Bullet RPM and performance

[Darryle]

Hopefully I am not beating a dead horse here or asking an ignorant question.

I have read about twist in relation to bullet stability and velocity in relation to bullet performance at long range. I just can't recall about reading if RPM contributes to bullet performance (i.e mushrooming and transfer of energy).

I know we have all read instances of penciling, the ELD-X bullets are one I experienced first hand and at a variety of ranges.

My questions are:

Have designers taken barrel twist/bullet RPM into consideration when designing these bullets and as twist rates get faster, ranges increase, bullets get longer and velocities get higher are these contributing factors to bullet design and performance?

Can you spin a bullet too fast to inhibit performance, increase failure or increase the chances of getting penciling?

Does spin have an affect on performance and has a single design been subject to testing at differing velocities, RPMs and ranges for terminal performance or has the data been gathered?

Is there a way to know you are in the sweet spot in relation to twist and bullet/load selection other than accuracy at the range?

I am in the process of deciding on a new barrel and other than stability for the chosen bullets, I am wondering what other effects twist will play with my selection.

Thanks Darryle

 

[MW204]

This may help, there are a lot of links for different questions in this link alone.

What is my optimal barrel twist rate?

Optimal barrel twist rate depends on what type of bullet you are using. Generally, fast twist barrels are used for longer bullets.

bergerbullets.com

 

[Kmccord]

Take a look at this, it will give details on bullet RPMs and what becomes over spinning and what that will do.

 

[Joejack]

I can assure you that that running them to fast they will blow up my bartlin 1&7 twist 22 creedmoor is blowing up 85.5 Bergers at 3260fps

 

[FEENIX]

Hopefully I am not beating a dead horse here or asking an ignorant question.

I have read about twist in relation to bullet stability and velocity in relation to bullet performance at long range. I just can't recall about reading if RPM contributes to bullet performance (i.e mushrooming and transfer of energy).

I know we have all read instances of penciling, the ELD-X bullets are one I experienced first hand and at a variety of ranges.

My questions are:

Have designers taken barrel twist/bullet RPM into consideration when designing these bullets and as twist rates get faster, ranges increase, bullets get longer and velocities get higher are these contributing factors to bullet design and performance?

Can you spin a bullet too fast to inhibit performance, increase failure or increase the chances of getting penciling?

Does spin have an affect on performance and has a single design been subject to testing at differing velocities, RPMs and ranges for terminal performance or has the data been gathered?

Is there a way to know you are in the sweet spot in relation to twist and bullet/load selection other than accuracy at the range?

I am in the process of deciding on a new barrel and other than stability for the chosen bullets, I am wondering what other effects twist will play with my selection.

Thanks Darryle

Follow the bullet manufacturer's minimum twist rate (per their ballistician) recommendation and you should be GTG.

 

[FEENIX]

I can assure you that that running them to fast they will blow up my bartlin 1&7 twist 22 creedmoor is blowing up 85.5 Bergers at 3260fps

Please elaborate; was this through a ballistic gel test? Thanks!

 

[BallisticsGuy]

It's quite possible for rifling design, twist rate, RPM (which is an emergent effect of twist rate and velocity) to affect how a bullet performs in terms of terminal ballistics. It would be nonsensical to make a blanket statement that it does or does not affect terminal performance because details matter. As an example: One of my rifles in .223 caliber uses polygonal rifling with 1:7 twist rate. The polygonal rifling doesn't actually engrave the bullet but instead swages it. I can shoot a particular 45gr bullet out of that rifle and it's just fine and effectively converts squirrels into atmospheric gasses. If I take the same ammo and put it in another rifle of mine that has the same twist rate but has standard Enfield type rifling that does engrave the bullet, the bullets turn into a grey puff of smoke before they get to 25m out of the muzzle. The rifling design in one weakens the jackets on a bullet model with an intentionally weak jacket and the excessive spin (a 1:14 twist is perfectly sufficient at the velocities I'm pushing with them). The bullets are within spitting distance of 400,000rpm from a 1:7 twist barrel and they're really not designed for not much more than ~250,000rpm. I only get away with it in my polygonally rifled barrel because the jacket isn't substantially weakened by the swaging. From the conventionally rifled barrel the bullet jackets are substantially weakened, so much so that they expand from sheer radial velocity. So, if they even made it to a squirrel from that barrel they would be very expand-happy.

 

[RockyMtnMT]

Stability/rpm is a major component to proper terminal performance. Through our testing we have figured out that ballistic stability and terminal stability are two diff things. The stability calculators will calculate ballistic stability. Basically, how much spin is required for a given projectile to pass through the atmosphere without upset. Has nothing to do with after impact. There is no calculator for terminal performance. Over time we have come up with our minimum required twist for Hammer Bullets. This revolves around terminal performance. We use the Miller formula calculated at standard atmosphere at sea level for 1.5 sg as the minimum for hunting. Higher stability/rpm is better for terminal performance. This will keep the bullet oriented forward longer after impact for deeper straight line penetration. I personally like my hunting rig running closer to 2.0 sg at sea level to ensure proper terminal performance and deep straight line penetration.

There is a line of too much twist for any bullet. Jacketed bullets struggle with separation of the core from the jacket if spin is too high. Resulting in failure. We have yet to find the line of too much twist for Hammers. There has to be a line, we just haven't reached out yet. Since they are pure copper there is no separation issue. The only issue I can foresee would be bullet slipping in the rifling. So far out PDR drive band technology has not failed. The only down side to faster twist that we have seen is with heavier larger calibers with lots of horsepower causing more rifle torque. Like running 400g .375 cal bullets over 3000 fps in an 8" twist. You wind up having to hang onto the rifle more to control it. It doesn't come into play with lighter bullets.

 

[sedancowboy]

Have designers taken barrel twist/bullet RPM into consideration when designing these bullets and as twist rates get faster, ranges increase, bullets get longer and velocities get higher are these contributing factors to bullet design and performance?

Yes but the twist follows the bullet design. As bullets get longer and sleeker the twist to stabilize them has to increase.

Can you spin a bullet too fast to inhibit performance, increase failure or increase the chances of getting penciling?

You can not over stabilize a bullet. However you can spin the jacket off of cup and lead core bullets.
While there are some articles out there about bullets hitting the target nose up if the twist is too fast Bryan Litz disputes that completely. Fast twists tend to cause better expansion instead of penciling.

Does spin have an affect on performance and has a single design been subject to testing at differing velocities, RPMs and ranges for terminal performance or has the data been gathered?

Absolutely spin has a lot to do with performance. Steve from Hammer bullets believes and has tested that bullet performance is increased with more stability instead of minimum stability.

My personal experience with ELD-Xs are the opposite of yours they tend to blow up before penetrating far enough. As with most things in life guns are a compromise of all the desired traits we want in a gun. We want light weight but low recoil, we want short barrels for carrying but high velocity for performance. Well you get the idea. I choose a bullet first, then I choose a cartridge that meets my velocity goals. Then I choose the barrel that will give me the performance and stability I need, then the action and stock.
Henry

 

[Mike D Texas]

The biggest issue comes in finding a twist that will work for both cup/core bullets AND monos. You have to sacrifice somewhere and there will be trade offs.

If you pick a fast twist, you potentially box yourself into only shooting mono bullets. If you pick a slower twist then you have to shoot a lighter weight mono typically and sacrifice bc. Whether or not penetration is compromised is another topic.

 

[RockyMtnMT]

I think many of the stories of bullets failing to open and deform for proper terminal performance are not bullet failures, but a failure to have enough stability to keep the bullet point on long enough to get fluid into the hollow point to cause the bullet to open from the inside out. If a bullet impacts with a yaw then it is much more likely that it will not get fluid into the hollow point for proper deformation. Tipped, exposed lead, or open hollow point, are all hollow points. The tip is a plug in the hole that must evacuate to allow fluid in to cause opening from the inside out. Exposed lead tip basically has fluid already in the hole. Just needs point on pressure to cause the lead to become liquid and cause the inside out opening. This is likely the easiest to deform depending on lead hardness and jacket. Then the exposed open hollow point. Size of hollow point and material hardness will dictate consistency of opening. The smaller the hollow point the more critical the angle of impact to get fluid into the hollow point. 1mm hollow point we found too inconsistent to market as a hunting bullet.

For us the chase for higher bc is hard to resist. We are long range hunters. We continue to test but will not sacrifice terminal performance for anything. We will be doing some more tip testing soon. To date we have yet to find a tip that will perform as well at low velocity as the open hollow point. Why have a higher bc bullet that will not perform at as low of impact velocity? You are better off running the consistent performing open hollow point with lower bc that will perform at lower velocity. High bc bullet that needs more velocity to open reducing it's range is counter productive. Let alone the risk of the tip not evacuating the hollow point at higher velocity increasing the chance of failure compared to an open hollow point. Keeping the chance of failure as low as possible is most important.

I hope that someday I will have to eat crow when it comes to tipped bullets. I am not holding my breathe.

 

[RockyMtnMT]

The biggest issue comes in finding a twist that will work for both cup/core bullets AND monos. You have to sacrifice somewhere and there will be trade offs.

If you pick a fast twist, you potentially box yourself into only shooting mono bullets. If you pick a slower twist then you have to shoot a lighter weight mono typically and sacrifice bc. Whether or not penetration is compromised is another topic.

I think lead core bullets hold up to faster twist much better than even ten years ago. Certainly better than 20 years ago. Long range and horsepower have pushed the products to better capabilities.

 

[FEENIX]

You can not over stabilize a bullet. However you can spin the jacket off of cup and lead core bullets.
While there are some articles out there about bullets hitting the target nose up if the twist is too fast Bryan Litz disputes that completely. Fast twists tend to cause better expansion instead of penciling.

I agree, esp. coming from a Ballistician like Bryan Litz.

 

[DJ Fergus]

Hopefully I am not beating a dead horse here or asking an ignorant question.

I have read about twist in relation to bullet stability and velocity in relation to bullet performance at long range. I just can't recall about reading if RPM contributes to bullet performance (i.e mushrooming and transfer of energy).

I know we have all read instances of penciling, the ELD-X bullets are one I experienced first hand and at a variety of ranges.

My questions are:

Have designers taken barrel twist/bullet RPM into consideration when designing these bullets and as twist rates get faster, ranges increase, bullets get longer and velocities get higher are these contributing factors to bullet design and performance?

Can you spin a bullet too fast to inhibit performance, increase failure or increase the chances of getting penciling?

Does spin have an affect on performance and has a single design been subject to testing at differing velocities, RPMs and ranges for terminal performance or has the data been gathered?

Is there a way to know you are in the sweet spot in relation to twist and bullet/load selection other than accuracy at the range?

I am in the process of deciding on a new barrel and other than stability for the chosen bullets, I am wondering what other effects twist will play with my selection.

Thanks Darryle

Theory & testing has indicated: more twist equals more mushroom.