Let's design a new service rifle and cartridge...

[Grand58742]

Wondering where you are getting the "Grendel does not do as well in similar barrel lengths" info from? Don't want to rehash the dozens of debates on TOS, most by people with minimal experience with either cartridge.

If you noticed, I posed the question rather than make a qualified (which I'm not able to do) judgment on either cartridge and instead read as much as I can about both. And from all accounts, the 6.5 is dependent on longer barrel lengths for better performance.

The Grendel charts from Alexander Arms:

http://www.alexanderarms.com/images/...ballistics.pdf

Shows a significant decrease in velocity from a 19.5 inch barrel to a 14.5 inch barrel. And nothing listed under the 14.5 inch barrel.

And per the AA website:

This mainly refers to the 6.5 Grendel as the .50 Beowulf® is best in a 16" barrel. The two 6.5 Grendel barrels that are best suited for hunting are the 24" and 20" barrels. The shorter tactical barrels may be applied, but, beyond slightly lighter weight and handier silhouette, they have no advantage and give up velocity. However, they are superb choices for general shooting or a utility rifle. The 24" 6.5 Grendel is well-suited to most hunting applications and, if one can live with the longer barrel, it is the most versatile of the 6.5 Grendel barrels. Accuracy is superb and the rifle is capable out to 1,000 yards with the right ammunition. The balance of the gun assists stability in most position-shooting and it is excellent for use from a bipod. This one barrel can bridge across a range of shooting tasks including long-range varmint work. The 20" barrel 6.5 Grendel is both shorter and lighter than the 24" gun, which is a big consideration if the gun must be carried. While it gives up some stability from a bipod, it is just a versatile.

Bold emphasis added by me. Which does support the longer barrel claim. However, a 24 inch barrel is not exactly the ideal length in today's combat environment and the numbers show the 16-19.5 inch as being suited for an all around barrel length.

As for the 6.8 barrel lengths:

https://www.m4carbine.net/showpost.p...22&postcount=6

and the very last paragraph here:

https://www.m4carbine.net/showthread.php?t=19878

Which does bring back the question I asked of the side by side comparison of the 6.8 vs the 6.5 out of similar barrel lengths as well as terminal ballistics. I know Doc has extensively tested the 6.8 and is a proponent. Not sure about the 6.5 testing but I might assume he has done some at some point. I know Alex Arms has some pictures of some gel tests done on their website under the ammo heading, but no accompanying data save the velocity and penetration numbers.

So we come back to the original point. Does the 6.5G have the same ballistic capabilities out of the same length barrel as the 6.8 at the same ranges? Not intended to argue one over the other, I'm genuinely curious.

Unfortunately, the 6.5G just hasn't been developed as much as other calibers and remains for the most part a niche cartridge where the best performance is achieved through hand loading. I would like to see more manufacturers get on board and wring as much potential out of the cartridge as possible.

[wild_wild_wes]

...if you are designing a new rifle I'd not start with either the Grendel or 6.8.... They are excellent attempts to optimize within the AR-15 constraints, but the best route would be to deal with some of those constraints...I'd go a bit longer cartridge length...I am sold on 6.5mm for bore size as it is optimum BC for the 115-120gr bullet weight range. Which is a very good AW / light rifle/carbine weight. Any larger, you either lose downrange performance or get into 7.62 NATO size/recoil/cost. Any smaller and you might as well stick with 5.56.

If we were just talking only using one of those cartridges as is, I think the Grendel would be the better choice, as it would allow for longer-range use out of LMGs as well as an assault rifle.

But if we were looking at a whole small arms cartridge family, neither would be optimal. In that case a more powerful 6.5 with longer case would rock, for use by both LMGs and DMs. And as was already said, a shorter case 5.56 would fill the need for most small arms users, in the form of a PDW.

[greenlion]

There's the 260 Remington and 6.5 Creedmoor. They are great cartridges, but you are creeping back up into .308 Rem territory as fas as size and weight of the cases.

There is also the .270-08 (6.8) wildcat. I would like to see a side by side ballistic comparison of that and the 260 Remington, since they are both based on the same 308 winchester case.

I think stepping down one half millimeter (6mm PPC/243 winchester), is going too far in the direction of the .223.

Stepping up one millimeter (7mm-08) is getting too close to the .308 in terms of size recoil and weight.

Something hovering around 6.5 or 6.8 seems to be magic in terms of low recoil and great ballistic coefficients.

[mig1nc]

What if somebody lengthened the 6.5G to the limit of the UCIW magwell?

As somebody already mentioned the Magpul 6.8 PMag which is designed for the UCIW magwell and won't fit in a normal AR lower.

[RyanB]

If we were just talking only using one of those cartridges as is, I think the Grendel would be the better choice, as it would allow for longer-range use out of LMGs as well as an assault rifle.

But if we were looking at a whole small arms cartridge family, neither would be optimal. In that case a more powerful 6.5 with longer case would rock, for use by both LMGs and DMs. And as was already said, a shorter case 5.56 would fill the need for most small arms users, in the form of a PDW.

The last thing we need is to feed the same cartridges into a rifle and machine gun. Ask a logistician if you can interchange ammo from an M4 to a SAW or vice versa. Aside from the mag feed system that causes trouble, you can't. Different items for linked and unlinked ammo. Furthermore you'll want different ammunition. The DMR should fire special ball with potential for groups in the range of 1 minute or .3mil. The machine gun might as well fire copper washed steel to save money and increase penetration. The accuracy of the weapon need only allow for a 2 mil beaten zone. You'll also want a bullet suited for penetration.

The original SAW should have been in 6mm SAW or at least used 5.56 with an 80 grain bullet loaded to 2.55".

[Bubba FAL]

Surprised no one's mentioned the .280 Brit developed in the '50s. Might not be having this discussion if it had been adopted by NATO instead of the 7,62x51.

[greenlion]

I don't even get the "Grendel does better out of longer barrels" argument. Everything does!

If the Grendel is going faster to begin with, and has better ballistic coefficients than the 6.8 or 7.62x39 in a 24 inch barrel, it will still be going faster and have better ballistic coefficients if they are all in a 14 inch barrel.

I don't know of any rifle bullet that gets faster in a shorter barrel. Please explain how ONLY the Grendel is hampered by being in shorter barrels...

[greenlion]

Surprised no one's mentioned the .280 Brit developed in the '50s. Might not be having this discussion if it had been adopted by NATO instead of the 7,62x51.

I personally think it would have been great if the NATO had adopted the 6.5x55 Swedish Mauser, or even the 7x57 Mauser instead of the 7.62x51. They would then have had the 60 years worth of development the 7.62 has seen. We would have tons of rifles with better ballistic coefficients and less recoil for sporting use.

[SomeOtherGuy]

I don't know of any rifle bullet that gets faster in a shorter barrel. Please explain how ONLY the Grendel is hampered by being in shorter barrels...

Grendel fans don't like seeing their cartridge lobbing bullets at 300BLK velocities despite requiring special, weak bolts and special magazines. That's about it. (Yes, of course, 6.5G will lob a better BC bullet at a given velocity than 300BLK, but for many loads you're still looking at rainbow trajectories. Remember, .45-70 will make it out to a kilometer easily, just not with a little 8 mil elevation adjustment.)

[SomeOtherGuy]

Keeping the 5.56 case dimensions but enlarging to a 6 or 6.5mm bullet would provide an incremental increase in barrier penetration, wounding, retained energy at long range (due to higher BC), and relative performance in short barrels (due to higher expansion ratio), all while allowing continued use of everything except barrels. If there was a need for a right-away improvement, this would provide it, just nothing spectacular. It would also cause only a slight increase in ammunition weight, which would be very important to the infantry.

6.8 SPC provides more & better for all those good things, but requires bolts, magazines and barrels to be changed (and belt links for belt-fed weapons). It also causes a greater weight increase, especially if you want the more effective 110-120gr bullets. It could be a good option too.

Anything bigger or heavier than 6.8 SPC probably makes the ammo standard load too heavy, and may make the rifle bigger and heavier too, for ordinary infantry or PDW type use.