6mm ARC vs 6.5 Grendel Sanity Check

[Fess]

The Grendel is generally a better deer-hunting cartridge. Hunters want a clean kill which typically is said to be 750-1000 ft-lbs of energy at the target (it is, of course more complicated than that). The 6mm ARC was not optimized for hunting deer, though. It just needs to disable and potentially kill a human. When NATO was looking for a PDW cartridge, they determined that a "disabling" energy could be under 100 ft-lbs, which is how the P90 supposedly had an effective range of 400m. Hornady designed the 6mm ARC to meet whatever convoluted definition was in its contract. Working backwards from what Hornady has written and what has been said in podcasts, the equation they were given values hits per weight of ammo over kinetic energy at and beyond some specified distance, likely over 600m. That is why they can claim that the 6 ARC is more lethal than 7.62x51 and the 6.5 Grendel. They admitted that they started with the Grendel, but the need to reduce drop and windage at the specified distance pushed them to 6mm.

[StoneHendge]

When the barrel goes sub 20" I kinda don't understand the arc at all vs grendel honestly.

My 16" Lilja 6 arc barrel does 105 gr RDFs with a G7 of .280 at 2450 near max with Varget. My 22" JP Grendel barrel does 123 gr Scenars with a G7 of .265 (what I use) at 2450 with a near max load of Shooters World Precision (which is a Varget clone, but more dense so case capacity isn't an issue using a drop tube). Both shoot sub MOA all day long.

My climate, geography and type of competition rule out use of ball powders, but I did get close to 2600 with CFE and the 123 Scenars in my 22" JP Grendel (which went from 2600 to a very uncomfortable 2650 when the temperature went from 95 to 105) before maxing out years ago. Based on the experience of others, I don't doubt that I would be able to get in the 2600 ballpark with CFE or Lever with the 105s in my 16" arc.

[LRRPF52]

How does the 6mm ARC shoot with 105 RDFs?

Hodgdon’s data shows that 31.2gr under a 123gr SMK is 48,700psi at 2602fps from a 24” barrel. I think even with the temperature shift, you’re still fine.

123gr SMK is a very similar bullet to the 123 Scenar, as both were competitors with each other back in the 2000s. Form factor and BCs are almost identical. Not sure about jacket thickness though. When Bill did his initial testing with both bullets and published the load data for them, they both had identical charge weight across the powders.

You can see that in Volume I of the Grendel Handbooks.

[LRRPF52]

The Grendel is generally a better deer-hunting cartridge. Hunters want a clean kill which typically needs 750-1000 ft-lbs of energy at the target. The 6mm ARC was not optimized for hunting deer, though. It just needs to disable and potentially kill a human. When NATO was looking for a PDW cartridge, they determined that a "disabling" energy could be under 100 ft-lbs, which is how the P90 supposedly had an effective range of 400m. Hornady designed the 6mm ARC to meet whatever convoluted definition was in its contract. Working backwards from what Hornady has written and what has been said in podcasts, the equation they were given values hits per weight of ammo over kinetic energy at and beyond some specified distance, likely over 600m. That is why they can claim that the 6 ARC is more lethal than 7.62x51 and the 6.5 Grendel. They admitted that they started with the Grendel, but the need to reduce drop and windage at the specified distance pushed them to 6mm.

From what I’m seeing with the 6.5 Grendel 100gr ELD-VT factory ammunition, it looks like it duplicates 6mm performance when I compare my real world with the Hornady 4DOF program and their 6mm bullet Radar data tracks. 6.5mm should spit a 100gr out faster as well, since it has more bore volume to evacuate, and more case capacity.

[LRRPF52]

I feel like you can approach that with the 130gr class of bullets in a 16" which would put you on par with the 115 in terms of BC. Interesting thought experiment. I feel like sierra is lying about the bc of the 105 blitzkings though. They were bucking wind way better and dropping far less than they should.

When the barrel goes sub 20" I kinda don't understand the arc at all vs grendel honestly. Grendel just makes more sense to me. Is the arc easier to load/more forgiving than grendel? I mean grendel is pretty easy. Shove enough lever in the case until you feel uncomfortable then send it with a projectile of your choice. Shoot sub moa all day. (Generalizing but you know it's about 90% accurate)

It’s very common for the Sierra bullets to have under-stated BCs, and other companies to have over-stated BCs. Here’s a great example of that:

123gr SST on the left, 120gr TGK on the right:



Just from looking at the boat tails and overall length, you can see which one has a better form factor.

Hornady lists the 123gr SST at .510 G1 BC. Litz had it at .462 G1.

Here are Sierra’s specs on the 120gr TGK:

.466 @ 2,360 fps and above / .482 between 2,360 and 1,660 fps / .420 @ 1,660 fps and below

From what I’m seeing from Hornady’s Radar data and actual drop/drift, I’m suspecting the published BCs for some of the 6mms are inflated. They did update their BCs with Mach value bands showing how BC changes with speed downrange, while also pushing people to use the Radar data that isn’t based on theory, but actual Doppler Radar tracks of each bullet.

[biodsl]

How does the 6mm ARC shoot with 105 RDFs?

Sadly, just as with the Grendel, I can't get the RDFs to shoot well. Above is the Hornady 105 BTHP. Below is the 105 RDF. 1.2 or 1.5 MOA is typical for the RDFs. Barrel is my Rebby-spun Lilja 16".

105Horn_v_105RDF.jpg

[Fess]

It’s very common for the Sierra bullets to have under-stated BCs, and other companies to have over-stated BCs. Here’s a great example of that:

Hornady lists the 123gr SST at .510 G1 BC. Litz had it at .462 G1.
...
From what I’m seeing from Hornady’s Radar data and actual drop/drift, I’m suspecting the published BCs for some of the 6mms are inflated. They did update their BCs with Mach value bands showing how BC changes with speed downrange, while also pushing people to use the Radar data that isn’t based on theory, but actual Doppler Radar tracks of each bullet.

I am fairly certain that the 123 SST predates Hornady's doppler radar system, but so far, they have not updated the info for that bullet on their website. Their marketing department seems to follow the the motto: "Always higher, never lower" when it comes to numbers. They are catching quite a lot of flak lately for the 7mm PRC factory ammo coming no where near the velocity printed on the box (ammo used for the launch was fairly close). An official response was put on a YouTube comment thread that admitted that they are unable to get the higher-performance propellant and have made a substitution, but so far, they have refused to put a sticker on the box with velocities that reflect its contents.

I have not looked into 4DOF. Is there a place that actually shows the BC or does it just spit out results for distances? I suspect that it would not be terribly difficult to back-calculate to get close to an actual BC. Heck, looking at results and back-calculating is how they formerly did it.

[LRRPF52]

Yes, the legacy BC values were based on 300yd tracks and extrapolated back to the form factor compared with baseline shapes.

One of the Hornady outliers is the 129gr SST is under-stated at .485 G1, whereas Litz found it to be .495 G1. Several of the 6.5mm and 7mm bullets had higher BCs than advertised, but most of the rest were over-stated across the industry when Litz did his first analyses.

Nosler and Speer were notoriously-bad for over-stated BCs as well, especially the bullets with the token boat tails that might as well be flat bases like on the ballistic tips, or the Sciroccos.

Lapua first published a free Doppler Radar program for their Scenars in the 2000s called Quick Target Unlimited.

Hornady’s 4DOF program is free and available right on their site with the ballistics program. You just click the switch from standard ballistics model to 4DOF.

When you’re in the 4DOF program, you select the projectiles from a drop-down list, which has a library of all Radar-tested bullets, including many from competitors now. I think they’ve done an overall great job with that, especially putting competitor bullets on it that are better BC-wise than a lot of their flagships.

I think everyone is having problems with consistent propellant sourcing nowadays.

8208XBR seems to have become a lot more energetic when you look at Hodgdon’s data.

Lot-to-lot variations are getting weird with ammo too.

I was shooting some 110gr PPU the other week through my 12” and the impacts seemed like they were higher than my 100yd center dot at 400yds.

A guy just registered 3000fps with 123gr ELD-M from his 16” Grendel in a string of normal 2450-2460fps velocities, with pierced primers in the hot readings.

This isn’t specific to any particular company, but something I’ve observed:

We have the skills gap hitting hard with Boomers all but retired or dead (born from end of WWII to early 1960s, so ages 79-60), the Gen X nepotist owners off on another vacation, Millennial managers on 2hr lunch or mental health breaks off on their phones, while the few Gen Z kids who actually show up to work run the machines. What could go wrong?