smg762 said:

what was the barrel length on the textron rifle. it looks like 16, and yet the round apparently didnt hit 80k PSI

The Textron rifle you're referring to is likely the Textron Systems Cased Telescoped Weapons and Ammunition system, which was developed as part of the U.S. Army's Next Generation Squad Weapon (NGSW) program. The NGSW program aimed to replace the M4 carbine and M249 Squad Automatic Weapon with a new rifle and automatic rifle that featured advanced ammunition designs.

The Textron rifle uses a cased telescoped ammunition design, where the cartridge case is completely enclosed by the projectile. The barrel length of the Textron rifle can vary, but it is typically around 16 inches (40.64 cm) in length. This is a shorter barrel length compared to traditional rifles like the M4.

The ammunition used in the Textron system is designed to operate at lower chamber pressures compared to traditional brass-cased ammunition. This is achieved through a combination of factors, including the use of a more efficient propellant and a different case design. While the specific chamber pressure values can vary depending on the exact design, it is generally designed to operate at pressures that are safe for the firearm and consistent with the desired performance characteristics.

The goal of the Textron system, and the NGSW program as a whole, is to provide a lightweight and compact firearm with improved performance and ammunition efficiency while maintaining safety and reliability. The use of cased telescoped ammunition and other innovations in the system help achieve these goals. https://gunsandgadgets.co.za/

I would love to see a comparible velocity curve for M855A1, although I would bet it is classified, if for no other reason than stopping politicians making trouble!

Am I reading too much into it, to suspect that M855 was optimised for the 20 inch barrel of the time, or is it just the way the testing was conducted with the hardware available?

PRM2 said:

Is there also similar data for 7.62 ball and 5.56 ball and AP

M855A1 was $0.46 and MG linked M80A1 was $0.91

Thank you, rough estimate is that 6.8 ball is over twice as expensive as 7.62 ball and close to 5 times more expensive than M855A1. Perhaps Colonel Powerpoint in the DoD can justify this with higher kill probabilities at overmatch ranges, as the conclusion of the succinct 300 slide presentation.

The new ammo isn't exactly cheap, when you consider it is being made in new, theoretically more efficient manufacturing facilities, albeit with a more complicated construction. Cue cost saving work arounds to do most of training using 5.56, 7.62 and simulators! 

PRM2 said:

I would love to see a comparible velocity curve for M855A1, although I would bet it is classified, if for no other reason than stopping politicians making trouble!

I think that M855 velocity curve was generated from private testing for a magazine article, so it should be possible to create a similar (unclassified) chart for M855A1.

PRM2 said:

Am I reading too much into it, to suspect that M855 was optimised for the 20 inch barrel of the time, or is it just the way the testing was conducted with the hardware available?

Since SS109/M855 was developed by FN when most 5.56 rifle and LMG barrels were in the 18-20" range, I imagine the loading was optimized for barrels of that length.

The dominating 5.56 mm weapon at the time of the NATO trials was the M16A1 with a 20 inch (508 mm) barrel. The standardization document for the 5.56 mm NATO (STANAG, now AOP, 4172) specifies a 508 mm test barrel. 

PRM2 said:

Thank you, rough estimate is that 6.8 ball is over twice as expensive as 7.62 ball and close to 5 times more expensive than M855A1.

Well to be fair the 6.8 is brand new, whereas the others have had a decade+ of economy of scale to lower production costs. Within 10 years or so theres no reason why 6.8 brass vs 7.62 brass EPR wouldn't cost about the same.

The real variable I think will be the cost difference between brass case and the SIG hybrid case, and how much more the hybrid adds to the price. 

(also  to JPeelen)

I really hope that as M855A1 was designed as lead free ammunition which would also improve the effectiveness of the M4, there isn't a similar velocity dip, this time around 16 inches barrel length. Marine Corps usage in the M27 springs to mind here. I don't think I have ever seen any actual data of M855A1 performance in longer barrel lengths than the M4.

PRM2 said:

M855A1 performance in longer barrel lengths than the M4.

I believe the specs were 2970fps from the 14.5" M4 and 3150fps from the 20" M16.

But really the performance gain is the much lower frag velocity, going from ~2700fps for M855 to ~1700fps for M855A1. That's where it's vastly improved performance comes from. 

Here is the only M855A1 velocity data I could find:

14.5" - 2912 fps

20.0" - 3185 fps (average)  /  3211 fps (corrected)

The M855A1 (smallarmssolutions.com)

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P.S.  I think I inadvertently caused us to go off track from your original question:

"how about...a heretical 16 inch barrel to give an AP projectile a fighting chance?"

M855A1 is Ball.  M995 is AP.