Am I doing it wrong?

N

Nick_C_S

New member
So today, I'm playing around with my AR-15 and noticed in very small print on the barrel, under the keymod shroud, it reads: "5.56 x 45 NATO 1/7."

I knew it was a 1:7 twist (but don't remember how I knew this); but did not know the upper was chambered for 5.56 NATO.

So far, all my load workups are with 223 Remington data. Am I doing it wrong?

Background: I had my AR built by a knowledgeable friend back in '17. I told him I want a nice one and don't mind paying a few more $'s. He told me what parts (I already had the blank lowers) to order online and I did so. I took all the stuff to his house and he built it out. This was over 5 years ago and my memory of the whole thing is vague. And so is his: A couple months ago, I asked him about it and he had forgotten entirely that he built out my AR. (I know we never discussed twist rate - being completely new to rifles back then, I didn't even know twist rate was a thing.)

That was California in 2017. Now that I live in a free state, I have finally gotten around to doing load work ups.
 
No worries. .223 Rem will shoot perfectly well in 5.56mm. Your barrel will also support standard 5.56x45 ammo. It also means that you can shoot something other than 55gr FMJ through it. Faster twist means that heavier bullets will stabilize.
 
Thank you Markr6754. I know they're dimensionally the same and that the 556 has a higher max pressure, so I wasn't too worried there. But I wasn't sure if there's something about which I don't even know to be worried - if that makes sense. I don't want to be "missing something."

Faster twist means that heavier bullets will stabilize.
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Yes, this I have since learned. Unclenick has been really good at explaining procession and all that (being an avid bowler in a past lifetime has helped in understanding as well). In fact, having too much twist for my ammo was a potential problem: I was loading 55gn varmint bullets. Their light construction doesn't bode well to extreme RPM. Fortunately, my loads have been mild so bullet deconstruction never manifested as a problem. Further, I have no desire to push the pressures. Good cycling and consistent chronograph results are my goals.
 
There is even a small complication about the 'higher pressure' rating of the 5.56 NATO versus .223 Rem. The measurement techniques used by NATO and SAAMI are slightly different - someone who knows more than I do (UN) might come by to explain it more clearly - so the pressures are not directly comparable.

Anyway what Markr6754 said is exactly right.

Also, my 1:7 barrel does quite well with 52 grain Sierra Matchkings, as well as 69 grain and 80 grain Matchkings. So your 1:7 is no problem at all.
 
1:9 will stabilize 75 grn. bullets.
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I've only loaded up to 65 grain bullets; specifically, Sierra's 65gn SBT (#1395). They even state "10" twist or faster" on the box. I've chosen this to be my tactical bullet. I do plan on trying Hornady's 75gn BTHP (#2279) some day; but I need another propellant for that one. Right now, all I have is AA2230. It's great stuff, but rather fast. I'll need something slower for the heavies.

The 5.56 has a longer throat/leade.
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What does that^ mean?
 
The 5.56 brass is exactly the same as 223. The firing chamber is the same. The difference is the 5.56 barrel is sized for longer bullets. Think Freebore, as in Weatherby cartridges: there is a bullet jump from the cartridge until the rifling is engaged.
The 65 grn Sierra is an excellent bullet. There are stability calculators that will predict stability based on input variables. It boils down to a wide range of bullet weights can be used in 223/5.56. With 1:7 75 gr bullets are stabilized. Conversely, some say 30 or 36 grn Varmint Grenades (Barnes) will self-destruct if spun too fast. Not in 1:9. I have gone up to 4000 fps, and get resounding thumps on steel plates over 100 yards away.
 
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Okay, so there's just more distance from the case mouth to where rifling begins? (Not sure how else to phrase that.)

I'm curious why that would be. To accommodate longer bullets? Which ones would those be? Aren't max OAL's the same? Academics, I suppose. But inquiring minds do want to know.
 
It was to accomodate the current NATO standard 62 grain with a steel cone "penetrator" which allowed the ammo to penetrate a steel helmet at 600 yards, meeting milspec.
This also required a new powder WC846 to be used in place of the WC844 used in the 55 gr FMJ training round.
 
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No you did not do anything thing wrong. Just keep shooting!! And have fun too. A/Rs are so variable that you can do almost anything with them.
 
Thanks Marco Califo. Good information, clearly presented. I appreciate it.

My inquiry was out of an abundance of caution. I have been doing work ups (picking up where I left off back in April '17) all summer. I'm in a cooler climate now and I think "shooting season" is nearing an end. Making good ammo for the AR was my priority this year. And in the past few months, I have made a number of loadings that cycle reliably and are certainly as accurate as I need them to be. I have since put hundreds of rounds downrange. Mission accomplished.

Just keep shooting!! And have fun too.
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I will Ed4032. Thank you.
 
Both the .223 and 5.56mm were "finalized" long before the long, heavy bullets and fast twist barrels existed.

If I remember correctly the difference between the two chamberings is in the rifling leade, with the 5.56mm done the way it is to accommodate a certain military bullet (I think a certain tracer) but I'm no longer certain, and too lazy to try and look it up. :D

I dont think it matters for any commercial loadings. And as to the pressure difference between the rounds, I think its machts nichts.

The higher listed pressure for the 5.56mm is still well withing the safe working limits of any .223/5.56mm chambered rifle, and well below proof levels.

Remember, with modern firearms in sound condition, slightly higher than listed standards is rarely dangerous. I'd say "never dangerous" but want to leave some wiggle room...:rolleyes:
 
The above is true. Where the 7" military twist comes from is the need to stabilize the 63.7-grain M856 tracer, which is 27% longer than the 62-grain M855 bullet whose trajectory it is supposed to mark. That extra length affects stability significantly. But target bullets, up to that same length, all have enough greater weight to be stable in my 8" twist barrel in normal match weather conditions.

As to pressures, when you look at the development history of 5.56 NATO, you find using the NATO channel transducer apparatus, SS109 (M855 is our ballistically compatible equivalent) was developed to have 6% higher pressure than M193. The new M855A1 runs higher pressure still because they moved to a faster burning powder to cut down on muzzle flash and blast from the M4's 14.5" barrel. But reports are that barrels are shot out much faster on it.

Meanwhile, note that the CIP did not adopt SAAMI pressure standards for 223. They adopted the SS109 pressure standard. So if you buy European 223, you have that 6% extra pressure. Considering that the proof pressure range for 223 is almost 34% above the SAAMI MAP, that 6% increase, while it will probably introduce a little extra throat wear, is certainly not enough to burst or beat the gun up badly.
 
And whilst we are talking about twist . . .

Can I assume that - with all else being equal - a higher twist rate will also mean a higher pressure?

At least, that's how it works in my mind . . . it takes more effort to push the bullet down the barrel if it has to twist harder.
 
Yes, but the effect is small. If you want to, you can take the revolutions per second times two pi to get the angular velocity and calculate the angular (rotational) kinetic energy from that, and you will find it rather small compared to the muzzle energy.
 
If you're going to go as deep into calculations as Uncle Nick describes to find how much pressure might be raised by a faster twist rate, don't forget to also calculate the rotational energy for the same load in the slower twist barrel for comparison.

A certain small amount of the force pushing the bullet down the barrel is "used up" engraving the rifling into the bullet jacket as the bullet enters the rifling. After that, the amount of force changes, because the bullet is now "cut" for the rifling and "riding on the rails" so to speak.

These are going to be very small numbers and only a tiny percentage of energy used firing the bullet.
 
I’ve always been under the impression the longer leade is to prevent malfunctions in case of carbon buildup during extended fire fights when you can’t clean your chamber. At least I believe I’ve read that somewhere in the distant past.
 
Yes, but the effect is small.
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Good to know. When I look at load data, I look at the test gun's twist. I have been keeping it in my head to assume slightly more pressure in my gun, if the twist rate is less in the test gun. I'll just keep doing that.

If you want to, you can take the revolutions per second times two pi to get the angular velocity and calculate the angular (rotational) kinetic energy
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I don't want to :p. Quip aside, it IS neat to know the formula - thanks. I guess the "times two pi" part is why my Marlin 44 Mag rifle has a very gentle 1:38 twist. The large caliber (centrifugal force), coupled with the lighter construction of pistol bullets.
 
That formula gives you the angular velocity. For angular kinetic energy you multiply that by half the axial moment of inertia. That is best measured on a vertical torsion pendulum rather than attempting to calculate it for the bullet's shape, since you don't normally know how jacket and core thicknesses and distribution vary inside the bullet. You can find the method described by several physics professors on line. Basically, it involves making a little platform for the bullet that you suspend with four or five feet of very fine steel wire. I use 0.0025" tungsten wire I have on hand. Then, with the bullet sitting upright on the platform, you give it a quarter turn twist and start your stopwatch and see how long it takes for twenty oscillations back and forth to occur. You can calculate Ia from that, though you have to calibrate the pendulum first with a couple of known moment of inertia objects. I use pin gauges for that.
 
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