Bullet RPM and performance

[jpndave]

I have been chasing this a bit lately ordering a couple of barrels, one 6.5 CM and one 300 PRC. The general recommendations have been to keep it under 300,000 RPM to avoid bullets shedding jackrts. 9 to 8 t gain wist in the 30 Cal should be safe but the 6.5 is pushing it at 8 to 7. Ended up calling Bartlein to change to 8.25 to 7.25 as I kept worrying about it. The gain twist with 5R rifling is about as gentle rifling as is available in these barrels. I want to be able to shoot the heavy solids but not have the twist so fast regular bullets blow up so conventional will work too.

Litz gets into this in his books. If I am remembering correctly Nathan Foster might as well. It's been a bit since I read his stuff.

You can't really over spin a solid. Jacketed bullets run into two potential issues. The coming apart has been mentioned. The other is inconsistent jackets and the bullets getting thrown as they leave the barrel. The faster you spin the more exaggerated that can be. Modern match bullets that is minimal with the better jackets.

 

[Mike D Texas]

I only got 4 rds down range today in my 22creed and it would blow up every bullet after that cleaned it it was not that bad shoot 4 then it would blow up bullets ran them over magneto speed 3210 3203fps. 26" 1&7 twist bartlin barrel 240rds on it now throat looks good

That's odd. I've shot 50 grain VMAX at 3950 out of my 22-250AI with a 7 twist Bartlein and they hit their intended target.

 

[nralifer]

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

Bullet spin does add performance to all copper bullets that petal well, primarily because the petals are sharp so they cut through the tissue, rather than simply displace it, as the bullet spins, much like a propeller. The penetration in the expanded state is much greater. Spin adds stability and overspinning monometal bullets adds to their performance on game. As long as the stability factor is between 1.5 and 2.0 you should be good. Bullets that expand down to 1700 fps impact velocity are the greatest beneficiaries of this effects because they are designed to open more reliably.

 

[nralifer]

I have been chasing this a bit lately ordering a couple of barrels, one 6.5 CM and one 300 PRC. The general recommendations have been to keep it under 300,000 RPM to avoid bullets shedding jackrts. 9 to 8 t gain wist in the 30 Cal should be safe but the 6.5 is pushing it at 8 to 7. Ended up calling Bartlein to change to 8.25 to 7.25 as I kept worrying about it. The gain twist with 5R rifling is about as gentle rifling as is available in these barrels. I want to be able to shoot the heavy solids but not have the twist so fast regular bullets blow up so conventional will work too.

Litz gets into this in his books. If I am remembering correctly Nathan Foster might as well. It's been a bit since I read his stuff.

You can't really over spin a solid. Jacketed bullets run into two potential issues. The coming apart has been mentioned. The other is inconsistent jackets and the bullets getting thrown as they leave the barrel. The faster you spin the more exaggerated that can be. Modern match bullets that is minimal with the better jackets.

Remember, the RPMs achieved with any bullet is not only a function of barrel twist, but also the MV. So with a tight twist barrel, you will need to slow the lead core bullet down some if you see jacket separation issues at high speed. Also be aware the fast lead core bullets can fail miserably and hardly penetrate at all at high impacts speeds at short range. Especially true of the high BC lead core bullets. Best to keep the MV of lead cores between 2800-2900 fps

 

[hoffbill]

Bullet RPM's are the result of the combination of twist rate and muzzle velocity. A heavier bullet fired from the same case will likely have less velocity, and therefore may require a faster twist rate to achieve sufficient RPM's to be stable. For the same reason a lighter bullet fired from the same case will tend to have higher velocity thus the fast twist rate may create too many RPM's resulting in deforming or disintegration. Any bullet is going to have a velocity range that give optimum terminal performance on impact which will be based on the construction and materials of that bullet. Seems to me that optimum terminal performance will depend on impact velocity and whether that bullet is stable at that velocity. That is what will give best mushrooming and RPM has little to do with that other than providing the stability since the RPM will go to zero almost instantly on impact.

 

[Chrsm]

I want to thank everyone for the education, even if it is outside of my complete comprehension.

Yup definitely making me DIZZY 🌪

 

[RockyMtnMT]

Terminal performance is more dependent on rpm than that. The same bullet fired at the same velocity from a faster twist will out penetrate the slower twist. There is terminal stability that is different than ballistic stability. As a bullet penetrates the rpm's degrade rapidly. At some point during penetration the retained shank will run out of rpm. When that happens it will get to a point of instability and then tumble. Same bullet fired with higher stability will straight line penetrate farther before it tumbles. If a bullet is fired with just enough stability to fly ballistically but not enough for proper terminal performance you will see a bullet failure. If the bullet has a bit of yaw or wobble on impact and the nose is not point on at impact then it will not get fluid into the hollow point and expand properly. This is when you see bullets recovered that look like a banana. This can result in pretty destructive results if the bullet tumbles through on the line of travel. It can also result in a bullet that turns drastically and never gets to the vitals. It can also result in a bullet that does nothing and pencils. We learned this when we first started impact testing bullets in the beginning of marketing Hammers. We were low velocity testing to confirm proper performance at 1800 fps. Launched a bullet at 1800 fps and recovered it looking like it could be loaded again. The only distortion was engraving from the rifling. Very disappointing as this is our advertised min. We decided to increase the vel to find out the min for this particular bullet, but could not get any more powder in the case. We had another rifle of the same caliber with a larger case so we grabbed it. First launch was right at 1800 fps. I remember thinking " That was a wasted shot". Went and dug the bullet out of the media to find a perfectly deformed shank. Hmmm... The only diff was barrel twist. Lots of lightbulbs went on at that moment. Ran the stability numbers and the first built was about 1.2 sg and the second bullet was over 1.5 sg.

Just because you can get a bullet to an animal accurately does not mean the bullet will perform properly. Marginally stable for ballistics can shoot extremely accurately but significantly increase the odds of failure to expand.

 

[Greyfox]

Bullet RPM's are the result of the combination of twist rate and muzzle velocity. A heavier bullet fired from the same case will likely have less velocity, and therefore may require a faster twist rate to achieve sufficient RPM's to be stable. For the same reason a lighter bullet fired from the same case will tend to have higher velocity thus the fast twist rate may create too many RPM's resulting in deforming or disintegration. Any bullet is going to have a velocity range that give optimum terminal performance on impact which will be based on the construction and materials of that bullet. Seems to me that optimum terminal performance will depend on impact velocity and whether that bullet is stable at that velocity. That is what will give best mushrooming and RPM has little to do with that other than providing the stability since the RPM will go to zero almost instantly on impact.

I agree with this. For this reason, a LR hunter using a load that has the likelihood of being used on game at ranges from 100-1000 yards needs to choose his load carefully. For example, my 6.5x284 load demonstrates stability to my max range, but also will be also have sufficient retained velocity to expand well and release energy well on a broadside shot directed to the larger target/chest area at the furthest range. On the other hand, at short to mid-range that same bullet driven much past a MV of 2950-3000FPS may come apart and not effectively dispatch the game unless the bullet is surgically placed in a soft tissue vital area, a much easier task at the short/mid-range. IMO, all this requires a carefully chosen bullet/BC/velocity/twist rate that balances these all these effects, and needs to be well tested. Often, to meet the demands of the hunter that may encounter game over a wide range of distance, the traditional definitions, characteristics, and designs of what we define as hunting bullets and the load/rifle parameters goes out the window. IMO.
One of several VLD/Target /High BC, style bullets retrieved from game at the longer ranges(62% wt ret). It's rare to find much more the fragments from game shot at short to mid range. All spinning no more then 285,000RPM…..But they are all dead in short order….

 

[jpndave]

Remember, the RPMs achieved with any bullet is not only a function of barrel twist, but also the MV. So with a tight twist barrel, you will need to slow the lead core bullet down some if you see jacket separation issues at high speed. Also be aware the fast lead core bullets can fail miserably and hardly penetrate at all at high impacts speeds at short range. Especially true of the high BC lead core bullets. Best to keep the MV of lead cores between 2800-2900 fps

Yes, I am aware. Rotation = muzzle velocity * 720/twist. Wasn't trying to make a comprehensive post. The high RPM can come from twist, high muzzle velocity or usually a combination of the two. You can drive the lead core bullets faster (and it's done a lot). But the twist has to be lower to not over rotate the bullet.

For my rifles, both I am planning on ~ 3000 fps. So at 3000fps the 7-1/4 twist is pushing the 300,000 rpm limit @297,931rpm. If that same rifle gets a lightweight varmint bullet at say 3500fps it's over the edge by a bit @347,586rpm.

 

[david g ranes]

Probably due to not as much bearing surface contacting the rifling. David

 

[Lou270]

I think hammers probably require more twist to go straight because they blow the front off and end up a tube. The density of an animal is many times greater than air so i doubt any sg value we think of for external ballistics is applicable to same degree but seems like ups odds of not tumbling once front blows off A more traditional lead bullet expands and had a mushroom or some deformation up front along with a shorter shank. Even something like a barnes with a longer shank most of time has a mushroom. In either case the center of gravity is more forward so less likely to tumble than a skinny 'tube'. Partitions often end up backwards but have no issues expanding as they are pretty soft nosed. Any case, I am intrigued by hammers and have a bunch to try but I dont think all bullets are equal and to get the beneifts of hammers may need faster twist than other bullets by their unique design

Lou

 

[orkan]

Long ago I remember it vividly when I cracked the code of unbelievable ballistic effects when running light bullets in fast twist barrels.

You can render a prairie dog into bits no larger than a dime by running a 50gr V-max in an 8 twist 22-250. If that bullet comes in contact with so much as a blade of grass, it will detonate. Pointless to use on coyotes, as it will virtually guarantee a splash wound. Conversely, a 52gr SMK is ruthless on coyotes in an 8 twist due to its heavier construction. It detonates the same, but does so inside the coyote for lightning strike kills.

It probably goes without saying that the horrific wind performance of the little bullets means I will certainly stick with heavy bullets with those fast twist rifles. However, for specific things where fun is more important than consistent impacts at longer ranges... the fast twist light bullet combo can not be beat. The most savage impacts I've ever witnessed.

Large overbore wildcats can accomplish similar things in 6, 6.5, and 7mm on much larger animals.

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[Lou270]

Im playing devils adovcate here because I have thought about this a good bit and fail to see a benefit for big game hunting . A faster twist would increase the centrifugal force. So bullets fragments would zip off at more right angle to penetration. Bullet deformation happens pretty much immediately on impact so I can see why the faster twist shows up as more 'pop' on a small animal like ground squirrel. I cant see how that helps on a big game bullet where you want to scramble all the vitals not just the near side.

Lou

 

[RockyMtnMT]

Im playing devils adovcate here because I have thought about this a good bit and fail to see a benefit for big game hunting . A faster twist would increase the centrifugal force. So bullets fragments would zip off at more right angle to penetration. Bullet deformation happens pretty much immediately on impact so I can see why the faster twist shows up as more 'pop' on a small animal like ground squirrel. I cant see how that helps on a big game bullet where you want to scramble all the vitals not just the near side.

Lou

Our shed pedals will tend to track with the retained shank, not outwards. Even with faster than needed twist. Often we will see an exit with 3 or 4 little holes around it. I would not say that centrifugal force is the aid. The aid is keeping the bullet oriented point on for as long as possible.

I will add that it is not a Hammer phenomenon that bullets need stability to work properly for terminal performance. We just happened to be the 1st to bring up the issue. I saw pictures of banana shaped bullets, and herd complaints about lead core bullets not opening long before we started making bullets. The market was driven by longer range pursuits, causing bullet companies to market heavier bullets to chase the bc. Only one way to make a bullet heavier. It has to get longer, requiring more twist to be stable. Pretty amazing that a couple of farm boys can get the rest of the bullet industry to start talking about terminal stability. Flies in the face of lazy marketing gurus that tie their wagon onto the wagon in front saying that heavier is better and higher bc is better even though the factory barrels can't properly stabilize the bullets they are pushing. Knowing or not caring, or even worse not knowing that it is detrimental to terminal performance.

There is a reason that it is difficult to find anyone with an issue of terminal performance with Hammer Bullets. Because terminal performance is the reason that we started making bullets and it is the most important job that a hunting bullet has. It must do it's job on arrival, period.