Reloading Press: 6.5 Grendel
The Reloading Press is an at-least-weekly feature here on Gaming Ballistic for 2016. Each week it looks at some interesting real-world cartridges and presents them with hopefully-useful information in GURPS Format.
6.5x39mm (6.5 Grendel)
The 6.5 Grendel was designed with the goal to give the AR-15 platform “legs” out to 200-800 yard, and even farther if possible. It started life as a variant of the 6.5 PPC, both of which were derived from the .220 Russian cartridge. As you can see from the cut-away, it features a fairly short and squat chamber case – which is excellent for combustion – and a long bullet.
Almost ridiculously long, actually. Though it’s not shown, the 120gr solid copper bullet has an overall length (OAL) of 35.56mm (1.4″) and an actual bullet diameter of 6.71mm (0.264″), for an aspect ratio of about 5.3, compared to about 3.9 for a 5.56x45mm. This is done purposefully, as high sectional density is a great way to get long range out of the projectile.
And get long range it does. With heavier bullets (130gr rather than the highest-energy version modeled as standard here), the penetration at 1,000 yards is modeled as 5% higher than even the 7.62x51mm NATO standard 147gr projectile (and a scant 1.25% less than the 168gr match bullet). So mission accomplished from that perspective.
But what do you give up? There’s always a trade-off, right?
Ballistic’s Calculator Inputs
Basic inputs for the calculator are as follows, selecting 406mm for the (test) barrel length.
| INPUT | ||
| 6.5 Grendel | ||
| Chamber Pressure | 50000 | psi |
| Barrel bore | 6.71 | mm |
| Case Length | 38.7 | mm |
| Chamber Bore | 10.87 | mm |
| Barrel length | 609.6 | mm |
| Bullet Mass | 130 | grains |
| Aspect Ratio | 5.3 | L/Bore |
| Burn length | 21.62 | mm |
| Projectile Caliber | 6.71 | mm |
| Total Accelerated Mass | 130 | grains |
| Expansion Ratio | 2 | expansion |
| Projectile Load | 1 |
That being said, the standard rifles with this cartridge are delivered can toss out 123gr projectiles out of an AR-15 platform that are still supersonic at 1,200 yards.
Out of the 24″ test barrel (610mm), the bullet develops 823m/s from the muzzle, and has a higher 1/2D range than the 150gr 7.62x51mm NATO round by about 55yds.
The projectile is just small enough that you have to worry about a pi- to pi transition from a front-on perspective, but the darn thing is so long that if it yaws in flesh it’ll do some ugly stuff. It’ll drop below 600m/s at 185yds with a 14.5″ barrel (such as the M4 carbine) and stay there to 300yds with the 24″ sniper barrel.
Barrel lengths sold by the vendor include 16″, 18″, 20″, and 24″. So all of the available platforms will range from 6d-1 pi to 6d+1 pi, but can reach out as far as much heavier rifles and rounds.
- The velocity is at the muzzle. The velocity with a 24″ test barrel is tuned to match real-world data at 2700fps with a 120gr solid copper projecile, the highest energy load listed on the Wiki page. I do this because GURPS damage is based on kinetic energy, so no GURPS PC worth their salt will choose anything less than the highest energy if pure penetration/lethality is at issue. If you’re doing suppressed subsonic shooting, that changes.
There are heavier and lighter bullets available; I’ve read that heavier bullets have issues due to stabilization, and if you start shooting a lightweight bullet here, I suspect you give up the substantial sectional density advantages of a long, heavy bullet.
This seems to be a very, very niche cartridge. It does what it does very well: it extends the effective range of an AR15 platform out to 800-1,000 yards. This gives you advantages in ammunition weight and magazine capacity (the 6.5 Grendel mags hold 26 rounds each), and potentially a very light rifle.
However, the restrictions on the performance sweet spot of the round (120-130 grains, apparently) and that it seems to do what it does best out of fairly long barrels (20-24″) means that this is going to be a firearm that mostly designated marksmen and snipers will love . . . but if you’re only sending one or two rounds at a time downrange, you might as well carry something more substantial. And if you’re looking for a round that does as well out of a carbine as it does out of a sniper/marksman’s platform, my understanding is that this isn’t your go-to.
Properly loaded and matched to a barrel/powder combination, match bullets easily hit 0.75 to 0.85 MoA at range – again, very handy for an AR-15.
You need a purpose-built upper receiver (or at least a barrel, chamber, and bolt change) to shoot this cartridge. The proprietary cartridge and upper seem to only be sold by Alexander Arms, the inventor of the round. The rifle can run you from $1,150 to $3,200 depending on parts and accessories, but if you already have a lower receiver, you can get into 6.5 Grendel for $650-$2,000. The higher priced models are presumably with tighter tolerances and better parts overall, such as free-float barrels.
Don't forget that owning a 6.8 SPC or a 6.5 Grendel often comes with "Intolerance, people who own the other round".
I actually deleted a section that sounded too much like "if you want to reach out to 700-1000 yds with an AR platform, buy this. For everything else, there's 6.8SPC." But that sounded too snooty, so poof, delete key. 🙂
I am a little confused by your statements, "This seems to be a very, very niche cartridge" and "if you want to reach out to 700-1000 yds with an AR platform, buy this. For everything else, there's 6.8SPC". From your write up here and on the 6.8 SPC page it seems like the Grendel has the same or strictly better performance than the SPC at all ranges and from all barrel lengths down to about 10". It has the same or better energy, carries that energy farther, and never drops down to GURPS pi- damage. Also, in your SPC write up it says that the mag capacity for the SPC, for a mag of the same dimensions as the 5.56, is 25 rounds and the Grendel mag holds 26 (which actually surprised me, I thought the SPC mag held 30). So it seems to me that the niche cartridge is the 6.8 SPC; it is for short ranges and for using submachine gun length barrels with a rifle round. Why do you see the 6.5 Grendel as the niche round?
-phayman53 (sjgames forum name)
I explain why in the following paragraph. The 6.8SPC chambering is fairly widely available from many vendors. The 6.5Grendel from only one. The 6.8SPC is optimized from 16" barrels and does very well in much shorter ones and with a larger variety of bullet weights and lengths – so it does short well and can do long. The 6.5 is top-of-the-class with a 125+/-5 grain bullet out of 20-24" barrels, which is a DMR weapon in conventionally laid-out rifles.
If the world over used bullpups, so you could get that 20" barrel from a 30" weapon, the story would be different. But if you one one rifle chambering with many different potential barrel lengths and bullet weights and sizes and types, I'll stand by my statement: the 6.8 is more flexible. If you fit the parameters of the 6.5Grendel for barrel length and bullet weight/size, it's your huckleberry. For wide issue to general troops? I'd say no.
That's a real-world assessment, though. From the GURPS perspective, my calculator probably does not reflect the local optimum for powder and bullet size for the 6.5G as well it might, largely because differences in powder burn profile can't be modeled in the program.
Thanks for the reply, that clarifies things a bit. When I read your paragraph below the part I quoted I did not realize that you were saying that the drop off in performance was worse than your ballistic calculator indicated. So, basically, if I wanted to "realistically" model the drop in performance of the 6.5 Grendel as barrel length shortens in a GURPS game, I should pobably have the damage decrease more quickly with barrel shortening than your chart above indicates, correct?
One other question: you mentioned that the narrow sweet spot in Grendel bullet weight, 125 +/- 5, is a drawback. My knowledge of ballistics is not as good as it could, but I was wondering why this is a huge drawback? It is arather weighty bullet with great range, accuracy, and penitration. Why would one want a bunch of options when, with this round, there is a clear best (in other words, what does it sacrifice by not being able to go hravier or lighter)?