What RPM stabilizes a 75 grain .223 Rem bullet?

[sirgilligan]

What RPM stabilizes a 75 grain .223 Rem bullet?

You see printed on the boxes of bullets: Requires 1:9 twist or Requires 1:7 twist.

But isn't it really the bullet requires a certain RPM?

I was talking with my cousin and his 1:9 twist .223 will not stabilize 75 grain bullets while my 1:9 twist rifle does. So, I started doing some math.

MV 1:9 1:8 1:7
2500 200,000 225,000 257,142
2600 208,000 234,000 267,428
2700 216,000 243,000 277,714
2800 224,000 252,000 288,000
2900 232,000 261,000 298,285
3000 240,000 270,000 308,571
3100 248,000 279,000 318,857
3200 256,000 288,000 329,142
3300 264,000 297,000 339,428

A test of a .223 Rem rifle where the barrel was cut 1” shorter for each test firing M855 ammo showed these velocities:

26” 3280 fps
25” 3229
24” 3188
23” 3169
22” 3158
21” 3117
20” 3097
19” 3060
18” 3052
17” 2972

That is an average loss of 30 fps with each loss of one inch of barrel.

So, a 1:9 twist 26” barrel is producing approximately the same RPM as a 1:8 twist 18” barrel.

Does anyone know the RPM required for .223 bullets? Is there some study or information somewhere that tells this information?

RPM = MV * 720 / Twist
RPM = 2500 * 720 / 9 = 200,000 RPM

It would be a lot easier to try and figure out if a bullet would work or if you are on the edge, etc, if the RPM needed for stabilization was listed.

I have shot Hornady Superformance 75 grain match ammo through my rifle and it stabilized. I do not have a chronograph, but the ammo states an MV of 2930 fps. The Hornady info doesn't tell what length barrel was used to get that velocity, but the Hornady reloading manual says they used a 26" barrel, so I am going to assume that is what was used making their Superformance ammo.

My rifle has a 26" barrel.

So, I deduce that an RPM of 232,000 is sufficient to stabilize the 75 gr BTHP bullet.
If you had a 22" barrel, to get to 232,000 RPM you would need to have a 1:7 twist barrel, a 1:8 twist would be right on the edge.

Does anyone have a list of RPM's needed to stabilize bullets?

 

[higgite]

I don't know of any one table since there are so many variables involved, but if you google "bullet stability calculator", you'll find a bunch of calculators.

 

[Rule3]

This page will give you recommended twist rates for bullet weights and velocity fps I do not know about RPM's in bullets. There are links at the bottom for you to research

http://www.6mmbr.com/223rem.HTML

 

[sirgilligan]

There are at least two ways to change the RPM of a bullet.
1) Change the twist rate of the barrel and keep the same velocity.
2) Change the velocity and keep the same twist rate.

 

[moxie]

You're off to a really good start, but you absolutely need a chrono to complement your math.

I'm not aware of any "list of RPMs" for bullets. You've accounted for some of the variables involved, but there are others such as friction between the bullet and barrel, BC, etc. But you need actual velocity of a specific bullet, driven by a specific load, exiting a specific barrel to begin the search for your answer. And to do this requires a chrono. Deductions "might" get you in the ballpark, but maybe not.

 

[higgite]

There are at least two ways to change the RPM of a bullet.
1) Change the twist rate of the barrel and keep the same velocity.
2) Change the velocity and keep the same twist rate.

Or change both.

 

[LocoGringo]

Gilligan, what length barrel was your cousin's rifle that doesn't stabilize the 75 grain bullets? It sounds like your rifle is a bolt action rather than an AR and are your rifles actually chambered for .223 Remington or are they chambered some other way like 5.56 NATO or .223 Wylde?

I ask this because I shot an AR-15 recently that has a stainless Wilson Combat 1:9 twist 16 inch barrel that stabilized 2 different commercial loadings with 75 grain bullets and the Federal GMM match 69 grain loading, so I'm POSITIVE it didn't get to the RPMs necessary as you assert. I'm not trying to say you're wrong, just curious to solve the puzzle. I didn't actually expect the 75 grain loadings to be stabile, but the box of the Hornady ammo said "for 1:10 to 1:7 twist rates". I was surprised to read the 1:10 twist rate included.

 

[moxie]

LocoGringo,

Very good observation.

Fact is, some bullets are just simply designed better from a stability standpoint and don't need as much spin to be stable as other bullets of the same weight/length. Bullet weight vs. bullet length is also a big factor.

 

[Rule3]

You want straight answers from the guys that know their stuff? Call these guys. They are most helpful.

http://www.sierrabullets.com/ask-sierra/talk-to-a-tech/

 

[taliv]

it's probably worth noting that the marking 1:7 or 1:9 or whatever isn't very precise and may not be all that accurate either. depending on the method for making the barrel, the twist can and does vary along the length (i.e. it might be 1:6.8 in one spot and 1:7.2 in another).
and that could certainly affect any "cut an inch off at a time" type barrel tests too

if you are studying the matter, i would recommend measuring the last few inches near the muzzle and using that in your calculations.

 

[ClarkEMyers]

Short answer - RPM depends on air density and also on bullet speed so there is no simple answer to a complex question - the bullet makers give a useful answer.

(should be easy to believe aerodynamic effects vary with speed see e.g. area rule for supersonic aircraft - pushing bullets faster to increase RPM doesn't really work see e.g. John Barsness writing in Handloader for explanation with little math see Bryan Litz for explanation with all the match anybody wants - for instance consider the wind drift - ballistic coefficient - in a .22 rimfire is proportional to lag time that is the difference between flight time in a vacuum and flight time in atmosphere and notice that the faster a bullet starts the faster a bullet slows down changing effective ballistic coefficient)

See the Berger Bullets website for a nice implementation of a standard calculator with advice and explanation by Bryan Litz - or see his own web site and books. Folks can enter their own data and compile their own list of required RPM for the conditions they specify. But again figures for standard meteorological conditions are about as useful as using the TV weatherman's barometric readings as normalized to sea level equivalent to know the local air pressure for a cooking recipe say in a house on the front range. The bullet makers reflect reasonable experience and testing to give users a satisfactory experience with the maker's bullets. They are not out to lose sales but they don't want to sell to a customer who won't like the results. The mention of know what the actual twist of a given barrel really is by actual measurement is equally correct. Of course hammer forged barrels what is on the tooling is what you get; button rifled maybe not so sure. The .223/5.56 is of course made in a wide range of twists so it really pays to measure that as well as .244/6mm Remington and others that have varied a lot over the years.

Factors include air density which means altitude, temperature, humidity and all the rest impact external ballistics. For this among other reasons experience does not transfer well - indeed results may vary from day to day.

It's also true that a barely stabilized bullet has more drag than a fully stabilized bullet that is gives up a lot of ballistic coefficient to a fully stabilized bullet - complex notion but I don't know a better way to say it.

As a practical matter it means that although I can drive the lovely Hornady plastic tipped boat tailed that is long 53 grain bullet fast enough from a 26" barrel Swift 1/14" twist at 5000' on a dry summer day to get round holes in paper I do NOT get the advantage of the a high ballistic coefficient and less wind drift from a barely stabilized bullet so all I gain is the ability to shoot more expensive bullets poorly.

Bottom line pushing bullets faster to get a longer bullet to barely stabilize gains nothing in performance over shooting a shorter bullet that is fully stabilized.

 

[sirgilligan]

Gilligan, what length barrel was your cousin's rifle that doesn't stabilize the 75 grain bullets? It sounds like your rifle is a bolt action rather than an AR and are your rifles actually chambered for .223 Remington or are they chambered some other way like 5.56 NATO or .223 Wylde?

I ask this because I shot an AR-15 recently that has a stainless Wilson Combat 1:9 twist 16 inch barrel that stabilized 2 different commercial loadings with 75 grain bullets and the Federal GMM match 69 grain loading, so I'm POSITIVE it didn't get to the RPMs necessary as you assert. I'm not trying to say you're wrong, just curious to solve the puzzle. I didn't actually expect the 75 grain loadings to be stabile, but the box of the Hornady ammo said "for 1:10 to 1:7 twist rates". I was surprised to read the 1:10 twist rate included.

My cousin's is a Les Baer and I think he said a 20" barrel. Mine is a bolt action with a 5.56 Chamber, 26" barrel, target crown.

What I have noticed is that when we are talking about 75 grain bullets we are not saying which bullets. I have to ask my cousin which bullet it was.

I shot the Hornady 75 Grain BTHP. It is shorter in length than the 75 grain A-Max.

I think the longer they are the more RPM's are needed.

 

[sirgilligan]

Short answer - RPM depends on air density and also on bullet speed so there is no simple answer to a complex question - the bullet makers give a useful answer.

(should be easy to believe aerodynamic effects vary with speed see e.g. area rule for supersonic aircraft - pushing bullets faster to increase RPM doesn't really work see e.g. John Barsness writing in Handloader for explanation with little math see Bryan Litz for explanation with all the match anybody wants - for instance consider the wind drift - ballistic coefficient - in a .22 rimfire is proportional to lag time that is the difference between flight time in a vacuum and flight time in atmosphere and notice that the faster a bullet starts the faster a bullet slows down changing effective ballistic coefficient)

See the Berger Bullets website for a nice implementation of a standard calculator with advice and explanation by Bryan Litz - or see his own web site and books.

Factors include air density which means altitude, temperature, humidity and all the rest impact external ballistics. For this among other reasons experience does not transfer well - indeed results may vary from day to day.

It's also true that a barely stabilized bullet has more drag than a fully stabilized bullet that is gives up a lot of ballistic coefficient to a fully stabilized bullet - complex notion but I don't know a better way to say it.

As a practical matter it means that although I can drive the lovely Hornady plastic tipped boat tailed that is long 53 grain bullet fast enough from a 26" barrel Swift 1/14" twist at 5000' on a dry summer day to get round holes in paper I do NOT get the advantage of the a high ballistic coefficient and less wind drift from a barely stabilized bullet so all I gain is the ability to shoot more expensive bullets poorly.

Bottom line pushing bullets faster to get a longer bullet to barely stabilize gains nothing in performance over shooting a shorter bullet that is fully stabilized.

The concepts of External Ballistics are something I don't fully understand, but I do have an above average grasp of the subject. I wrote a ballistics calculator, and I read many different papers, and had to get my dusty Calculus and Physics books out. It was very rewarding.

The thing that you say that really should sink home is your last statement. It is better to fully stabilize.

I was just trying to explain to my cousin why his 1:9 wasn't behaving exactly like mine. I expect that the 6" more barrel I have accounts for 100 fps or more and that extra speed results in more RPM.

 

[ClarkEMyers]

Agreed on the extra barrel length gives more RPM and so improves stability

Agreed on the extra barrel length gives more RPM and so improves stability.

And the extra barrel length gives more speed and so increases the air resistance - more molecules hit per unit of time - so rpm is increased AND rpm needed is increased for not much net gain.

My own 26" barrel comes closer to stabilizing the Hornady bullet I mentioned (lovely bullet with a high nominal and I emphasize nominal ballistic coefficient) than a 24" barrel does for a shooter who lives down the road. That is on a good day he gets round holes on most days I get round holes. It's still not a wise choice of bullet for either of us.

Again using the calculator on the Berger Bullets web site - or roll your own, or implement on a spread sheet or buy books and discs from Bryan Litz - or even just paper and pencil it's not hard to check sensitivity by plugging in different numbers and seeing what comes out.

For most people most of the time the practical differences in conditions - air density - overpower any differences in velocity. Folks do use a long barrel to keep the .308 supersonic all the way to 1000 yard targets but mostly fast twist and normal velocity range is on the firing line.

 

[moxie]

Yes, ceteris paribus, the longer bullet needs higher RPM. But you will never encounter ceteris paribus in the real world.

 

[MtnCreek]

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

 

[judgedelta]

If you are talking about 3000 feet per second, times 60 seconds per minute, you would get 180,000 feet per minute, at least to start with. If the bullet turns every 6 inches you would get 360,000 rpm's, at least to start with. That will turn a soft-nose 62 grain bullet into smoke at about 75 yards...

 

[Archie]

The problem with trying to increase velocity is the air resistance is increased with more velocity. Air resistance is the primary cause of instability of projectiles.

I have a Remington 700 in .22-250 Remington; 1-14 twist. I had the bright idea that 69 grain BTHP bullets would be useful. They wouldn't stabilize. So I found a powder that would shoot them faster. They were unstable at a higher velocity. (Seriously, the best groups were six to eight inches at 100 yards with most of them straight on, rather than sideways.)

However, a 70 grain flat based, round nosed bullet shot fairly well. (50 to 55 grain bullets shoot very well.)

As you seem to suspect, the type (shape) of bullet makes a difference. Length is the controlling factor for spin and stability, not weight.

So, increasing velocity to get stability from incorrect twist rate is not very successful.

 

[joneb]

I ask this because I shot an AR-15 recently that has a stainless Wilson Combat 1:9 twist 16 inch barrel that stabilized 2 different commercial loadings with 75 grain bullets and the Federal GMM match 69 grain loading, so I'm POSITIVE it didn't get to the RPMs necessary

At what distance were these bullets stable?

 

[LocoGringo]

100 yards. I say stable because the holes in the paper were round rather than oblong or obvious keyholing.

 

[joneb]

100 yards. I say stable because the holes in the paper were round rather than oblong or obvious keyholing.

With that bullet in .223 Rem. some would expect good results at a far greater distance.

 

[LocoGringo]

Not sure what you mean joneb. I was just testing this rifle as it was new-to-me and was curious if it would stabilize the two types of 75 grain ammo I had. I had heard that 1:9 was marginal for 75 grain, so I wanted to test it. This rifle seems to be able to handle it. I know that ammo is meant for much farther, I was just testing. It may be that the ammo began to lose stability at 200 yards since it would be going slower at that distance. I don't know.

I was just playing around to find out and had tested 8 different ammo manufacturers at 100 yards through that rifle that day to get an idea of what it liked and didn't like. Turns out it likes Federal GMM 69 grain ammo, HPR and Hornady 75 grain ammo and PMC X-tac 55 grain.

 

[joneb]

Not sure what you mean joneb.

Stable at 5-600 yards with enough velocity.

 

[LocoGringo]

Ok, for that application, I have no idea. I really wouldn't try that with a relatively lightweight 16" barreled carbine.

I had fun testing though.

 

[iagbarrb]

Nice topic. But RPM is just a part of the whole equation. Higher twist velocities will move the bullet to the right. If air preassure increases you will notice this effect even better. Air density increases with speed, etc etc etc. So.... RPM is one of all the factors you shoud consider for bullet stability.