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Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by SapperRIP, Oct 25, 2018.
Bullet stability affects accuracy and bc. Over spinning the bullet will negatively affect accuracy. Under spinning will negatively affect bc.
A quote from Brian Litz: applied ballistics
"On the other side, over spinning a bullet is a bit of a myth. So if your barrel twist rate is far faster than recommended, you only need to make sure you account for the added spin drift"
Unless the bullet flies apart from being overspun
I can assure you there is a reason 6ppc guys are on the ragged edge of stability. For the purpose of long range hunting I will take a sg of 1.7 over 1.3 any day but I do not get hung up on it. If going custom you might as well do it right.
I have heard that there is now testing being done on expansion of a bullet in regards to twist rates.
It's been noted that a higher twist rate at the same speed as a slower twist rate will show different expansion between the two. (Faster twist equating to more expansion) --- iirc, it is not yet proven just has been seen and noted and I'm pretty sure its it's actually being tested to see if it holds true.
Also, some say that too fast of a twist can slow your muzzle velocity, but iirc this has been disproved except for in extreme twist rate situations.
The root of the reason why over-spinning a bullet CAN (not will) degrade accuracy is because any imperfections in the bullet itself will be magnified more. For instance, if a bullet has a slightly thinner/thicker jacket on one side of the bullet, or any other defect that makes it effectively lopsided, then the higher the rpm's of said bullet, the farther it can be pulled out of the group. So if your over-spinning a bullet, your groups CAN open up vs. a bullet that is spinning just fast enough to properly stabilize, IF the bullets you are shooting are not concentric.
On the other hand, you can over-spin the hell out of a bullet that is perfectly concentric, and will have zero adverse affects aside from increased spin drift.
That is, unless your spinning the bullet fast enough to begin to tear the bullet apart. For instance, I had this happen while shooting fire-forming loads of 85 grain Sierra varminter bullets out of my 8 twist .260 AI pretty fast, they would shoot stupid good, as in .2's with 5 shot groups while fireforming, unless my barrel got warmish. Then they became erratic. But with the extreme spinning rpm's, and the relatively high velocity (3300+ if I remember correctly) and a very frangible varmint bullet, they turned prairie dogs into a pink mist.
Also I will add, that it is becoming more common to find that people shooting ELR type stuff are aiming for a higher than regular s.g., because it is believed that it aids in the bullets transfer into the sub-sonic range. Not sure on the science behind this, just what I have heard from a couple ELR guys.
IMO, hunting adds another dimension to the equation. Besides stability, I think there are other considerations that relate to accuracy, barrel/throat wear, and terminal performance(expansion characteristics) of the particular bullet on game over the expected range of distances that game will be encountered. While sufficient bullet stability is critical, simply choosing the fastest twist available might produce unwanted collateral effects depending on your particular requirements.
I think codyadams pretty well nailed it, by making it a bullet construction issue. I had a .358 AI, with a faster than usual twist, fire forming loads shot great, turned up to its potential the wheels fell off,
As we see more turned mono's in elr, and hunting applications it will be interesting to see how this plays out.
I can see why the poll is split pretty well down the middle, my answer could change depending on how the question is posed just a little.
Torquing of the shooting system is another reason to limit spin to that actually needed.
All matters of recoil matter for each shot.
Also, if it didn't matter then all barrels would come in 6tw.
Agreed, yes there is such a thing as too fast a twist. Voted no because it doesn't have anything to do with SG.
I like an sg of at least 1.7.
Running sg of 2.68 in my 338 and it shoots very well