Why would short bullets be inappropriate for fast twist barrels?

[arakboss]

Question in title and I will add this question. Does length of bullet change the rpm of the bullet leaving a barrel of a given twist rate? I would think not.

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[BigGame]

Length of bullet does not affect RPMs. Only the twist rate and velocity affect it.

Some say that 'over stabilizing' a bullet (gyroscopic stability factor greatly in excess of 1.5) can be detrimental to accuracy. I have not seen this and it may only be relevant if you're shooting competitively. Others say that extra-high stability can be beneficial at very long range when the bullet slows to sub-sonic. Only relevant if you're shooting out past 1000 yards.

For most practical shooting, I don't think there's any problem with shooting short bullets in fast twist barrels.

 

[po18guy]

Nope. However, think of the stability of a very short aircraft at high speed versus that of a longer aircraft. Or a very short wheelbase car versus a long wheelbase car. Longer is inherently more stable. But, cars and aircraft are not twisting in rifling. Short bullets have far less bearing surface than longer, heavier projectiles. Thus, they have less jacket to be engraved by the lands of the barrel. In some cases, if there is insufficient engagement, the bullet can strip its rifling from accelerating faster than the rifling will permit. Longer bullets can also tolerate faster twist rates without stripping the rifling, and the gryoscopic stability from the faster twist rate stabilizes the bullet for a more accurate trajectory.

 

[Pepe-lepew]

The velocity of the bullet leaving the muzzle changes the speed the bullet turns for a given twist rate. Easy to calculate the angular speed using the twist rate and muzzle velocity.

Lighter bullets out of faster than needed twists will just be spinning faster than necessary to stabilize.

You can spin them too fast. Bullets of light construction will come apart if spun to fast.

 

[arakboss]

Is it reasonable to assume that 40gr 224 bullets in 1:7 twist barrels should be fine out to at least 300 yards (minus wind factors)

 

[arakboss]

The velocity of the bullet leaving the muzzle changes the speed the bullet turns for a given twist rate. Easy to calculate the angular speed using the twist rate and muzzle velocity.

Lighter bullets out of faster than needed twists will just be spinning faster than necessary to stabilize.

You can spin them too fast. Bullets of light construction will come apart if spun to fast.

Nevermind, misread your comment.

 

[eldorado]

Nevermind, misread your comment.

No. The rotational velocity is a linear function of linear velocity. Note that there is energy associated with bullet rotation. The more energy that goes into spinning the bullet, the less goes into forward velocity, for a given charge.

 

[2ndtimer]

In my experience, shooting lightweight (40 grain) bullets out of faster twist barrels (1 in 8" twist) has seen very little degradation of accuracy compared to longer bullets at 100 yards. I haven't compared them at my range's maximum of 200 yards, but will do so the next time I am out there when the wind is reasonable. It certainly is possible for lightweight short thin jacketed bullets to come apart and disintegrate prior to reaching a 100 yard target ( a proven fact with 55 gr Nosler .243 SHOTS bullets out of a 1 in 9.25" twist .243 Win at velocities in excess of 3600 fps). I think it might have more to do with thinness of jacket and roughness of bore than just twist rate and velocity. I have had no issues with 40 gr Ballistic Tips and Hornady V-Max and Z-Max at 3500fps in 1 in 8" twist barrels.

 

[Lloyd Putney]

Question in title and I will add this question. Does length of bullet change the rpm of the bullet leaving a barrel of a given twist rate? I would think not.

iT IS THE OTHER WAY AROUND LONG HEAVY BULLETS WON'T STABILIZE IN A SLOW 1:14 ETC BARREL.
Buy a reloading manual and there will be pages that explain that to you.

 

[EHJ]

Question in title and I will add this question. Does length of bullet change the rpm of the bullet leaving a barrel of a given twist rate? I would think not.

Good question...

Since 'weight' goes hand in hand usually with length - and with simple length it's hard to differentiate the differing length characteristics (Ogive as it relates to actual projectile contact surface with barrel and rifling...) with what the weight of the projectile is affecting.

Go light - usually less (shorter) engagement with lands but faster velocity's or, heavier and longer (more real estate for engagement - sometimes...) and lower velocity...?

 

[oremike]

One thing not mentioned above is that a bullet loaded short weather by design or construction will have a greater jump to the lands and that can affect accuracy. I kind of do the opposite or at least different by loading round nose bullets to magazine lengths IMHO getting the bearing surface closer to the lands. I did this in a 300 Savage, the Hornady 150 RN's shot better than the Hornady 150 SP's loaded the same with the same OAL. Your mileage may vary.

 

[po18guy]

The proof is in the pudding. Load up some lightweights and some heavyweights. Target them. "If" the light bullets group well in a fast twist, good!

 

[NW Backpacker]

Assuming a longer barrel gives higher velocity than a shorter barrel, the spin rate will be higher with the longer barrel if both barrels have the same rate of twist, right? If so, then it's possible that a longer barrel, with a slower twist rate might give a higher spin rate than a shorter barrel with a faster twist rate, but you'd have to do the math to determine if that is actually the case.

Edit - articles added - I have not verified their information:

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Bullet Stability

The details of gyroscopic and dynamic bullet stability

bisonballistics.com

 

[ThunderMtnOrd]

If a lighter bullet is "over stabilized", it will not arc over like a perfectly thrown football. It will impact nose up. That WILL affect accuracy on the downslope of the trajectory, especially as the bullet goes sub-sonic.