming in the barrel can have some aditional pressure that would be hard to calculate as a constantor not?The extra drop and tune change was not the only reason I think it affects in bore time but when I am tuned for 100% compensation with higher baro at 1 mile there is little to no poi change as a matter of fact the drop is completely negated and the bullets are automaticly aimed higher remaining at the same poi and the only way I can see it aimed higher is that the muzzle is in a differing position at launch because the bullet is slowed in the bore enough to be aditionally compensated in a vertical respect with just a baro change.So the drop that is normally seen with regular muzzled rifles with higher baro is way more than the calculated drop and with 100% vertical compensation the poi is the exact same or even higher is some cases but not all. I have never seen a poi remain a constant on a standard muzzled rifle with baro change.I have some pictures of the air being compressed in front of the bullet and will try to find them,but by the time the bullet gets to the end of the barrel it is at higher pressure in front of the bullet and a lower pressure behind the bullet.I can certainly be wrong but some of this vertical compensation starts to show some things I have never seen before.That is why I am thinking what I am.

Tim in Tx

Tim,
It is curious that poi does not shift with an increase in barometric pressure, and does suggest that internal ballistics is affected. Otherwise, the denser air should cause more drop. Considering for a moment that the greater pressure on the nose of the bullet slows it down, the bullet should exit later. In Al's simulations of Lynn's long range rifle, bullets exited just before the peak in muzzle projection. So if the bullets slow down and exit closer to the peak, then this would decrease poi shift. But compensation for muzzle velocity differences would decrease, because the rate of muzzle angle change decreases as the peak is approached. Is this what you are seeing - no poi shift, and an increase in vertical dispersion?

Thanks,
Keith

 

[tim in tx]

Hi Keith ,Only when I am fully tuned for 100% compensation then I see no shift or drop due to baro,if I have vertical I have poi changes as well.I think I am still away from the peak or taper off of compensation ,I am actually at the bottom of the slope with the ammo from the mile test session. The reason is the faster velocites are at the bottom of the slope [2960]and the slower velocities [2820] is when the compensation starts to taper off at 1000yds.I normally try to keep stuff for the mile at 2930 fps or exit at the bottom end of the slope .If possible I want the bullet exit to be in the middle of the velocity range within the compensation swing , Unfortunately high frequency likes to hang at the muzzle when the bullet exits in that area so slightly before the peak or right off the bottom to the middle is probably a good thing .


Tim in Tx