Grendel Case Taper

[LRRPF52]

Who is the father of making the case less tapered to increase capacity? Why PO Ackley, of course.
On searching I found this from https://www.shootersforum.com/wildcat-ca...thing.html

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The process of a case gripping the walls of the chamber during powder combustion is called obturation. It's one of the devils that plagued the M-16 in its early days. There is a lengthy article about this phenomenon, written by the military man whom discovered it. Can't remember his name, but he wrote a letter to the top brass about the failures of the spent cases to extract from the chambers during firefights in Vietnam. He referred to his letter as the "letter read 'round the world." Might be able to find it using that in your search string. Add "M16" to it, and your luck should improve. Very interesting reading. The failures to extract got a lot of good men killed. Sad, as well as being something that stokes the fires of rage that the rifle and round were not more thoroughly tested before being fielded. Add to that, the troops were told the rifle was "self-cleaning." Nothing could have been further from the truth, given the powder recipe they used in those days.

I think an Ackley case obturates more than a standard case because it has that small amount of extra cylindrical value. I do not know if Ackley cases are less tapered than a standard case. Maybe, to wipe out the original chamber. Might be why they stick better than standard cases. Opinions and comments welcomed...

The man's name is Richard Culver. Marine. Lots of stuff about his M16 experiences on the 'net..."


Case taper is the same on the SPC as the 5.56 but the case is shorter. Shorter cases should result in less chamber "sticktion" if that's a word.
Grendel case is shorter, so maybe less also. I would think Bill Alexander has this all worked out but I haven't read anything about it.

When I shot my Howa bolt last week, the bolt handle was difficult to lift with factory Wolf ammo which is what got me thinking.

The early AR15s were one of the most tested service rifle designs in US military history. Even when Army Ordnance sabotaged them, they still out-performed the M14 in terms of reliability and accuracy.

The problems came from not chrome-lining the chamber, and changing the propellant from what the AR15 was validated on during all the trials and testing. The AR15 was engineered around small stick powders, one of which was IMR 8208M. The rifle ran fine with this propellant and the original cartridge specs.

The Army changed the propellant spec without notifying Stoner, ArmaLite, and Colt. They knew the validated ammo worked, so when they were in production of the Colt Model 602 (partial fence lower, 1/12" twist barrel, slick side upper, tri-prong flash hider, bakelite/black furniture) they used the proven ammo to test those rifle batches before they were sent to SEA.

Problem was, they sent them to SEA with the new propellant ammo, which was ball powder. The ball powder load generated gas port pressures that were 8-10,000psi higher than the rifle was engineered around. This caused such an increase in the cyclic rate, that extraction was happening much too early while the case was still obturated.

Now factor in that they had corroded chambers with very rough textures after just a few days in the rainy season, and think about what the brass is doing under pressure. This is why they had incidents of case head separations and failure to extract, with guys taping cleaning rods to the sides of the weapons.

After the investigation, several changes were made to the AR15 (type classified as the M16). The most important ones were chrome-lining of the chamber and increasing the buffer weight with a new buffer design to act as a rate reducer for the higher port pressure powder. The original buffer was the Edgewater buffer, very lightweight with Browning A5-type conical stack and telescoping spring guide.