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Projectile Lengths

33K views 27 replies 16 participants last post by  Don Fischer 
#1 ·
Purpose of this thread is for the membership here to use for calculating twist rate requirements without having to go buy a box of whatever bullet just to find out it wont work.

Feel free to measure any bullets you use, handgun and rifle alike and post the measurements here. Make sure you list the bullet diameter along with the length measurement.

Here is a handy link to a twist calculator I use quite often and it will fit in with the use of this thread quite nicely.
http://kwk.us/twist.html
 
#2 ·
Here are the ones I have in stock.

200 LSWC Dardas, .452" dia., .652" length
200 Berrys TCFP, .452" dia., .531" length
185 Hornady XTP, .451" dia., .530" length

240 Hornady XTP, .430" dia., .703" length

125 Hornady XTP, .357" dia., .547" length

125 LRN Dardas, .356" dia., .602" length

100 Berrys HBRN (.380 ACP), .356" dia., .485" length
102 Golden Saber (.380 ACP), .355" dia., .473" length


174 Sierra MatchKing, .311", 1.206"

180 gr Winchester FailSafe, .308" dia., 1.303" length
175 Sierra MatchKing, .308" dia., 1.232" length
168 Hornady Amax, .308" dia., 1.278" length
165 Hornady InterBond, .308" dia., 1.312" length
155 Hornady Amax, .308" dia., 1.214" length
110 Sierra Varminter, .308" dia., .803" length

140 Hornady Amax, .264" dia., 1.380" length

105 Hornady Amax, .243" dia., 1.234" length
85 Sierra GameKing, .243" dia., .938" length

69 Sierra MatchKing, .224" dia., .895" length
62 Barnes TSX BT, .224" dia., .939" length
53 Sierra MatchKing, .224" dia., .700" length


I will make it a point to update as I get new projectiles.
 
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#4 ·
Not at all. its the total length of the bullet that makes it require a specific RPM, and Bullet RPM is a fuction of velocity and rifling pitch.

Take the 165 Hdy IB I listed above VS the 180 gr Failsafe also listed above. The failsafe requires 13.2 twist (180 gr bullet measuring 1.303" at 2700 fps, which is .30-06 velocity for that weight) and the IB requires 13.4 twist (165 gr bullet measuring 1.312 at 2800 fps, which is also .30-06 velocity for that weight) and the Failsafe has almost twice the bearing surface because it doesnt have a Boat tail. Now slow them both down in reduced loads to 2500 fps and the FS requires 12.7 and the IB requires 12.7, so internal ballistcally speaking they are equals in thier requirements as far as barrel twist is concerned.
 
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#6 ·
yes, because the velocity multiplied by the rifling pitch is what gives you the required RPM. Its why .300AAC blackout barrels are generally 7 or 8 twist.
 
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#9 ·
copper clad lead core jacketed bullets are 10.9
 
#14 ·
how do we determine the specific gravity of said projectile? Sierra's website does not list that data
Specific gravity is the ratio of the density of the subject object to the density of water. Since the density of water is, by definition, 1.00, the density of the bullet in question is the specific gravity.

To measure it, fill a graduated cylinder with water to some level less than full, and record the level. This will be in mL, but at room temperature a mL is the same as a cc. Drop in a bullet and record the new level. Since all but really weird bullets sink, this new level, minus the previous level, is the volume of the bullet.

Divide the weight of the bullet by its volume, and you have its density. Of course, bullet weights are given in grains, and densities are reported in grams per cubic centimeter, so we have to do a bit of math, but it's not hard. Since a pound is 7000 grains, the weight of the bullet divided by 7000 is its weight in pounds. And since one pound is about 454 grams, multiplying the bullet weight in pounds by 454 yields its weight in grams. A simple division of the weight in grams by the volume in cubic centimeters (ccs) yields the density of a bullet.

Of course, most normal people don't own a graduated cylinder - I do, but I'm a bit odd - so this isn't terribly practical. Another option is to fill a standard measuring cup with water about halfway and record the level. Then toss in a hundred bullets and record the change in level. The difference will be the volume of 100 bullets, so you can simply divide by 100 to get the volume of one bullet.

I have to ask, though, why anyone would care about the specific gravity of a bullet. It's essentially a density problem, and there is very little variation in density among all the various bullet types of a given weight. A 100 gr hard lead bullet will have a density only slightly higher than a copper-clad FMJ bullet of the same weight, and I really doubt that the difference will have any influence on anything important.
 
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#16 ·
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Gundoc: New to this so plz explain. If the round spins per the rifling, how do you accomplish the "needs a higher twist rate?" Thanks....
Run it at a faster velocity. If the cartridge cannot be pushed fast enough to stabilize the round without overloading the cartridge to dangerous pressures then you will need to rebarrel to a faster twist rate.

a 1:9 .223 barrel will stabilize up to 69 gr bullets at .223 velocity. The same twist rate in a .22-250 will effectively stabilize up to 80 grain bullets because the .22-250 is capable of 600ish fps more velocity than the .223 is. And they both use .224" bullets. Its also why the .22-250 usually has a 1:12 twist barrel instead of the 1:9 a .223 has.

Also smaller diameter bullets are harder to stabilize than larger diameter ones.

Then you have the problem of overstabilization, which is an issue considered when shooting precision long range.

Basically you want to select a bullet that will stabilize precisely where the rifling pitch and velocity capability of the cartridge put the RPM for optimal accuracy. This is why most .223 barrels 18-22 inches long with a 1:9 twist are ridiculously accurate with 69 grain sierra matchkings. Its right where that bullet likes the rotational forces to be.
 
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#17 · (Edited)
JLA: Would it be correct to say, to use a chronograph and target, the best accuracy would not be necessarily be the max velocity at a specific distance. That best round for that distance may not be the best round for a significant increase in distance. Or did I read to much into your comment?
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Opps, forgot the purpose of this thread. Got into the read of this and went on a tangent, my bad.
 
#18 ·
Its all good.. Learning is occurring and THAT is what this forum is all about.

Accuracy does not mean a specific velocity or a specific pressure in general. It is a specific pressure that produces a specific velocity that yields a specific barrel time for THAT rifle. And there is absolutely no way to know exactly what that load is without testing for it.

I started a thread up in the stickies of ammo and reloading forum titled Load Development: Ladder Test. Check it out it gets into detail on how to easily and effectively test your rifle for just that load.

The purpose of this thread here is to determine specific projectile length, so one can calculate the proper velocity/rifling pitch required to stabilize it.
 
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#19 · (Edited)
So is the twist rate generated by the calculator the minimum rate meaning that a faster twist would be good (up to a point)?

Here are a few I measured:

.223 Rem
Hornady No. 2266, .224 dia, 55 gr SP W/C, 0.690"
Hornady No. 2267, .224 dia, 55 gr FMJ-BT W/C, 0.739"
Sierra No. 1390, .224 dia, 55 gr HPBT, .722"
Hornady No. 2270, .224 dia, 60 gr SP, .745"
 
#20 ·
Here are a few pistol bullets I have in stock

.45 ACP
Rainier, 230 gr, plated (enc) RN, .452 dia X .658
BBI, 230 gr, LRN (moly coated), .452 dia X .677
Hornady, 185 gr FMJ-SWC, .451 dia X .528

.40 S&W
Precision Delta 165 gr FMJ-FN, .400 dia X .543
Precision Delta 180 gr FMJ-FN, .400 dia X .587
Hornady 155 gr XTP, .400 dia X .617

9MM
Precision Delta 115 gr FMJ RN, .355 dia X .562
Hornady 115 gr. XTP. .355 dia X .541
 
#21 ·
Here are some I have:

.45 ACP-
Remington- 185 gr, JHP, .452 x .550
Extreme- 185 gr, Plated Flat Point, .452 x .494
Berry's- 185 gr, Plate Hollow Base Round Nose, .452 x .603

.40 S&W-
Berry's- 180 gr, Plated Round Shoulder, .400 x .581
Berry's- 165 gr, Plated Flat Point, .400 x .552

.38 Special-
MPS- 148 gr, Lead Wadcutter (DEWC), .359 x .564

9mm-
Berry's- 124 gr, Plated Round Nose, .356 x .589
Berry's- 115 gr, Plated Round Nose, .356 x .560
Winchester- 115 gr, Silver Tip Hollow Point (STHP), .355 x .535
Hornady- 90 gr, XTP, .355 x .448
 
#22 ·
So is the twist rate generated by the calculator the minimum rate meaning that a faster twist would be good (up to a point)?

Here are a few I measured:

.223 Rem
Hornady No. 2266, .224 dia, 55 gr SP W/C, 0.690"
Hornady No. 2267, .224 dia, 55 gr FMJ-BT W/C, 0.739"
Sierra No. 1390, .224 dia, 55 gr HPBT, .722"
Hornady No. 2270, .224 dia, 60 gr SP, .745"
Yes exactly.

A calculator will provide a minimum twist rate requirement for the given velocity and projectile. Faster is OK to a point.
 
#23 ·
I advocate using twist rate formulas like the linked one or even just the Greenhill original formula but the proof of the pudding is in the eating. The formulas give you a starting point. You still have to test to find the bullet the gun likes, by weight and construction.

You can use fast twist with short bullets if the bullets are up to it. I have seen short bullets shot in fast twist disintegrate before reaching the target from the centrifugal force of rotation. Others report here of bullet/veloctiy/twist rates of bullets not suppose to stabilize having fine accuracy. Science is wonderful and near perfect when all the fact are used as input information. But with guns and bullets we don't have all the scientific data needed for it to be perfect. The variables for gun/bullet accuracy are huge and simulations and science only get us close. Testing gives the true answer. The formulas only help discard the bullet/velocity/twist rate combinations that are a far way out of the ball park.

LDBennett
 
#25 ·
Gotta agree with LD. I use several different formulas when developing loads, especially for those unknown and obsolete cartridges. But, sometimes the unexplainable happens. That is, what shouldn't work, works.
 
#27 ·
Post #14 could use a small addition--ALMOST added just before the word same.....This will be in mL, but at room temperature a mL is the same as a cc.

ml and cc ALMOST the same.

Poster #2 and several others-Im assuming you are from Tex buy your info.. I also see you use Dardas bullets. I was in Matts production area back in May. Neat. I got to see some of my bullets being made/lubed.

Matt is a champion shooter from some years ago. I bet he used his own bullets, that would make it double neat.
 
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