AR-Stoner 6.5 Grendel barrel short chambered.

[BjornF16]

There are two types of people - those who say:

Wow - higher than normal velocity! Where do I get one?

And those who say...

Whoa - there's no free lunch. Higher velocity means higher pressure.


Are the pressures unsafe? No problems reported.
How high are they? No one has measured.

Lesson learned: know your chamber, even if you don't reload.

Question:

How was the 50K psi rating determined and with what type of equipment and or labs?

Answer:

Operating pressure is 50,000 psi

It is derived from the requirement for a complete durability cycle within the AR weapon system. Determination was from old fashioned fatigue analysis of the design and trials of the weapons. Ultimately the fielded rifles have the biggest effect as the fleet durability is what matters.

Additional info:

And finally hand loading. LR is correct that the the Grendel must be approached with caution. If you simply load it up and then look for flat primers you have already killed your parts. The MAOP is a very mild 50,000 psi, proof is 67,340 psi. I cannot see the difference on the case or the primers when firing the two. That said it is quite capable of meeting the performances we built in while remaining durable. I have test mules that have run over 8000 rounds and are still ticking along. But a small case will bite without much provocation. I know how my throat works and we set up the loads and OAL to suit. Anyone ever notice that the 100 Berger could be loaded out longer than the factory set up? We set the jump to control the pressure. Same with the 123 Scenar. Sub a 120 SMK and then set to the lands minus a bit and the pressure will skyrocket with very little load adjustment. Tiny case = sensitive.

And finally:

...5R barrels within a specific pressure level will allow faster velocity that is why the competition guys love them (they shoot good groups as well). Good lapping will also help for the same reason by allowing the bullet to move easily down the bore. For a given load they will drop the pressure and consequently the velocity. Bring them up to spec and they are fast.

Polygonal form rifling (BHW makes excellent examples of this and fine barrels) extends the logical progression of 5R. It will drop the pressure slightly more and so you get a slight reduction in velocity.

So the "results" using the term in the broadest sense.

5R unlapped Liberty, most pressure fastest, variable throat and bore fouling so pressure jumps the most and the ES is up.
5R lapped, Very consistent bullet moves a little easier drops a bit of MV nice ES
Polygonal, Smooth, lowest engraving force and least bore resistance, slowest. (good barrel for hand loads that might well allow some improvement over factory velocities)

...Satern 5R demonstrates that it is a superb barrel as always.