Grendel Case Taper

[bj139]

Using the datum dimensions for the .308 Win and Grendel, I see that the .308 taper is 0.0120 in/in while the Grendel is 0.0123
in/in which means the Grendel has a bit more taper than the .308 Winchester.

Grendel:

Near Base 0.4426
Near Shoulder 0.4328

Difference 0.0098

Datum Length 0.8000

Taper 0.0123

.308 Winchester

.308 Win

Near Base 0.4703
Near Shoulder 0.4553

Difference 0.0150

Datum Length 1.2500

Taper 0.0120

Are you actually measuring this? You are quite a bit off from SAAMI.

[bj139]

I see. Well heres my best advice for building a single shot(gas-less) grendel AR that extracts well(or .223 for that matter)
1) Works smooothest when bolt and barrel are a factory matched set. This lets the bolt rotate easier during extraction. I have hypothized that if using a non-matched set, you could lap in the bolt/chamber interface, but have never tried myself.
2) You really need a side-charging upper. I have used a Gibbs and a BCA. The side charger really allows you to perform a clean, continuous, firm pull to to extract.
3) Loads. light to moderate loads will extract much easier. If using hot loads extraction will be difficult, even with the tips above performed. I have two good nodes with a few loads, but typically use the lower one for this reason.

2) I have found the side charger is more difficult to extract due to the force on the bolt not being in line with the bolt but to the side by more than an inch.
A charging handle has a more direct line of pull on the bolt being above the bolt by about 1/4 inch.

3) I agree with you on the light loads being easier to eject.

[JASmith]

Are you actually measuring this? You are quite a bit off from SAAMI.

Those numbers are lifted directly from the datum points shown on the SAAMI chamber drawing for the Grendel.

I see now that I indadvertently used the case drawing for the .308 Win.

The correct data is:

308 win

Near Base 0.4714
Near Shoulder 0.4564

Difference 0.0150

Datum Length 1.2500

Taper 0.0120

My conclusions remain the same.

BTW The Case head and shoulder dimensions are derived from the point of intersections of the lines and cannot be measured. They are for reference only.

The body and shoulder datum dimensions are located at points that can be measured. IIRC these are the points used to validate gages and, in turn, verify that the chamber meets specs.

[SightedIn]

2) I have found the side charger is more difficult to extract due to the force on the bolt not being in line with the bolt but to the side by more than an inch.
A charging handle has a more direct line of pull on the bolt being above the bolt by about 1/4 inch.

3) I agree with you on the light loads being easier to eject.

To each his own I guess. I have used t-handle extraction method and the side-charge handle extraction, and i personnaly prefer side pull.
I extract with palm side up, giving a good firm pull, but not a jerk, all the way back, and release. Never a struggle for me to extract in that manner.
Hope that helps

[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

"nvshooter's Avatar
nvshooter nvshooter is offline
Beartooth Regular

Join Date: Oct 2010
Location: Nevada
Posts: 4,884
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.

[bj139]

Could be the two BHW barrels shown above have much smoother chamber walls as well.
I am expecting a 20" Ballistic Advantage barrel from UPS today.
I will assemble it with no gas block and see how difficult ejection is. The chamber should be smoother than my BCA Grendel.
Maybe time to take a few gas blocks off others and shoot to see.

I shot the Ballistic Advantage barrel today and had to whack the charging handle with a 2x4 most times to get Wolf steel rounds to eject.
Then I shot some Hornady Black and I was able to eject by pulling on both sides of the charging handle.
The effort seemed to be slightly more than the more tapered cartridges.
The barrel shot very well especially with Wolf steel ammo.

[bj139]

I just bought a BCA 7.62x39 AR15 barrel during the Memorial day sale.
I will post my results on manual extraction difficulty when I shoot it.

[bj139]

I was searching the web and found this comment about case taper.

Factory Valkyrie brass is small primer.
Good - that's what I'd assumed / hoped, but was told it was large. (it's surprisingly difficult to tell which is in use looking at the photographs on
Accurate Shooter and elsewhere!). We have one Valkyrie user that I know of in the UK, but he has reformed SPC brass to get shooting as the genuine stuff is unobtainable here as yet.
(He is using a straight-pull AR. The 224 Valkyrie is a potentially good choice for that number as we know the parent SPC case extracts sweetly in manual operation whilst the high pressure Grendel was found to give hard extraction unless downloaded so severely that it gave no real benefit over 223s shooting 77s.
As these ARs are used in 'Civilian Service Rifle' and similar competitions involving rapid fire, magazine changes, weak shoulder operation and 3-position shooting, hard extraction is toxic as struggling to yank the operating handle back causes so much disruption to shooter position and rifle hold / aim.)

Laurie, Apr 3, 2018

[bj139]

To each his own I guess. I have used t-handle extraction method and the side-charge handle extraction, and i personnaly prefer side pull.
I extract with palm side up, giving a good firm pull, but not a jerk, all the way back, and release. Never a struggle for me to extract in that manner.
Hope that helps

I put my side charging upper on my 458 Socom since the port is large enough to clear the 458 case.
The port on a standard, milspec rear charging upper is not large enough for the 458 case to fit through, making extraction difficult.
I have an ambi charging upper as well to which I may move my new 7.62x39 barrel if extraction seems easy with the standard upper.

[bj139]

I shot the 7.62x39 barrel today with Tula steel case ammo.
Ejection was difficult and I had to mortar out all 5 rounds that fired. Strictly manual mode ejection with no gas block.
I had 4 other failures to fire since the lower had an aftermarket light hammer spring single stage trigger.
I should have brought some brass cased ammo and a mil spec lower but I didn't. Next time.
So, increased case taper made no difference in ease of extraction with steel cased factory rounds.

[BluntForceTrauma]

BJ, interesting experiment!

[bj139]

One factor I noticed in shooting a few different barrels and different cartridges is that steel case ammo seems to stick more than brass case ammo.

There is also friction between parts of the bolt which vary depending on how smooth these have worn.

It may eject manually OK when worn in.

I used a new parkerized bolt for the most recent new 7.62x39 barrel so there may be more friction.

I used a NiB bolt for my two 6.8 based wildcats and brass ammo only since I had to form it from 6.8SPC brass.

The new NiB bolt should have smoother lugs than the parkerized bolt.