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Author Topic: Ballistics question regarding bullet length  (Read 223 times)

Online Madgomer

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Ballistics question regarding bullet length
« on: October 17, 2018, 09:40:39 AM »
Apologies in advance, this question might get a little long.  In terms of figuring out what bullet might be a good match for a given gun, we often start with a consideration of bullet weight vs barrel twist rate to ensure there's enough spin rate for stabilization at your desired range.  My understanding is that bullet length is actually the critical factor rather than weight, although generally more weight equals more length so the weight discussion is directionally correct.
My question is:  let's say I have a spire point or hollow point at an equal overall length and weight to a ballistic tip pill and they're all similar in ballistic coefficient, does that lightweight nylon tip create the same stability characteristic as the spire or HP pill of the same weight and length, or does it act like a shorter pill with better stability at lower spin rate?  I probably need a physics lesson from Dutch or PA on this.
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Offline BoBallistic

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Re: Ballistics question regarding bullet length
« Reply #1 on: October 17, 2018, 07:01:50 PM »
Mad - I'm no Dutch or PA but I will try and answer this....now the answer may get a little longer......First a history lesson - if you look back at the Nosler Ballistic Tip Pill/Bullet it had a similar questions that was asked way back when, about 25 years ago....When it comes down to the bullet length, even the Greenhill Formula includes the polymer tip in the overall length of the bullet when it comes down to spin rate.......folks at all the major bullet manufacturers run their own test and if you look at the latest and greatest Hornady ELD-X bullets design, they include the polymer tip when they calculate ballistic coefficients....

Lets look at two of today's most popular rounds, the 6.5 Creedmoor and the 260 Rem, both are very similar in ballistic coefficients and performance, although the bullets on the 6.5 CM are usually heavier this is because the CM has a longer neck than the 260 does so therefore you can seat longer bullets in it....the old Europeans caliber and rounds of yesterday look almost disproportion to their American calibers...they seat the bullets way out this is due to the longer necks of their calibers....remember if a bullet cannot wider, then they can only get longer.....if you look at any bullet chart that tells you the length of the bullet it always includes the tip as well and the Ogive, the shank, the body, the boat tail angle and design of the bullet

How does that fit into this conversation, all I am trying to say is that the folks do include there tips in their overall lengths of their rounds, look at what Hornady has done, the old Ballistic Tip heats up and the point deforms at about 400 yards, that depends on ambient temperature, elevation, BP, bullet CG and a host of other form factors....but because they (Hornady) changed material of their points. If you go on-line and look at any bullet designs the CG of the bullets is the difference, as the bullet gets longer the CG changes and this does include the polymer tip of the bullet as well.....

But it all boils down to what bullet works in your rifle....you are going to have to put your range time in to find that out...and your targets will let you know instantly if the bullet if the right one for your rifle.....so your twist rate in your barrel does matter, believe me. Using the Greenhill Formula as a go-by and that how I start off with a caliber that I am unsure of, it that I will run the bullet diameter and the bullet length into this formula and will go from there......



 
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Offline yari

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Re: Ballistics question regarding bullet length
« Reply #2 on: October 17, 2018, 07:12:52 PM »
OK. believe what bo says.

if your question is will a spire or HP be better at long range, my impression and common sense/horse sense says the spire would be more accurate at long ranges
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Online Madgomer

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Re: Ballistics question regarding bullet length
« Reply #3 on: October 17, 2018, 10:14:40 PM »
Thanks Bo - your mention of the Greenhill formula gave me something to go dig into, and in doing that I found this article (link below) which covers exactly the point I was trying to understand.  Might make your head hurt a little, but the bottom line is that a plastic tipped bullet of equal total length and weight to an all-metal bullet of the same caliber will have better gyroscopic stability (can handle lower twist rates in the barrel or lower velocities without tumbling).  This is simply a result of the lower density of the plastic tip making the bullet behave like a shorter all-metal bullet in terms of gyroscopic stability.  You still have to pay attention to the true overall length in terms of max COAL for your magazine, etc, but from a stability standpoint it gets treated differently.

Now I need to do a little more digging and get my hands on the actual "Equation 1 and Equation 2" that they reference in order to figure things out for my specific situation.  I got into this question while trying to find a good bullet for 2 of the 3 different .223s that I reload for.  Those two have relatively slow twist rates so they beg for a smaller pill.  I have them both shooting decent with 40 grain ballistic tips but would prefer to have something a little heavier if I can get away with it.  The 53 gr Sierra soft point is one that many are speaking well of even though the BC is relatively low (it's very short).  I've done a little work with that in my Ruger M77 and it does look promising.  I would expect 95% of my shooting to be at 200 yds or less, so a lot of this is probably going to turn out to be an academic study that doesn't matter much, but always interested in learning more.

https://arxiv.org/ftp/arxiv/papers/1410/1410.5737.pdf

Thanks again guys!
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Offline gobihawk

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Re: Ballistics question regarding bullet length
« Reply #4 on: October 18, 2018, 08:23:06 AM »
I use the Nosler Varmageddon 55 grain in my .223 Wylde and .22-250. Shoots great in both. Pdogs hate them. May want to take a look at these.
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Offline BoBallistic

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Re: Ballistics question regarding bullet length
« Reply #5 on: October 18, 2018, 11:04:42 AM »
Mad - As we all know heat destroys barrels and bullet tips....as Dutch pointed out in a previous post that barrel life only last a few seconds!! That statement was echoed by PA a few years ago....and it you think about the flight time of a bullet it goes a 1000 yards in what a second or 1.5 seconds....amazing fast and the point of a bullet gets extremely hot no matter what it is made of....as I mention in the early days, the Nosler Ballistic Tip tip deformed after about 400 yards and then the accuracy goes out the window, the friction builds up enough heat in flight to deform the tip of the bullet.....

If you look at the SR-71, the famous spy plane of the 70 through the 2000's, it had to be fueled after take off because it leaked all over the place due to friction....it would seal up those leaks after getting airborne....heat is a funny element....

Now here come Hornady along and changes the game with its heat resistant bullet that does not deform through out the flight and heat friction it causes.....I bet all the manufactures were Hornady's first customers!!! Most manufacturers have copied Hornady Tip material and somehow incorporated into their own design....

There was one exception to the all of this was the 1960's Winchester Ballistic Tip...the tip was made of bronze and I have about a half of box left from way back then and wonder how they would hold up to the test they run on their bullets today? humm....

If you shoot only 200 yards, The Miller and Greenhill formulas are sort of a waste of time for you, but as you said it is good that you read this because you and I both learned something about the bullet tip....with a 22-250 and a 223 Wylde that Gobi shots and you shoot a a 223 like most of us do, then at 200 yards that is only a chip shot for those calibers...whether you use a tipped bullet or just a spire point or a hollow point, at 200 yards it should not matter what the tip ends up at....

Now has anybody thought about the type of deformed tip that the bullet encounters? It is different for every bullet at every time you pull the trigger...so what is the melting point of the spire points and hollow points?? Just depends of how long the shot is and the a bunch of weather conditions that can add up to a much different conclusion that what you had thought....

But like you Mad would love to dig into the Equation 1 and Equation 2 that they reference to see if it really matters to any of us.....thanks for your question....it gives us all pause and time to think about things!!

   

 
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Offline BoBallistic

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Re: Ballistics question regarding bullet length
« Reply #6 on: October 18, 2018, 05:56:15 PM »
After plundering through my old ammo boxes found a box of Remington 270 130 Gn Bronze Points, I stand corrected, it was not Winchester, but Remington Box I have similar to this box shown in the picture.....

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Offline yari

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Re: Ballistics question regarding bullet length
« Reply #7 on: October 18, 2018, 09:38:31 PM »
After plundering through my old ammo boxes found a box of Remington 270 130 Gn Bronze Points, I stand corrected, it was not Winchester, but Remington Box I have similar to this box shown in the picture.....

after seeing pics of your ammo collection that must have taken all day to find that specific box. @--0--0105 --9-908
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Offline Dutch-Hunter

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Re: Ballistics question regarding bullet length
« Reply #8 on: October 19, 2018, 07:59:04 AM »
Here's an article from Sierra to ponder. https://sierrabulletsblog.com/2015/05/07/understanding-twist-bullet-stabilization/

Another from AccurateShooter, http://bulletin.accurateshooter.com/2013/07/over-stabilization-of-bullets-why-is-too-much-spin-a-problem/

These articles should make an interesting read. When possible I'll expound on this in greater detail. This living on the road really hurts my online life.

Bo has done a good job of hitting the nail on the head. The mystery of why one gun shoots better than another has been a mystery for, well ever. This is one of the reasons why knowing your "actual" twist is important when chasing ultimate accuracy. The actual twist rate to the fraction is very important.
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