Although I've used the jbm calculator a few times I couldn't tell you if it were true on output ? Whether there's any difference on tips, shapes, bearing surface or other points? I've mainly used for test on lengths vs diameter on twist requirements.
Twist vs Bullet Weight Question
- Thread starter Small Lady
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Calvin45
Well-Known Member
I feel like I can somewhat relate…not regarding forum people but family and friends. I've learned the wisdom of keeping my mouth shut as I get older but a younger me was someone my friends and family HATED watching movies with…cuz I'd always point out the plot holes (even in supposedly realistic ones) and ruin the show!
Hugnot
Well-Known Member
Would/should this occur upon shortening a hollow point bullet then plugging in a nice plastic point?
Imagine - the 178 grain Hornady hollow point boat tail bullet shortened by .14 inches, then having a .14-inch-long plastic point replacement resulting in the bullet having an unchanged length. The weight loss of .14 inches of copper jacket is estimated to be 2.5 grains (hollow point jackets thicken at tip end). The 2.5 grains of removed jacket were subtracted from 178 grains resulting in 175.5 grains. The plastic weight was interpreted as being zippo.
Both bullets were subjected to the Miller Sg estimator that showed:
.308 178.0 grain Hornady hollow point boat tail, Sg = 2.02
.308 175.5 grain Hornady plastic point demo, Sg = 2.46
Replacing the plastic tip with the extremely increased density lead would shift the CM forward, add weight, & affect Sg. Next installment will make an attempt to demo this.
Hornady has or claimed to have defeated the problem of melting metplats with the ELDM & ELDX bullets. I have blazed away at distant rodents using the 6mm 87 VMax at just under 3,500 fps at 3000-4000 ft elevation at temperatures in the mid-eighties and have noticed no abject trajectory aberrations. The 6mm 87 Vmax has the old style, more orangy colored plastic tip. The aerodynamic qualities of the bullet should have changed but my crude observation showed no change. Hornady has a 90 grain 6mm ELDX bullet that costs more than $10 per hundred than 87 VMax - don't think I will use it. Should bullets have angled pop out fins to move CP aft? What about rotation applied to something like that.
When I was a kid, one of my pals was Jewish. Whenever I could not figure out how or why stuff worked, I went to him for help. If he could not come up with a solution, his family would help. Lots of really high power, smart thinking, combined with extensive education & experience. I would parrot the solution, occasionally without a complete understanding. This really annoyed & PO'ed folks.
Playing around with the Berger, JBM, & my spread sheet shows the same or extremely close Sg values for the same bullet with identical data input. Berger is not into plastic points & uses standard aero/altitude tables for pressure vs. actual barometric pressure measured in inches of mercury.
Imagine - the 178 grain Hornady hollow point boat tail bullet shortened by .14 inches, then having a .14-inch-long plastic point replacement resulting in the bullet having an unchanged length. The weight loss of .14 inches of copper jacket is estimated to be 2.5 grains (hollow point jackets thicken at tip end). The 2.5 grains of removed jacket were subtracted from 178 grains resulting in 175.5 grains. The plastic weight was interpreted as being zippo.
Both bullets were subjected to the Miller Sg estimator that showed:
.308 178.0 grain Hornady hollow point boat tail, Sg = 2.02
.308 175.5 grain Hornady plastic point demo, Sg = 2.46
Replacing the plastic tip with the extremely increased density lead would shift the CM forward, add weight, & affect Sg. Next installment will make an attempt to demo this.
Hornady has or claimed to have defeated the problem of melting metplats with the ELDM & ELDX bullets. I have blazed away at distant rodents using the 6mm 87 VMax at just under 3,500 fps at 3000-4000 ft elevation at temperatures in the mid-eighties and have noticed no abject trajectory aberrations. The 6mm 87 Vmax has the old style, more orangy colored plastic tip. The aerodynamic qualities of the bullet should have changed but my crude observation showed no change. Hornady has a 90 grain 6mm ELDX bullet that costs more than $10 per hundred than 87 VMax - don't think I will use it. Should bullets have angled pop out fins to move CP aft? What about rotation applied to something like that.
When I was a kid, one of my pals was Jewish. Whenever I could not figure out how or why stuff worked, I went to him for help. If he could not come up with a solution, his family would help. Lots of really high power, smart thinking, combined with extensive education & experience. I would parrot the solution, occasionally without a complete understanding. This really annoyed & PO'ed folks.
Playing around with the Berger, JBM, & my spread sheet shows the same or extremely close Sg values for the same bullet with identical data input. Berger is not into plastic points & uses standard aero/altitude tables for pressure vs. actual barometric pressure measured in inches of mercury.
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Mikecr
Well-Known Member
Stability would go up, but BC would go down.
You could also fire bullets base first to put CG closer to CP. But again, BC would take a hit.
Doom2
Well-Known Member
For those interested it is important to understand that the twist equations that are normally used (Greenhill, Miller) are not geometry specific to the bullet and are geared to generate a minimum required twist rate. They are semi-empirically derived and contain approximations based on experimental data. To properly analyze the Sg and required twist requires more data about the bullet than is typically available. Miller was kind enough to allow his work to be posted on the JBM site.
Hugnot
Well-Known Member
During WWI, occasionally, .303 Brit & US Caliber .30 (.30-06) rounds had bullets pulled then reinserted point down. This exposed the lead core at the open end of the cup base. Sniper activity was common. Occasionally thick steel plates having small openings were used to observe & spot targets. The plates became targets & the thinking was that a hit from base forward bullet would deliver more energy to the steel plate & inflict injury to an observer.
Don't think this works - velocity would decrease & impact energy would decrease because energy uses velocity squared but time for bullet to consume its mass upon impact would decrease. Use of a pointy hi vel bullet with a depleted uranium core would work better. A local gun club sez NO to my .22-.250 for shooting steel.
Looking at the Miller process - 30 is multiplied by the bullet weight then the product of that value is divided by a blast of arithmetic including exponential math, ratios, & differences - sure looks like "semi-empirically derived and contain approximations based on experimental data." - around the common value of 30.
Don't think this works - velocity would decrease & impact energy would decrease because energy uses velocity squared but time for bullet to consume its mass upon impact would decrease. Use of a pointy hi vel bullet with a depleted uranium core would work better. A local gun club sez NO to my .22-.250 for shooting steel.
Looking at the Miller process - 30 is multiplied by the bullet weight then the product of that value is divided by a blast of arithmetic including exponential math, ratios, & differences - sure looks like "semi-empirically derived and contain approximations based on experimental data." - around the common value of 30.
Hugnot
Well-Known Member
I liked the video - it showed CP & CM.
Then -"These bullets spin real fast (over 200 thou rpm) because the gyroscopic forces are needed to keep the bullet flying in a good point first path (no tumbling) plowing thru air (treated like a liquid) makes the center of pressure (CP) in front of center of mass (CM)"
Various projectiles used against armor have fins to maintain CP in front of CM. Pop out bullet fins with no rifling?
The Berger video uses the term "center of gravity". Calculations for kinetic energy use the term "slugs" for mass value, where the mass gravity acceleration on earth's surface is 32 ft/sec/sec. Bullets flying over the moon's surface would allow slower twists for stability.
Time for lunch & a cold beer.
Then -"These bullets spin real fast (over 200 thou rpm) because the gyroscopic forces are needed to keep the bullet flying in a good point first path (no tumbling) plowing thru air (treated like a liquid) makes the center of pressure (CP) in front of center of mass (CM)"
Various projectiles used against armor have fins to maintain CP in front of CM. Pop out bullet fins with no rifling?
The Berger video uses the term "center of gravity". Calculations for kinetic energy use the term "slugs" for mass value, where the mass gravity acceleration on earth's surface is 32 ft/sec/sec. Bullets flying over the moon's surface would allow slower twists for stability.
Time for lunch & a cold beer.
Small Lady
Well-Known Member
I'm just glad that I asked for a simple answer
Hugnot
Well-Known Member
Just wait until we get into the heavy stuff.I'm just glad that I asked for a simple answer
