Max pressure

[Bubbacop]

What is the max pressure for the AR15 platform, I’m surprised the loading manuals don’t emphasize for bolt or gas gun.

[grayfox]

When you say "for the AR15 platform", you need to specify the caliber and cartridge. It varies considerably.

[tdbru]

from SAAMI.org:

image.png
(copied from the SAAMI.org website)

So the 6.5 Grendel has as a max avg pressure 52,000 PSI. Note the 6.5 CM is 10,000PSI higher for max avg pressure.

So both developers of load manuals and sporting arms ammo manufacturers have to adhere to these limits in their data and ammo.

One of the advantages I feel for the 6.5 Grendel at only 52K PSI is that the barrel life is about 3x to 4x longer than the barrel life for the 6.5 CM for example. If you want 6.5 CM (or 260 Rem) performance, then that is what you need to buy. The 6.5 PRC has even more performance, along with the 264 Win Mag and the Weatherby 6.5-300. but those are really hard on barrels at the higher pressure and larger powder charges. One of the down sides of eeking out max performance in any caliber is that the barrel life tends to be short. the 6.5 Grendel pressure limit(s) were setup so that the bolt thrust (force = pressure x area) would not over stress the bolt lugs or the barrel extension lugs on the AR given the greater base area of the Grendel as opposed to the standard 5.56x45 for the AR rifle. If your run Grendel pressures up to the same level as the 6.5PRC for example, you would have similar short barrel life on a bolt gun. and you'd over stress the barrel extension lugs and the bolt lugs on an AR style rifle. but why? much simpler to just go by a 6.5 CM or 6.5 PRC if that's the performance level you're striving for, rather than overloading the 6.5 Grendel, in my opinion. Of course you're free to do what you want with your reloads, but the ammo factories are not. they need to adhere to the SAAMI spec.

best of luck,
tdbru

[Bubbacop]

Thanks tdbru that’s what I was looking for.

[LRRPF52]

The 2nd-fastest muzzle velocity with 123gr SMK is done with only 48,700psi, so I’m a big fan of powder selection working in your favor vs just chasing pressure.

The short Grendel case design with 30 degree shoulder does a great job of achieving a full propellant burn, unlike a lot of other cartridges (5.56x45 as one example).

Manufacturer

Powder

C.O.L.

Grs.

Vel. (ft/s)

Pressure

Grs.

Vel. (ft/s)

Pressure

Hodgdon

CFE 223

2.260"

28.4

2,352

36,800 PSI

31.2

2,602

48,700 PSI

Hodgdon

LEVERevolution

2.260"

28.2

2,420

40,200 PSI

31.0

2,649

50,300 PSI

Winchester

StaBALL Match

2.260"

27.0

2,319

40,900 PSI

29.7

2,492

49,100 PSI

Hodgdon

Varget

2.260"

25.2

2,242

37,800 PSI

28.0

2,501

49,900 PSI

IMR

IMR 8208 XBR

2.260"

24.4

2,246

36,900 PSI

27.0

2,504

49,900 PSI

Hodgdon

H335

2.260"

24.4

2,244

41,100 PSI

26.9

2,446

50,100 PSI

IMR

IMR 4198

2.260"

20.0

2,213

42,900 PSI

22.2

2,369

50,400 PSI

Hodgdon

H4198

2.260"

19.8

2,184

41,200 PSI

22.0

2,365

50,200 PSI

As to the AR-15 with a standard barrel extension, the lug teeth aren’t that long, so you’re pushing it when you start going to 55,000psi and above.

The dimensions were built around the 222 Remington in 1957, which was a perfectly-fine cartridge for the task. Army Ordnance Board kept moving the goal posts for the steel helmet perforation requirement, so Stoner asked Remington to blow the shoulder forward to get more powder in the case, and use a boat tail design to retain more impact speed at distance.

That created the 222 Special Cartridge, which was later named the .223 Remington and type classified in the military as 5.56x45mm, with M193 Ball and M196 Tracer being 2 of the first production types of ammo. There were variations within M193 with the first large lot of ammo using IMR 4475 if I recall, which could not be made consistently enough and in large enough lots to meet the velocity requirements, so they switched to WC846 ball powder, which was one of the main factors in early failures with 601s and 602s in Vietnam. 601s and 602s had been doing fine with the early lot stick propellant M193, but then experienced excessive cyclic rate increases with WC846, even though chamber pressure was lower. Port pressure increased due to the longer burn curve of WC846 ball, which the AR-15 was not engineered around.

There were early bolt failures recorded by USAF and US Army before major combat forces were deployed to SEA in 1965, so problems with the pressure containment vessel were already a thing in the early 1960s. USAF and US Army did some joint testing and analysis, and discovered that the hard chrome bolt was suffering hydrogen embrittlement (surface too hard, leading to failure nodes mostly at the cam pin hole, then the teeth adjacent to the extractor). They removed the hard chrome requirement for the bolt, and had new bolts made with a phosphate finish, added shot peening, and Magnetic Particle Inspection.

So you could make an argument that the bolt and extension have always been a bit under-sized even for 5.56x45mm. They addressed that with metallurgy and processes designed to make the bolt more durable, and this happened during Colt 602 production. The Colt 603 (M16A1) benefitted from a lot of hard lessons-learned in the ammunition changes, bolt failures, bore and chamber corrosion, gas port diameter reduction, and heavy rifle buffer to prevent carrier bounce.

A big factor in bolt durability is cyclic rate and the timing of the action, to reduce radial or rotational stress on the lugs while they are still under residual chamber pressure load. Gas system timing is important for several reasons, reliability being first, and bolt durability being second. It’s why Rifle-Length Gas System guns with optimum dwell time do so well and don’t really see bolt failures.

With shorter barrels, we’re trying to choke the gas and spring the action heavier to create as close to a RLGS as possible.

[Fess]

As mentioned, the pressure of the cartridges of the AR15 are handled by the bolt and barrel extension, not the receiver. This means that there is no set max pressure for the platform. There are variants of the AR15 with bigger and stronger bolts and barrel extensions that can handle the Winchester Super Short Magnums that have a rim diameter of 0.535" diameter and max average pressure (MAP) in the chamber of 65,000 psi.

The maximum bolt thrust can be calculated as the pressure of a piston the max diameter of the interior of the case multiplied by the max chamber pressure. With a given bolt strength, the larger the diameter of the cartridge case, the lower the max pressure allowed. The 223/556 AR15 bolt has a significant safety margin for its 0.3759" diameter case. To keep prices low, the 6.5 Grendel was designed to use what is essentially the same bolt the 223/5.56x45 with the interior bolt face opened up for the larger rim. The fatter Grendel case required a lower MAP to maintain good bolt life. For this reason, the MAP was limited to 52,000 psi for the 0.441" dia case of the 6.5 Grendel and 6mm ARC.

Note that the important dimension for bolt thrust is for the case body, not the rim. This comes into play with cartridges that have rebated rims (rims smaller than the case body). The 284 Winchester, for example has case body of 0.500" dia but the same rim diameter as the slimmer (0.470") 308 Win. The 284 Win was designed to be chambered in rifles designed around the 308 and its 62,000psi max chamber pressure. In order to keep the bolt thrust nearly the same, the MAP of the 284 Win is limited to 56,000 psi.