6mm Grendel Group Design Project

[stanc]

Gentlemen, Warriors, and Vile Masses of The Horde: Let’s put our heads together and spec out the ultimate 6mm cartridge based on the Grendel case. Release of the new .224 Valkyrie got me thinking about Grendel-based wildcats. I think 6mm is begging to be a factory round.

I already have in mind my perfect 6mm round. I challenge you guys to do better.

Challenge accepted!

OK, here’s my thinking. . . .

Start with a Grendel case, which has a case length of 1.520" or 38.6mm.
Next, take the bullet with the longest nose length that you intend to use: the gorgeous Berger 6mm 105 Hybrid with a nose length of 0.701”.
Even though some magazines allow a COL of 2.300, let’s hedge a bit for the sake of reliable feeding and declare our max COL to be 2.290. (Factory loads with shorter bullets could be loaded to standard AR COL of 2.260.)
Now, COL of 2.290 minus bullet nose length of 0.701 equals a max case length of 1.589. Let’s just round it to 1.590" or 40.4mm.
THUS, WE NEED A BRASS MANUFACTURER TO INCREASE LENGTH OF GRENDEL CASES FROM 38.6MM TO 40.4MM!
Neck length of this brass will be on the short side for more case capacity, but should match the proven 6mm BRX of .210”.
Shoulder remains at 30 degrees, for feeding reliability. I just don’t trust 40 degrees for feeding from the magazine in an AR; I could be wrong, but that’s my opinion as of today without further proof.

I don't trust 30 degrees. When the US Army developed what became 7.62 NATO, they began with a 30-degree shoulder, then soon reduced it to 20 degrees for reliable feeding.

No military rifle cartridges have a 30-degree shoulder. The steepest are 7.5x55 Swiss at 28 degrees, followed by 6.5x55 Swede at 25 degrees. All others are 23 degrees or less.

Why not the 6.8 case cut down and necked down? Not as fat as a Grendel case, thus lacks capacity, thus lacks velocity.

High velocity is not necessary, according to the original (modest MV/high BC) design philosophy of 6.5 Grendel and the 6mm SAW (below).

Thoughts from The Horde?

I would opt to use the Valkyrie case, necked up to 6mm, and shoulder angle reduced to 23 degrees. Fits in existing M68 links, for machine gun use.