Optimized Caliber Proposal

[stanc]

I am sure you know this, but for others:

.224 in = 5.6896 millimeters
.277 in = 7.0358 millimeters
.284 in = 7.2136 millimeters

Cartridge designations are named for numerous reasons but rarely on pure bullet dimensions. Common examples are .223 Rem is .224", .357 Sig is .355", etc.

So, when DocGKR says that a 7mm bullet is ideal and then lists .277 examples, he is doing so based on true numbers and not a manufacturer's given name.

Sorry, but manufacturers' names are not the issue. The problem is the difference between bullet diameter and caliber (and the failure to let the reader know which is being discussed).

- Diameter ---- Caliber -
.224"/5.7mm - .22/5.56mm
.243"/6.2mm - .24/6mm
.257"/6.5mm - .25/6.35mm
.264"/6.7mm - .26/6.5mm
.277"/7.0mm - .27/6.86mm
.284"/7.2mm - .28/7mm
.308"/7.8mm - .30/7.62mm

Note that "6.5mm" and "7mm" are unique in that they can be used for two different bullet sizes, depending upon whether one is referring to diameter or caliber.

[HTR]

While I agree in general with some comments made about .277 bullets needing more aerodynamic specimens, consider this:

When you look at the outstanding performance of an 85 or 95 grain Barnes bullet fired from a 14.5 or 16" 6.8 X 43 barrel and then pair that with a 120 SST, or perhaps even the 135 SMK, fired from a 20" AR or Bolt gun, there is already a wide variety of performance from the 6.8 X 43 now. Most rifles can reliably handle up to 60,000 PSI in slow fire or single fire, and 55,000 PSI in rapid fire, without accelerated wear on components.

With all this information, including the ARDEC data, it is unfortunate that there has not been more movement w/r/t the testing and or adoption, but things take time, and while its passing, 6.8 X 43 continues to reproduce and consolidate these performance figures.

I just wish more of the people making these decisions would go out and fire the round more often, looking at all of the performance gains its made in the last 5 years.

[RUTGERS95]

While I agree in general with some comments made about .277 bullets needing more aerodynamic specimens, consider this:

When you look at the outstanding performance of an 85 or 95 grain Barnes bullet fired from a 14.5 or 16" 6.8 X 43 barrel and then pair that with a 120 SST, or perhaps even the 135 SMK, fired from a 20" AR or Bolt gun, there is already a wide variety of performance from the 6.8 X 43 now. Most rifles can reliably handle up to 60,000 PSI in slow fire or single fire, and 55,000 PSI in rapid fire, without accelerated wear on components.

With all this information, including the ARDEC data, it is unfortunate that there has not been more movement w/r/t the testing and or adoption, but things take time, and while its passing, 6.8 X 43 continues to reproduce and consolidate these performance figures.

I just wish more of the people making these decisions would go out and fire the round more often, looking at all of the performance gains its made in the last 5 years.

great post....spot on

[Clint]

From the sister thread over at arfcom: http://www.ar15.com/forums/topic.htm...=540655&page=1

If freed from the M4/M16 platform imposed design compromises, an “idealized” combat rifle cartridge, can be explored. As discussed ad nauseum, virtually every wound ballistic test in the last 100 years points to approximately a 7mm projectile as offering the ideal combat performance––be it the 1920’s era .276 Pederson, the post WWII .270 & .280 British cartridges, or the newer SPC program. 6.8 mm only makes sense to upgrage current 5.56 mm carbines; if a new rifle is selected, it should be built from the ground up in a new optimized 7 mm cartridge along the lines of 7x46 mm or perhaps in a 7 mm CTA.

The above quote is from DocGKR.

This “idealized” combat rifle cartridge is interesting.

The general solution would be to bump up the OAL to 2.5" or 2.55" and use a .277 or .284 dia bullet.

This requires new mags, gear and a new weapon.

The increased length allows a muzzle energy of ~2000 ft-lbs for the "OC" ( 120 gr @ 2750 fps ).

For comparison, the 5.56 has ~1200 ft-lbs, 6.8 ~1600 ft-lbs and 7.62x51 ~2500 ft-lbs

So my question is this: how much is too much when it comes to recoil / flash / weight / length / etc?

Part of the beauty of the 6.8 is the moderate recoil, effectively allowing shots as fast as 5.56.

A higher power round would give some of that up, but would perform like the hottest 6.8 and at moderate pressure.

Thoughts?

[stanc]

Since you asked, here are my thoughts, Clint.

The 6.8 SPC was designed for the express purpose of converting existing 5.56mm carbines and rifles. The thing is, in the entire history of the US Army, there has only been one time that a cartridge was adopted specifically to upgrade existing weapons. That was right after the Civil War, and due to very tight budgets, a .58 Rimfire round was used in .58-caliber muzzleloading rifle-muskets that had been modified into "trapdoor" breechloaders. Even then, the .58 Rimfire was superseded the following year by the .50 Government round. In the 145 years since the .58 Rimfire was fielded, the Army changed rifle cartridges seven times, but never again for the purpose of upgrading weapons in the inventory. Each time a new cartridge was adopted, so was a new infantry rifle.

The moral of the story is that there is no need for a next generation, brass cased cartridge to be limited by M16 magazine well dimensions, because a new individual weapon would almost certainly be adopted along with a new cartridge. As I understand it, this is also the reasoning behind Cris Murray's 7x46 and Tony Williams' 6.5x45 rounds, which have OAL roughly midway between the 2.25" of 5.56x45 and 2.80" of 7.62x51.

However, I think the 7x46 and 6.5x45 rounds are probably more powerful than truly necessary for a carbine, and the bulk of the 7x46 will necessitate an undesirably long magazine in order to keep a 30-rd capacity. Interestingly, this may present a golden opportunity for 6.8x43, which has heretofore been somewhat handicapped by the relatively short, stubby bullets required for the round to fit M16 mags. Freed from the M16 mag length constraint, 6.8x43 could be loaded with a much more streamlined bullet, say perhaps a scaled-down 7.62 M80A1. In concert with the increased velocities allowed by the SPC II chamber, long range trajectory ought to be excellent. Such a loading wouldn't enable 6.8x43 to replace both 5.56x45 and 7.62x51, as Murray and Williams seek to do, but that likely doesn't matter. IMO, the probability of replacing 7.62x51 is nil, as there is no dissatisfaction with its performance.

[HTR]

Since you asked, here are my thoughts, Clint.

The 6.8 SPC was designed for the express purpose of converting existing 5.56mm carbines and rifles. The thing is, in the entire history of the US Army, there has only been one time that a cartridge was adopted specifically to upgrade existing weapons. That was right after the Civil War, and due to very tight budgets, a .58 Rimfire round was used in .58-caliber muzzleloading rifle-muskets that had been modified into "trapdoor" breechloaders. Even then, the .58 Rimfire was superseded the following year by the .50 Government round. In the 145 years since the .58 Rimfire was fielded, the Army changed rifle cartridges seven times, but never again for the purpose of upgrading weapons in the inventory. Each time a new cartridge was adopted, so was a new infantry rifle.

The moral of the story is that there is no need for a next generation, brass cased cartridge to be limited by M16 magazine well dimensions, because a new individual weapon would almost certainly be adopted along with a new cartridge. As I understand it, this is also the reasoning behind Cris Murray's 7x46 and Tony Williams' 6.5x45 rounds, which have OAL roughly midway between the 2.25" of 5.56x45 and 2.80" of 7.62x51.

However, I think the 7x46 and 6.5x45 rounds are probably more powerful than truly necessary for a carbine, and the bulk of the 7x46 will necessitate an undesirably long magazine in order to keep a 30-rd capacity. Interestingly, this may present a golden opportunity for 6.8x43, which has heretofore been somewhat handicapped by the relatively short, stubby bullets required for the round to fit M16 mags. Freed from the M16 mag length constraint, 6.8x43 could be loaded with a much more streamlined bullet, say perhaps a scaled-down 7.62 M80A1. In concert with the increased velocities allowed by the SPC II chamber, long range trajectory ought to be excellent. Such a loading wouldn't enable 6.8x43 to replace both 5.56x45 and 7.62x51, as Murray and Williams seek to do, but that likely doesn't matter. IMO, the probability of replacing 7.62x51 is nil, as there is no dissatisfaction with its performance.

I could not agree more with this post. As I continue to explore the possibilities with longer OAL, and heavier, higher BC bullets, I am finding some surprisingly good long-range performance from the 6.8.

Consider this:

If fired from a bolt gun ( in this case we will use this example to approximate the ballistics possible if we are not constrained to the AR-15 mag well). Thus, when loaded to 2.40" the 6.8 X 43 can fire the 110 TTSX, at 2775 FPS from an 18 inch barrel. It can also fire the 120 SST at 2620 FPS, which is exceptionally close to the 120 grain / 2700 FPS mark, and this bullet has BC of .400. It is only a minor matter to produce a bullet with weight of 110-115 or 120 grains and possessing BC of .450. It can be done.

This leaves all the performance gained in the last 5 years, still intact, as the Mil could use weapons as short as 10" for entry carbines or PDW's, and longer guns with 1:10 twist to launch 115, 120 or 130 grain bullets at speeds of up to 2650 FPS. I have already personally fired the 135 SMK from my bolt gun (not to mag length) at 2450 FPS, and made consistent hits out to 600 yards.

Interesting times are ahead....

[stanc]

If fired from a bolt gun ( in this case we will use this example to approximate the ballistics possible if we are not constrained to the AR-15 mag well). Thus, when loaded to 2.40" the 6.8 X 43 can fire the 110 TTSX, at 2775 FPS from an 18 inch barrel.

This leaves all the performance gained in the last 5 years, still intact, as the Mil could use weapons as short as 10" for entry carbines or PDW's, and longer guns with 1:10 twist to launch 115, 120 or 130 grain bullets at speeds of up to 2650 FPS.

Interesting times are ahead....

Indeed. It's too bad there isn't a semi-auto platform sized for a 2.40" cartridge. I suppose the best available option for a "proof of concept" weapon is an AR-10 type carbine, with 6.8 bolt? (Maybe Remington's new AR that's convertible from 5.56x45 to 7.62x51?)

Not sure what -- if any -- currently available magazines might work, though.

[HTR]

I believe that Colt is coming out with a rifle using modular mag well, that can be converted from 5.56 X 45 to 7.62 X 51.

They won't make it, but perhaps that could spawn an intermediate magazine well, say and "AR12" size shooting 6.8 X 43mm, with OAL of 2.40 or perhaps even 2.50".

If you did that, the performance would be unmatched. You could shoot everything from 85 grain TSX's at 3100 + FPS, to 135 SMK's at 2450. It is certainly possible, since I have now done both, just not (the 135 SMK) in an AR-15 magazine.

Give me a SA rifle with magazine of 2.40" and 16" barrel and I will show you an optimized light battle rifle. Hell, its only .10" more mag!

[stanc]

I believe that Colt is coming out with a rifle using modular mag well, that can be converted from 5.56 X 45 to 7.62 X 51.

Right, it's Colt, not Remington. Bad memory.

They won't make it, but perhaps that could spawn an intermediate magazine well, say and "AR12" size shooting 6.8 X 43mm, with OAL of 2.40 or perhaps even 2.50".

Give me a SA rifle with magazine of 2.40" and 16" barrel and I will show you an optimized light battle rifle.

Yeah, it seems like it'd be perfect. Last night I was doing some on-screen measurements of the Remington 130gr AccuTip, and Hornady 130gr SST and GMX. It looks like all three would give your optimum cartridge length of 2.40" when seated to the cannelure.

Have you tried the 130gr GMX? I think it'd be a lot better for this purpose if it were 100-110gr, but otherwise it looks interesting. (FWIW, one of the Grendel shooters says he put .264" 120gr GMX bullets in a lathe and removed metal from the base, to cut weight down to 95-100gr.)


.277" 130gr GMX

[Clint]

I believe that Colt is coming out with a rifle using modular mag well, that can be converted from 5.56 X 45 to 7.62 X 51.

They won't make it, but perhaps that could spawn an intermediate magazine well, say and "AR12" size shooting 6.8 X 43mm, with OAL of 2.40 or perhaps even 2.50".

If you did that, the performance would be unmatched. You could shoot everything from 85 grain TSX's at 3100 + FPS, to 135 SMK's at 2450. It is certainly possible, since I have now done both, just not (the 135 SMK) in an AR-15 magazine.

Give me a SA rifle with magazine of 2.40" and 16" barrel and I will show you an optimized light battle rifle. Hell, its only .10" more mag!

So how was the recoil on that 2.4" load? Still suitable for CQB?

Also, does anyone have some dimensions for a few bullets that includes length to cannelure / full caliber length and OAL?