The Era of High Performance Intermediate Cartridges

[LRRPF52]

Since the introduction of 6.5 Grendel in 2003, we have been in an era of new High Performance Intermediate Cartridges, primarily driven by delivering the most performance possible from within the AR-15 receiver set, while still bringing double-stack staggered capacity magazines.

The demand for this performance is driven by both the private and military sectors with a lot of overlap in requirements. Those of us who got into AR-10s and SR-25s early-on became quickly familiar with the weight, bulk, and sight picture disturbance penalties paid with .308 and similar cartridges that fit into that form factor, so became a bit disillusioned with the large frame guns.

Since 2003, we have seen several cartridges introduced to work from within the AR-15 to deliver more energy on-target, while keeping recoil and sight picture under control. Here is a list of most of them that double-stack and have/had solid company support.

6.5 Grendel
6.8 SPC
300 Whisper/300 BLK
6mm Hagar
.25-45 Sharps
6mm AR (Grendel necked down)
224 AR (Grendel necked down to .224”)
7.62x40 WT

224 Valkyrie
24 Nosler
22 Nosler
6mm ARC (Grendel with shoulder moved back .030” necked down)
350 Legend
300 HAM'R
6mm Max
22 ARC (22 AR formalized by Hornady)

So we have 1x 6.5mm, 1x .277”/6.8mm, 3x .30 Cal/7.62mm, 5x .243”/6mm, 1x .257”, 4x .224”, and 1x .357”.

For 6.5 Grendel, we have bullet weights ranging from 80-160gr, with 124 factory options covering 85-140gr.

For 6.8, there are bullet weights ranging from 85-140gr, factory ammo 87.5-120gr. Almost all of it is 110gr or 115gr though.

For .30 cal, there are bullet weights of 110gr-220gr (subsonic 300 Whisper/BLK use heavier, supersonic use 110gr-125gr). 300 HAM’R uses 110-150gr.

For 6mm, bullet weights range from 55-110gr. The Grendel-based 6mm cases can use the full range of 6mm bullets. The longer cases were limited to 55-75gr, 55-90gr, or 55-100gr.

For all the bottleneck supersonic cartridges, the average bullet weight is 97.5gr with rough calculations.

Most of the above cartridges are dead or on the verge of dying. 6mm Hagar is dead, as is 25-45 Sharps and 7.62x40 WT (replaced by 300 HAM’R). 224 Valkyrie has some degree of following with Federal keeping ammo prices low to try to save it, but it doesn’t offer much over 5.56 when you look at 62-75gr velocities. 24 Nosler and 22 Nosler are all but dead. 6mm ARC and 22 ARC will put the final nails in their coffins I think.

That leaves us with 6.5 Grendel, 6.8 SPC, 300 BLK, 6mm ARC, 6mm Max, and 22 ARC. 350 Legend and 300 HAM’R are for niche hunting applications with a straight wall approach. HAM’R has a shoulder but it’s effectively a straight wall.

When I’ve compared the Grendel against the 6mms, I’m seeing a lot of overlap due to faster mv from Grendel due to piston area/bore volume. Doppler Radar data seems to indicate we’ve been misled with BCs again for many of the 6mm bullets, so my 100-110gr 6.5mm real-world trajectories have matched or exceeded the 6mms of the same weights. 18” Grendel seems to match 24” 6mm AR and ARC with 90-107gr weights, with a bit less chamber pressure as well.

6.5 Grendel continues to remain the most capable and versatile alternate chambering for the AR-15, with a ton of factory ammo support.

[mtnlvr]

Great writeup! I'm a fan of the 6.5G as it really is a versatile and seems to be a well-performing caliber in both gas and bolt guns, and at a variety of barrel lengths!

[lazyengineer]

I agree, and consider 6.5 Grendel to remain the best general purpose round. My biggest frustration with 6mm ARC is for some reason they appear to run a slightly shorter headspace - meaning it's a bit more spotty converting recovered 6mm ARC brass back to 6.5 Grendel... Grr... But back to your point, I consider a 6mm diameter bullet just a bit too small diameter for the terminal performance I'm going for (just a bias), and the .30 cal bullets just to slow and heavy and too much of a rainbow trajectory. 6mm rounds don't have any advantage until you get so long range, that I doubt I'll take an ethical hunting shot that far, with the trade-off of reduced performance at the distances I do. It IS a great PRS/gong/game round though. Where .300 rounds are just too short range for my interests. 6.5 Grendel is juuuusssst right and good at all of it. It's a fantastic do-all round. 6.8 was close, I feel bad for that round as it's a good round - but Grendel has better legs for those 800 yard gong range shots, while still giving comparable terminal performance at the closer range. What I really like about 6.8 SPC is the smaller diameter case body and higher pressure rating, but in the end, 6.5 Grendel is just the better do-all round.

The bigger question I have right now, is where will the new developments go in consumer grade higher strength alloys, that we are seeing in the SFAR (and I suspect Sig Fury guns). Which is allowing 6.5 Creedmoor to be fired in an AR15 sized barrel extension and Bolt (ish). That's impressive. For $905, I have one on order right now to play with.

So my follow-up to that, is can one get a 6.5 Grendel bolt and barrel extension in this new alloy now as well? And with that, can one now run a 63,000 PSI 6.5 Grendel? IMHO, that would be profound.

[LRRPF52]

The reason Hornady moved the shoulder back on 6mm ARC was because a certain bullet for the military customers required a COL that was a bit long for the Elander mags, which are not forgiving at all when it comes to even 2.260” COL, but they’re smooth mags that feed well if you move the COL back to 2.230-2.245”. At those COLs, the ogive and shoulder-neck-junction were able to be located back .030”, hence the relocation of the shoulder. One of the Hornady engineers described this without specifically calling out the Elander mags, but they’re the main ones that have that constraint.

The problem for the 6mm is the bore is tighter, which doesn’t allow you to throttle it as fast as a Grendel with the same bullet weights.

What POF and now Ruger are using in the small-frame .308/6.5CM rifles is AerMet alloy for the bolts and extensions. It costs about twice as much as your 9310 steels used in AR-15 bolts.

Sons Of Liberty Gun Works has AerMet Grendel bolts for 6mm ARC, but you also have to think about matching the strengths in the barrel extension and bolt when balancing the pressure containment systems.

If you look at the Ruger SFAR extension, it has 3 pressure relief vents for catastrophic failures, 2 of which perforate the walls of the receiver around the extension at 3 & 9 o’clock, so they are aware of the consequences for failures in hoop strength with the extension. Keep in mind that’s an AR-15 bolt with a .473” cartridge pocket in the face, with no changes to the diameters of the extension, which means really thin chamber walls.

See the pressure relief hole at 3 o’clock in front of the ejection port. They also vented the carrier to bleed off gas in a catastrophic event so the carrier doesn’t turn into a grenade. It’s an important consideration when you have 46gr of powder and 62,000psi chamber pressure when a case fails.



With high round counts, I would expect to see egging of the chamber, but we’ll see. I think Ruger knows that 99% of the customer base can’t afford to shoot .308 or 6.5CM in high volume, and the POF Revolution was high-volume tested with low-pressure Russian steel case ammo.

The legacy 107-110gr 6.5mm bullets are already competitive with the 103-108gr 6mms, despite what you see for advertised BCs. Berger probably has the most accurate BCs for their published numbers, but I have suspicions about many of the others after running them through the Doppler program, and seeing numbers that don’t match the advertised BCs.

To seal the deal, all you need is some updated 6.5mm bullets in the 100-110gr class with longer ogives, tipped meplats, and long boat tails. Hornady did this with the 100gr ELD-VT which is a factory round, and Sierra has some great ones as well with the 105gr dual-purpose Blitzking. Hornady isn’t pushing speed either with the 6.5 100gr ELD-VT factory ammo, as it is only doing 2740fps from a 24” barrel. Per Hodgdon’s data, you can drive 100gr faster with LVR and CFE223 at lower chamber pressures.

I think where the 6.8 SPC, 300 HAM’R, and 350 Legend make sense are for hunters who want that close-in “slap" on-target. 7.62x39 does it too, but Grendel hits with more FPE with comparable audible slap as well. Even at 400yds from my 12” Grendel, the audible impacts on steel are impressive with 123gr. 6.8 SPC 115gr OTM goes subsonic 200yds closer (675yds) than even 5.56 77gr Mk.262 (875yds), which is one reason why SOCOM didn’t show any lasting interest in it. Those are from 16” barrels too. A 12” barrel Grendel will stay supersonic past 900yds at sea level, so you get better performance than the 6.8 or 5.56 with much shorter barrel length, allowing use of a can in the same profile.

As to the pressure ratings, 6.8 SPC should not have been over 53,000psi SAAMI MAP due to extension tooth length, bolt lug length, and hoop stress. Anytime someone tried running high-volume pyramid testing with the hotter 6.8 loads, they blew up guns and broke bolts repeatedly. JSOC tested it like that on their own because they weren’t seeing competent total ownership from anyone in the industry, and ceased the testing with 10 carbines after they started failing. Same thing happened within Barrett, who was testing duplex loads even. The brave reloader at the bench can get away with a few ranges trips of high velocity loads pushing 55,000-58,000psi, then run back to the forums claiming these as the norm, but who has ever pyramid-tested that load and crossed the finish line with 10 intact AR-15 carbines? With the right powder selection in 6.8, you can still get really fast speeds and stay under 53,000psi.

If you used AerMet bolts and extensions in 6.5 Grendel in the AR-15, I would go with the WOA M4 competition barrel extension and barrel thread tenon with longer, more shallow threads. This pushes the thread relief cut farther forward away from the center of the propellant stack in the case where peak pressure occurs. One of the issues with the AR-15 is that the thread relief cut for the barrel extension and tenon is right over the center of the powder column. Also makes pressure testing a fool’s errand with a standard AR-15 action because of gauge placement needing to be over the propellant column center, otherwise you won’t get peak pressure reading.



As to running the Grendel case at those pressures, for military, it would make sense to look at the hybrid cases as well. Right now, we’re getting 93% of the performance of .260 Remington with 6.5 Grendel when you compare 123gr from 24” barrels. (2650/2850 = .9298) We’re getting that with 10,000psi less chamber pressure and 15gr less powder, with single digit FPE recoil energy, from almost half the rifle weight.

[lazyengineer]

That was a really great reply. Thank you.

FWIW, as a reloader, and one who sometimes visits gong ranges, I have mountains of 6.5 CM brass (it's more common than .308 brass at the places I go to, adn people just leave it on the ground at such venues by the hundreds now). So I have piles of 6.5CM brass, and bought the SFAR to basically just start pounding ammo through, rather than wrecking my RPR BBL.

I have a 6.5CM MDRx, but am retiring it. Despite the shortness of bullpup size, t the gun is wildly too heavy for a decent CQB gun (12 lb with scope), yet grossly inaccurate for it's 1000 yard gong-banger roll (3 - MOA and will throw a flyer out to 5+MOA pretty much ever string - sadly; the MDR has issues). So I'm probably going to just take a bath and sell it. And going to give SFAR a shot. We'll see how the chamber holds up - I respect Ruger and and will be surprised if the chamber develops issues - we shall see! I do have some PRS high pressure load ammo, so some of it's pretty hot - we'll see how those batches run too.

[LRRPF52]

I have the Savage MSR-10 smaller frame .308 and haven’t even shot it yet. I panic-bought it in 2020, as if I hadn’t been panic-buying ARs since the 1980s. Lol
That’s my 7th AR-10 and I don’t shoot the others I kept. I love the .260 Rem gasser with Bartlein barrel, which feels like you can’t miss with it, but it’s 14lbs and wears out primer pockets on the 3rd firing, unless you really drop the charge weights down and make it a glorified Grendel.

That little Savage MSR-10 though has all the features I could have asked for, and a small bolt carrier but appears to use a standard LR-308/SR-25 sized bolt and extension. The lighter you go with the rifle in the .308 class of cartridges, the more torque you see yanking the scope to the left (RH twist rifling).

But yeah, 6.5CM has become very ubiquitous over the past 10 years. Shelves are full of it, but how many of the factory loads are made to a dual-pressure point gas gun specification like 7.62x51 is?

For the AR-10s, I think 7mm SAW would be a fun project. 7mm-08 already does really well compared to both .308 and 6.5CM, but no target loads for it. Excellent barrel life and great 7mm bullet selection make it more ideal for the gas guns, since there is plenty of bore volume similar to .308, so evacuation and piston area work in your favor looking at velocity.

Every bullpup I’ve used has been heavier than expected, including the AUG. Everything looks amazing on paper with them until you put one in your hands and get that clunky feel.

[lazyengineer]

...
Every bullpup I’ve used has been heavier than expected, including the AUG. Everything looks amazing on paper with them until you put one in your hands and get that clunky feel.

Unfortunately this is true. I have and used to heavy run an AUG, but in the end, It's just a heavy gun. One to look at, is the RDB - from KelTec - of all things (i know.. KelTec). In all honesty, It's actually kind of great, and of late, people who have been doing head-to-head and really running the guns hard; are starting to say it's actually the best one. I believe so too. It's the lightest, best trigger, quite reliable, softest recoiling (AUG's recoil way more than they should), and actually pretty accurate (after broken in - their barrels start off pretty bad, but get better pretty quick). It's been my wish for awhile now that somebody somewhere would make a Grendel version, because then you'd have the best most compact efficient power in a small frame (6.5 Grendel - 17" BBL), in the most compact form factor (bullpup), and in the rendition that's the lightest weight and best overall gun (RDB). There's only 2 parts they need to make - the barrel, and the bolt. But, so far, no joy.

For reference:
Here's an RDB 17" next to an 18" (yes 18") AUG
The RDB is just a more svelt gun.

Here's an RDB (5.56 of course) next to an SBR'd 14.5" 6.5 Grendel (my night hunting rig) Look how drastically smaller it is, yet has a 17" BBL. I align at the trigger, for the photo comparison.


Here's how much one weighs with an upgrade metal handguard and a decently heavy optic choice (I.e. you could go lighter with some effort)


Here's the part (which is not serial numbered) that I wish someone would sell in 6.5 Grendel.

[LRRPF52]

One of the things that seems to separate High Performance Intermediate Cartridges is having upwards of 34gr case capacity.

It allows higher muzzle energy from shorter barrels, and depending on the projectile shape/BC, downrange ballistics that justify use in place where legacy 7.62 NATO weapons have been used.

[Lemonaid]

Any comments about the 6mm MAX? It seems about the same performance as a 6 ARC but uses the standard bolt of a .223.

[LRRPF52]

The overall 6mm MAX form factor would have made a better cartridge for the AR-15 back in the late 1950s, had it gone forward as the SCHV cartridge.

One of the challenges with it currently is it requires longer COL, so the magazines haven’t been worked out for higher caps.

Case length is very close to 5.56, so there isn’t much room for ogive length in the 100-110gr class of projectiles where the 6mm AR shines.

Army Ordnance had Stoner running around in circles as a distraction while they pushed for the adoption and production contracts for the M-14 in the late 1950s, but some of the cartridges he and his drafters came up or were asked to draft designs for had more case capacity, and they made tweaks to the receivers, magazine, bolt, and barrel extension to accommodate them on paper. Each time they responded with their drafts, they were then told to do another based on this little tweak here and there. Stoner discusses it in the sit-down interviews he did if you ever want to listen to them.



One was based on the 25 Remington, which would have been more ideal for performance, but 30rd mags would have been taller. It has a .422” case head like the .30 Remington, which the 6.8 SPC was based off of.



What I like about 6mm MAX is that it uses a .390” body diameter, which is smaller than the .422” of the 6.8, so magazines should not be that tall compared to 5.56 mags, but you still get more case capacity. 6mm MAX is a necked-down 350 Legend, which is in-between .223 Rem and 6.8 SPC in case body dimensions:



Everything is a trade-off though, so in order to get more case capacity, the case had to be longer, and subsequently the COL is pushed to the limits of the AR-15 magazine well, while sacrificing some ogive length for the VLD-type projectiles. AR-15 magazine well is 2.395-2.398” raw internal max length, not counting the follower tab LRBHO trip spine, or the drainage channel up front. This does not leave much room to play with for adding COL due to magazine wall thickness, and necessary stand-off between the case head and meplats/bullet tips. There has to be clearance for cartridges to smoothly ride inside the magazine, and an error factor for deviations in manufacturing for COL must also be considered in the stand-off clearance.



If I could wave a magic wand and erase .223 Rem/5.56 from history though, something like 6mm MAX would be in its place, but not in 6mm. One of the constraints for the SCHV program was that the rifles had to use a .224” bullet. They could have still been a small-medium bore with another diameter, whether it was 6mm or larger and still exceeded the requirements.

The initial requirements were for 3400fps mv, because ballistic testing showed that 3400fps projectile terminal effects within 300m were pretty brutal. Had they leaned on a combination of BC and high mv, with moderate chamber pressure, they could have done it all with more case capacity. 6mm bores with 34gr of powder tend to be tightly-bottlenecked with bore ratio relative to propellant mass. There’s a balancing point where you can evacuate the bore quickly without crazy chamber pressure, generate insane mv, without torching throats. 5.56 does amazingly-well with throat and barrel life since it isn’t overbore.

6.5 Grendel will spit a projectile of the same weight faster than any of them, whether it be .224 Valkyrie, 22 ARC, 6mm MAX, or 6mm ARC. It then becomes a question of BC, projectile construction, projectile availability, and intended use. Grendel has 20 years of magazine development behind it with 26rd mags, which is an advantage. It also has 124 factory ammo loadings currently and growing, which I don’t think any of the others will ever reach.