Grist for the mill.
Cool article, thanks for posting. As far as detrimental effects to M885 performance out of sub-14" barrels; doesn’t modern ammo choices alleviate some of that performance degradation?
The deviation of pressure on those barrels is crazy! Makes me wonder why anyone would rock a SBR pencil barrel…
Sent from my iPhone using Tapatalk; coherency may be in jeopardy.
Most improvements in “modern ammo” generally revolve around bullet design and construction; I don’t know that newly designed loads are traveling any faster down the bore, nor do I know if their superiority in design over, say, NATO ball is enough to make up for velocity losses when fired from shorter barrels.
Like ice cream, I’ve not met an AR variant I didn’t like, but I prefer a 16" gun for my purposes…
From the article:
This shift toward shorter barrels has resulted in the U.S. Army and Marine Corps adopting the 14.5-inch barreled M4 carbine with a re-design of the 5.56×45 from the 55 grain SS-109 to the 63 grain M855 ammunition to optimize this barrel length.
Huh???
The SS109 and the M855 are similar rounds, both use 62 grain bullets, and the M855 round was adopted many years before the M4 carbine was adopted.
Exactly…
That “article” is chock full o’ FAIL.
That’s an eye opener for me on a few things on the sbr. Thanks for posting.
ETA: would you further point out this fail. No kidding…
Off the top of my head here…
The 62gr. XM855 has always been known as a puncture wound round. It was designed to make it through a barrier and do at least some damage.
The 55gr. FMJ needs a minimum of around 2750fps to fragment. The faster the better. Works great out of a 16 inch barrel up to around 150 yards max.
There is always a trade off in this world of physics. I am thinking 75 and 77 grain bullets with a cannelure are the best bet in the SBR world.
Having 2 hits in a timely manner in a QCB environment will be better than getting your 16 inch barrel hung up on the door and getting none.
Good article. I liked the graph on bullet speed vs. barrel length.
I currently have a 11.5 inch SBR on order and am more aware of the limitations of a light weight flash bang toy.
You just cant have it all… Yet!
Interesting information. The graphs make it easy for us mentally challenged to interpret the information. Does anyone know what powder the M855 round uses? Something around the H335 or AA2230range? kwg
Didn’t Mr. Noveske say that he noticed the best accuracy with 18 in bbls?
I thought M855 was the bullet that is used in the SS109 cartridge. Am I wrong? 
You’re close, but it’s actually the reverse. NATO adopted Belgium’s 62gr. SS109 steel-core bullet design for the 5.56x45 NATO cartridge. Different NATO countries load the cartridge to their own specs. The US’s load is designated as M855, and it uses the SS109’s bullet design.
http://ammo.ar15.com/ammo/project/hist_ss109.html
Again, the US adopted the M855 round in the early 1980s, way before the M4. The article’s claim that the US adopted the “M855 ammunition to optimize this barrel length” of the M4 shows an obvious ignorance of basic US military weapons and ammo history. I make no claims about anything else in the article, though.
Great article. I found it interesting that 11, 12, and 13" barrels had approximately the same muzzle velocity.
Reading the test article still makes me wonder “so what”
The testers made no actual measurements of accuracy of the shortened barrel. And with the testing apparatus they were using it would not have mattered anyways because the barrel was all chopped up, resting on a bag, and had a huge piece of metal bolted to it. It seems like it would make a lot more sense to take a barrel, mount strain gauges on it at 1 inch intervals, and use a high speed camera to measure barrel deflection during firing. The strain gauges would not alter the barrel and would probably have very little, if any, effect on accuracy.
Yea, that was interesting, but there could be a couple reasons for this that do not correlate across all brands of ammo and test conditions. So for this test, yea they were all about the same, but as a blanket statement, probably not.
The other thing to remember is that the pressure between 11" and 13" was not nearly as close as the velocities. It looks like 1.5kpsi loss between 11.5" and 12.5".
What really got me was the considering the 20" was the initial design length had a chamber pressure of 5.7kpsi, it’s pretty remarkable the thing even works at 7" and 17kpsi.
Also the pressure going up from 16" to 17" could be like the above velocities, a function of this setup and this ammo that might not happen with different variables.
This was better covered in one of the gun rags, not a measure of accuracy but pressure and velocity on barrel length. Print article needed to be edited as well, several conflicting statements or erroneous statements based on wording.
It wasn’t chamber pressure they were measuring it was bore pressure, or the pressure in the barrel just before the round left the muzzle. The chamber pressure would be the peak pressure acheived in the chamber which would occur at the time or shortly after the round is fired. Also looking at the setup there is no gas tube on the barrel so the data is definately screwed. In addition “performance” is never really defined but after reading through it the performance they are looking at is for suppressor design.
I understood that, just wrote the wrong one. Not chamber, clear.
Fwiw, I don’t think a whole lot of volume is sucked up by the gas tube, meaning there shouldn’t be a remarkable amount of pressure difference, just my guess.
If you make the assumptions that the interior diameter of the gas tube is about 3/32 of an inch and that the length of the gas tube is half the length of the barrel, and there are no losses of pressure at the gas block, the volume of the gas tube is about 8.8% of the volume of the barrel.
I think the biggest issue is the tester only tested two things (pressure and velocity), did not use a very realistic apparatus. In addition the original poster added a new twist and said “performance” in the title.
In the article the tester stated his object was;
“Intuition has transitioned into the sound engineering practice of actually measuring pressures in the suppressor entrance chambers to calculate hoop stress and, by knowing the yield strength of the material, the safety factor. While some of these factors can be approximated through calculations, actual measurements are definitive. These issues are the subject of this paper.”
So again “so what” unless you are designing suppressors?
The M855 is just the American version of FN’s SS109 which was the victor in NATO standardization trials.
Yes, that’s what I mean when I say that they’re similar, and in post #11 I discuss it a little more.
The point is that the article is incorrect regarding the specs for the SS109 and M855 and they are also incorrect about the reason why the M855 was adopted.