Steel is a poor material for the 5.56/223 round as that material was never a consideration for the original cartridge. In fact, the 223 round was not so much “designed” as it was a wildcat. The guys who came up with the round wanted a certain velocity at a certain range. I read the 1971 Guns & Ammo article
“The 223 is here to stay” by Robert Hutton. Robert Hutton was technical editor of Guns and Ammo magazine and must have been very wealthy as he owned a big piece of real estate in Topanga Canyon California. It was called Hutton’s Shooting Ranch. Hutton’s article documents how he developed the 223 round. If you have any sort of technical background, it is apparent he is an amateur and his cartridge represents what an amateur would do. He took an existing cartridge, necked it up and down, blew the shoulder out, changed shoulder angles, he had a chronograph, got the velocity he wanted at distance. The crowning achievement in the article was punching holes in the wobble pot at 500 yards. That is about all the lethality testing Hutton did, punching holes in a helmet. He used the Powell Computer, a paper slide rule, to estimate pressures. He did not pressure test his cartridge he did not have a pressure curve. This cartridge was then adopted as the US service round.
No doubt Hutton meant well in creating a wild cat that could be a potential service round. But this also shows what happens when "back yard mechanics" create things. Hutton did not have the analytical capability to thoroughly study cartridge case design. William Davis, the Government Technical Expert at the Icord hearings, said on the History Channel that the technical data provided the Government on the 223 round did come with a pressure curve. These guys developed a cartridge and never thought of documenting what the pressure curve looked like. Pressure curve is absolutely critical to the timing of an automatic weapon. How long energy is available, the maximum pressure and how fast it drops off is fundamental to the design of a automatic gas mechanism.
This is from Chinn's Machine Gun series.
Hutton did not look at case hardness, taper, expansion or contraction. A professional would have looked at the expansion and contraction of the case in the chamber and adjusted case taper, thickness, and established case hardness in the sidewalls and case head. You would have to work with manufacturing to determine realistic hardness parameters throughout the case, but this is important as it affects the Young’s Modulus. As it turns out, the brass case 223 drags on extraction, there is not enough clearance between the case and chamber. Steel case is even worse. I have seen many failures to extract steel case ammunition on the firing line with AR15’s.
It turns out the 223 is fairly straight tapered. This was a fad, highly promoted by P.O Ackley, and widely copied. I am not a fan of very straight tapered cartridges. The one and only advantage of a very straight taper is maximizing the amount of powder you can get in the case. The wildcat era of the late 1940’s through the 1960’s was all about high velocity, and only high velocity. It was very one dimensional thinking, ignoring other aspects of cartridge design that are very important. One of the things you trade off for a straight case is that the cartridge does not “steer” well during feeding. Anyone can test this, which shape feeds better into the end of the tube, a taper, or a straight cylinder? Alignment to bore is important for feeding with all cartridges, but the really straight ones are going to jam up more often when alignment gets slightly out of whack. Straight cartridges will drag on extraction because the case walls are relaxing off the chamber walls in a straight line, not a diagonal. It turns out portions of the 223 case are still sticking to the chamber walls during extraction and a major reason for extractor lift.
Understanding Extractor Lift in the M16 Family of Weapons www.dtic.mil/ndia/2003/smallarms/din.ppt This is very undesirable as jams will get you killed in combat. Lots of good American Boys died in Vietnam with jammed M16’s in their hands. Ideally, the case will be fully relaxed off the chamber walls during unlock and there will not be any resistance between case and chamber during the residual blowback period. If you look at good case design, the Russian 7.62 X 39 and the recent Chinese service cartridge, both have more case taper than the 5.56 Nato and both were designed with steel as a case material. Both have nice thick rims, which is also important for machine gun rounds.
As an example of the well thought out nature of these Chinese rounds, the 5.8mm operates at a much lower pressure than the US service round. It only generates a 41,500 psi (284 MPa) chamber pressure which is marginally higher than that of the old single-base propellant used by the vintage 7.62x39mm and much lower than the 5.56mm M855/SS109’s 55,000 psi (380 MPa). The current pressures of the latest 5.56 rounds have been kept out of public view, but it seems to be in the range of 62,000 to 65,000 psia. Considering the proof round is 70 kpsia, the Army is operating its cartridges at pressures that are guaranteed to crack bolt lugs very quickly. You see, the AR15 was designed for a 50 kpsia round, not a 65 kpsia round. Lower pressures means fewer failures to extract when the weapon gets hot, or the Trooper is in a hot environment. It is always true that doing the job at lower pressures is better than doing the job at higher pressures.
The Russians took into account the material characteristics of steel as a case material, examining the expansion and contraction, along with the production technology, aiding the excellent function design of SKS's and AK47's. As such, these steel rounds are outstanding in feed and extraction. The 5.56 was created without spending any time or effort on alternatives, alternate materials, anything. As such, given the fact the case shape is not optimum for brass, it most certainly is not optimal for steel.
The 5.8mm’s corrosive powder is not particularly hot either. It only generates a 41,500 psi (284 MPa) chamber pressure which is marginally higher than that of the old single-base propellant used by the vintage 7.62x39mm and much lower than the 5.56mm M855/SS109’s 55,000 psi (380 MPa). A non-reloadable Berdan primer is used to prime the 5.8mm cartridge’s propellant.
Hutton did not spend time and money examining issues such as pressure curve, contraction or relaxation. Rifles and cartridges that did not go through an appropriate development will have issues with case materials. The AR15 and 223 Remington are an example. You can read all the issues they created here:
http://www.dtic.mil/dtic/index.html
Report of the M16 Review Panel Appendix 4 Appendix 4 Ammunition Development Program.
Report of the M16 Rifle Review Panel Volume 7 Appendix 6 review and analysis of M16 System Reliability.
Report of the M16 Review Panel Appendix 5 Procurement
Report of the M16 Review Panel Appendix 7 Vietnam Surveys
Report of the M16 Panel appendix 10 the small arms program
Report of the M16 Review Panel Summary Report.
So what you read, in lots of posts on forums, is shooters having issues with steel cases in their AR15’s. That is all due to the cartridge being a wildcat and the developers not examining steel as a case material, and adjusting case parameters for reliable function.
Oiling steel cases will help extraction function by breaking the friction between case and chamber, and also solvating lacquer. Might as well oil the steel rounds in your machine cannon:
Handbook on Weaponry - Rheinmetall - 1982
11.4.2 Ammunition for Automatic Cannons
Automatic cannons are usually understood to be automatic weapons with a caliber of from 15.24 mm on up. Based on the caliber and the design, automatic cannon ammunition must, strictly speaking, be included with artillery ammunition.
Brass or steel cases are used as cartridge cases for automatic cannon ammunition; the steel case has come to predominate in Germany. Through the use of serviceable slide varnishes and longitudinal grooves of the cartridge chamber, a uniform extraction process can be achieved. The slide varnishes were especially developed for the exterior varnishing of cartridge cases. The two measures noted here make it possible to fire continuous fire with “dry” ammunition, i.e. not oiled, where normal lacquers are used, unoiled steel cases have a tendency to jam.
Page 549
I believe the longitudinal grooves Reinmetal is referring to are the flutes found in the HK91 and in a number of German pistols. These flutes are essential to floating the upper 2/3’rds of the case off the chamber walls, precisely to break friction between the case and chamber.
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