IMR-4064 and the different data between AR & 223 loads with 69gr smk

[Metal god]

69gr smk
PMC cases
25gr IMR-4064
CCI #400 primers
COAL 2.255

So I've been loading this load for my dads AR ( 16" 223 Wylde 1-8 ) for a few years now . During load development I knew I was close to or a tad over max but it was by far the best grouping load and still is . I do not use this load for any of my firearms . No specific reason other then I usually load 77"s . I exclusively used his rifle during load development , I'm not sure if I've ever even tried this load in any of my rifles .

How ever today when loading some up I thought I'd look up the data again on this load in the manuals and online . Now I know there will be differences but I was a little surprised with what I found .

These will all be Max loads

Hodgdon ________________ 24gr C
Lyman"s 50th ____________25.5gr
Sierra AR data ___________24.1gr
Sierra 223 bolt action data__ 25.8gr

My notes show I started at 22.5gr and worked up to 25gr so I believe I'm fine but the load does seem over gassed based of the brass being thrown to 1 and 2 o-clock . Seeing how the Sierra 223 data is almost 2 full grains higher then there AR data for the same bullet and powder . Does the lower charge have more to do with the gas system and what works best then high pressure ?

Lets pretend for this argument that we are all in agreement that you can shoot 223 in a 5.56/223Wylde but can't shoot 5.56 in a 223 . Using that logic that 223 data of 25.8gr should be way over max . I'll add that the difference in twist rates are double with the 223 data having a 1-14 while the AR data has a 1-7 . Does that 1-14 really reduce pressure that much ??

Anyways I found that 223 data from Sierra interesting .

[Mobuck]

Every bore is different and the data is only representative of the specific bore used to collect that data.

[Jimro]

This is really normal for the heavier bullet end of the load spectrum for 223/5.56 load data. Different sources will give you a pretty broad swing of charge weights.

Jimro

[T. O'Heir]

All manuals will show slightly different results. All data in manuals are averages. All manuals reflect conditions on the day of the tests using specific components and firearm(if there was a firearm), only. Any data found in any manual will be safe to use.
25.8 grains of IMR4064 is only 1.8 grains over the Hodgdon max tested in a 24", 1 in 12 twist barrel. Both the barrel length and rifling twist matter.
The rifling twist has nothing to do with pressure. It has to do with stability of the bullet and velocities.

[Metal god]

The rifling twist has nothing to do with pressure. It has to do with stability of the bullet and velocities.

I've read an extreme difference like 1-7 to 1-14 does indeed makes a difference in pressure . More so a pressure spike when the bullet runs into a 1-7 twist as apposed to a 1-14 . In the 1-7 the bullet is forced to change directions much faster then the 1-14 slowing it down just a tad creating a higher pressure spike or you could say the 1-14 allows the bullet to be eased into the rifling . At least that's what I remember reading at some point .

That was my point as far as the 1-14 twist allowing more powder to create the same pressure .

Although I've seen data where one manuals max is another's start . My interest lies in the "theory" that the 5.56 has and or allows more pressure then the 223 . So why do I/we see so often 223 data with higher charges then AR or service rifle data ? In theory you should never see 223 data almost 2gr over an AR load . Based on many writings of Uncle nick here I believe there is actually no difference in actual pressures from 223 to 5.56 loads when measured with the same equipment .

My thought was maybe they shot the 25.8gr load out of the AR rifle but found it to be over gassed or some other reason then just to much pressure . I thought maybe one of you would have an answer or another theory on that difference . Remember now , that data came from the same source "Sierra"

[Unclenick]

MG,

I let my CAD software work out the moment of inertia for the bullet's longitudinal axis. At 3000 fps the 14" twist puts about 1.5 ft-lbs of energy into its spin. A 7" twist doubles the angular velocity, so it puts 4 times that energy, or about 6 ft-lbs into rotation. But that's out of 1377 ft-lbs at that velocity, so not much of it. About equal to to the difference a 4.5 fps velocity change makes to ME, which is less than normal shot-to-shot variation causes. Frictional differences due to the two twist rates will be too small to have detectable effect.

[Metal god]

Well you shot that theory down in flames I wonder where I read that because I've believed the twist did effect pressure for some time now . Oh well , always learning .

[SonOfGun]

5.56 and 223 operate at different pressure.

[Unclenick]

CAUTION: The following post includes loading data beyond or not covered by currently published maximums for this cartridge. USE AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assume any liability for any damage or injury resulting from use of this information.

Not really significantly different pressures. They were originally the same, as Remington, who helped with developing 5.56 merely copied it and gave it the commercial name .223 Remington for the civilian market, and with M193 specs, they still match commercial .223 MAP. The Belgian SS109 cartridge, adopted here as M855, measures about 6% higher pressure on U.S. copper crushers and conformal transducers than .223 and M193 do. On European style channel transducers, that difference apparently disappears, as they rate 5.56 NATO and .223 Remington both at 4300 bar (62,366 psi). At least, I believe that to be true as in an email exchange I had with Sellier & Bellot at one point they said the NATO and civilian round had the same spec in their copies of the published information.

So it seems those channel transducers show that 62,366 psi number for the same reference ammunition that shows 55,000 psi on a SAAMI style conformal transducer. Though I've seen one Australian military study contradicting that, it's the only explanation I've got. Federal says they load M855 to about 58,000 psi on the conformal transducer, so the match occurs somewhere in that range. In any event, if you buy .223 ammo made by a European country, it has been loaded with that 62,336 psi MAP limit as measured on their equipment. You can read that on the CIP homolgation site.

The only real screw-up I've seen on pressures are in the Western Powders load data. They list separate 5.56 loads rated at 62,366 psi. The problem is, unlike the Europeans, they didn't measure that number on a Kistler channel transducer. They used a SAAMI type conformal transducer, as one of their technicians verified to me over the phone. So they should have been using Federal's 58,000 psi number with that transducer.

If all this pressure bouncing around is confusing, keep in mind two things: One is that SAAMI assumes a 4% standard deviation in pressure to begin with. Second, makers of load manuals don't use the SAAMI Maximum Average Pressure (MAP) the way manufacturers do. The MAP is that maximum number you see commonly published. The way a manufacturer uses MAP is as the maximum an average value of peak pressure over 10 rounds can be with new ammo (there is a higher SAAMI number for aging lots). Individual rounds in that 10 round sample can go higher or lower as long as the average stays at that limit. They have another pressure number called the Maximum Extreme Spread that says how much variation is allowed withing the ten shot sample. The bottom line is that an individual round in that 10 rounds can theoretically be as much as about 18½% above the MAP (CIP allows 15%). That's still below proof pressure by a bit, so they don't worry about it.

The load manual authors don't use the MAP that way. They treat the SAAMI MAP as an absolute upper limit for any individual round. Hodgdon spelled this out in the last print manual of theirs I got a dozen years back. They don't allow any individual round in their tests to exceed that value. They explained this is why their maximum loads have different pressure values. The value they give is the average. The fact it is below another powder's average tells you one round in that 10 shot average peaked enough higher to reach the SAAMI number. Hodgdon advised that when you see one powder's maximum load producing a higher pressure than another's, you know that powder produced less pressure variation than the powder with a lower maximum did. Lyman's data suggests they do the same thing Hodgdon does. Hornady and Sierra data isn't exactly pushing the boundaries, either.

Load manual data is generally erring low by both SAAMI and CIP standard practices, which is probably done partly out of liability concern, but also probably because they know many people, in order not to have to purchase different powders for all their guns, will try to make do with a sub-optimal powder that produces a lot of pressure variation and could easily exceed SAAMI MEV if they didn't publish a lower limit load for it.


MG,

The effect of rifling isn't easy to pick out. If you unroll the bore to a flat sheet and look at the rifling angle, you might think, well, if I pushed a bullet against a slope like that the force vector for the angle is significant; over 10% for the 7" twist. But when you push a bullet against a slope like that, the vector moves the whole mass off a straight line. In spinning, the center of the rotating object has lower velocity in linear inches per second traversed around the circle than the outer edge does, so its like you have a longer lever arm for turning the center mass than the outside of the mass. As a result, there is less force applied in spinning the bullet than the sine of the flattened out rifling angle would apply to divert the whole bullet, and that's why the energy numbers are small.