Edmond ; I have a question for you .

Given the M-16 or M-4 or what ever version of the 5.56 mm round is now considered to light at 55 - 62 grains .

In your opinion what would make a short case capable round of as much muzzle velocity with near square energy projectile say with a weight of 140 grains ?. IE , 3K FPS 3K lb. energy

I was thinking along the lines of maybe a 270 Short Mag round ?.

What do you say ?. Hopefully staying in a Magazine size similar to the 5.56 platform ?.

Shoot Straight Know Your Target . ...

quote:

Originally posted by Dr.K:
Edmond ; I have a question for you .

Given the M-16 or M-4 or what ever version of the 5.56 mm round is now considered to light at 55 - 62 grains .

In your opinion what would make a short case capable round of as much muzzle velocity with near square energy projectile say with a weight of 140 grains ?. IE , 3K FPS 3K lb. energy

I was thinking along the lines of maybe a 270 Short Mag round ?.

What do you say ?. Hopefully staying in a Magazine size similar to the 5.56 platform ?.

Shoot Straight Know Your Target . ...

I'm not Edmond but read your note with interest - could you not be happy with the 6.8 Rem? It does not come up to the 3K x 3K you cite but is very much superior to the 5.556.
Consider:
6.8MMx47 REMINGTON SPC BALLISTICS
BULLET VELOCITY (fps) ENERGY (ft-lbs.)
Muzzle 100 yd. 200 yd. 300 yd. Muzzle 100 yd. 200 yd. 300 yd.
115-gr. FMJ 2,800 2,523 2,202 2,017 2,002 1,622 1,250 1,039
115-gr. BTHP 2,800 2,535 2,285 2,049 2,002 1,644 1,345 1,075
115-gr. MK 2,800 2,535 2,285 2,049 2,002 1,644 1,345 1,075
*24-inch Barrel
[Article at http://www.rifleshootermag.com/ammunition/remington_0303/]

I'm thinking if you go on up in bullet weight with the increased length you may run into OAL problems?

And the 6.5 Grendel - maybe if you jumped the bore to .270 or on up to 7mm? Performance is very similar to the 6.8 Rem - and both work in the M16 platform, I think.
Ballistic performance
Bullet weight/type Velocity Energy
90 gr (5.8 g) Speer TNT 2,880 ft/s (880 m/s) 1,658 ft•lbf (2,248 J)
120 gr (7.8 g) Norma FMJBT 2,700 ft/s (820 m/s) 1,942 ft•lbf (2,633 J)
123 gr (8.0 g) Sierra Matchking 2,650 ft/s (810 m/s) 1,917 ft•lbf (2,599 J)
130 gr (8.4 g) Norma 2,510 ft/s (770 m/s) 1,818 ft•lbf (2,465 J)
108 gr (7.0 g) Scenar (moly) 2,790 ft/s (850 m/s) 1,866 ft•lbf (2,530 J)
Test barrel length: 24 inches
Source: Alexander Arms Pressure-safe Load Data

A cartridge I have always loved and one that I think could well be a successful miltary round is the 260 Rem. This little round is very capable and I'm sure it too could be used with the M16 platform - maybe someone has already tried this? It is a larger case round, based on the 7.62 NATO [308 Winc.] parent case so this might well place it outside what you are trying here. As an aside, the .308 Winc has certainly lent itself to being the ‘parent’ for a lot of successful cartridges and it is still going strong.
I can think of no current combo of components that will give the performance you ask for here and do so with a case that will keep it within the confines of a 5.56mm magazine and the M16 platform. What say others to this? I see an interesting project developing here!

What about the 7MM TCU?

Grr; Made a long answer and connection timed out!

I did rebarrel an AR 15 in 7mm TCU many moons ago when I shot Metal Silhouette with a Contender. I was very happy with it.

Edmond ; As your a military Arms expert or at least a very KNOWLEDGEABLE ONE !.

I was not trying to reinvent the wheel , I was simply trying to extract an Idea from you , as to what practical round would fit into an M-4 magazine format . That will deliver a 120-140 grain projectile near square MV X ME at say around 3000 FPS with Equal energy .


The reason I'm asking is the Stoner Design may be going up in flames soon !.

The 6.8 isn't being looked at favorably .

So you think a 7mm TCU will fill that bill ?.

Small weapon powerful projectile with substantial weight , better than twice 5.56mm standard projectile weight .

Thank You for your input as to all of you who have responded . I didn't think this would be of interest to others . Shows what I know !.

Shoot Straight Know Your Target . ...

in the answer I made, I said that the Swiss tried many different experimental rounds in 5.6mm, 5.8mm, 6mm, 6.5mm during the 70s when they started working on their new service rifle.
The rifle was the SIG 541 in caliber 6.5 x 48.

It became the Sturmgewehr 90 but the Swiss Federal Military Commission adopted the 5.56mm round with a 63 grains bullet.

All that to say that what is more important is training.

when I hear BS about guys not going down in Afghanistan when shot with M4 or M9 because they wear heavy clothes in winter....

Someone in 2003/2004 made an AR 15/ M16 using the 7.62x39mm M 43 soviet round and the AK magazine, it was not a bad idea.

Spending billions on BS and having worn out magazine springs causing jams is telling about what is going on on your side of the pond.

The best move in my opinion would be buying the H&K 416 and training the soldiers.

quote:

Originally posted by Edmond:
in the answer I made, I said that the Swiss tried many different experimental rounds in 5.6mm, 5.8mm, 6mm, 6.5mm during the 70s when they started working on their new service rifle.
The rifle was the SIG 541 in caliber 6.5 x 48.

It became the Sturmgewehr 90 but the Swiss Federal Military Commission adopted the 5.56mm round with a 63 grains bullet.

All that to say that what is more important is training.

when I hear BS about guys not going down in Afghanistan when shot with M4 or M9 because they wear heavy clothes in winter....

Someone in 2003/2004 made an AR 15/ M16 using the 7.62x39mm M 43 soviet round and the AK magazine, it was not a bad idea.

Spending billions on BS and having worn out magazine springs causing jams is telling about what is going on on your side of the pond.

The best move in my opinion would be buying the H&K 416 and training the soldiers.

Why, do you mean, Sir, to abandon "Spray and Pray" and go to "Aim & Claim?" What an idea - surely that will never work!

maybe on your side of the pond..

7mm TCU is a good idea Ed.

Many years ago, (shut up! ) I had Myra's Gunshop make me a variant of their 222 based wildcats. They made several, the 243/222 MYRA, 25/222 MYRA and so on, up to 270, IIRC.

I asked him to do a 6.5 based on the 223, with an AI type shoulder. He rebarreled a Leader (an OZ made AR-18 copy) and made me a set of dies.
Worked just fine, no mods necessary to mags. A little fiddling with the cartridge OAL was necessary, but only to get it to fit in the mag. Feeding was fine.

Turned a marginal pig hunting rifle into a very effective one, with the 120 gr SP at around 2600 fps.

I can't speak for the AR-15 platform, but I should think that this might work OK?
JMHO.

Regardless of Aim and Claim or Spray and Pray or worn out followers springs or lack of training .

There is a REAL NEED for a heavier projectile !

I'm saying some where between 120-140 and I would like to opt. for the latter .

I've always liked H&K weapons as I own a couple they're function has never been an issue in my experiences .

In all fairness the M16- M4 or any variation of the Stoner design has NEVER favored Dirt Mud Sand for functional operation !. Now couple this with a light projectile , I see the hand writing on the wall .

It's saying Caliber and Design change inevitable !.

So Regardless of our personal opinions of these weapons , my intent was to explore another possible candidate for the position !.

Thank You All for the input .

Shoot Straight Know Your Target . ...

quote:

Originally posted by Dr.K:
Regardless of Aim and Claim or Spray and Pray or worn out followers springs or lack of training .

There is a REAL NEED for a heavier projectile !

I'm saying some where between 120-140 and I would like to opt. for the latter .

I've always liked H&K weapons as I own a couple they're function has never been an issue in my experiences .

In all fairness the M16- M4 or any variation of the Stoner design has NEVER favored Dirt Mud Sand for functional operation !. Now couple this with a light projectile , I see the hand writing on the wall .

It's saying Caliber and Design change inevitable !.

So Regardless of our personal opinions of these weapons , my intent was to explore another possible candidate for the position !.

Thank You All for the input .

Shoot Straight Know Your Target . ...

Well the M16 sure hasn't favored the dirt mud sand for quite some time now. Probably breaking the record for longest in service military rifle for the U.S.

It's unlikely the U.S. is going to go with a round with a 140 gr bullet. You are getting back into the full size battle rifle cartridge again. They want to keep the recoil down as much as possible also.

HK isn't the same company it was before since it changed ownership. Their PR department has gone to hell. The Army was pretty much happy with their 416 but since their PR dept sucked they were told to go to hell and looks like the FN SCAR is ticket now. The SCAR does have caliber change barrel interchangeability too.

I think the best way to go if you want an AR-Type rifle that will throw 120-140gr. bullets with authority..... is to go with the larger AR-10 sized action.

Cartridges like the 6mm x .223 (5.56x45mm) work OK in an AR-15 platform, but you're not really gaining anything with the larger diameter bullets in the small .223 case when the rounds are loaded to magazine length.

This may very well all be true .

How ever the M1A has been called back up in vast numbers !.



By 1963 US Army was purchasing the M16 for use in South East Asia and by various elite forces. The Army also ordered 85,000 rifles in 1963. An additional 35,000 were ordered in 1964, 100,000 in 1965, and 100,000 in 1966. These rifles were initially issued primarily to combat troops in the Dominican Republic and to Special Forces, Airborne, helicopter crews, Air Commando and other special category troops in Vietnam.

The M-16 was type classified standard A in 1965 and became the military's basic service rifle. By 1966 it was in widespread use. The M16 was called the "black rifle" and "Mattel toy" thanks to its appearance. Troops liked the light weight, but complained about insufficient range and lethality. While the M16 had been marketed as virtually "maintenance free, poor maintenance instructions (or even no instructions) and jungle climate together with the fouling-prone direct gas system caused trouble. Its high rates of fire in the jungle environment had a larger impact on increasing American morale than on actually inflicting enemy casualties. The move to high-velocity 5.56 mm was also subsequently adopted by the Israelis, the Soviets, and NATO allies. DARPA's most significant contribution to this program was its willingness to "think outside of the box" and try something new.

I personally was less than fortunate to have used an original M 16 ( Training ) as well as Version 2 which was better but far from stellar !.
I will admit Matty Toy was easier to carry than the M 14 how ever far less effective . IMO

Which still brings me to my original point . A short cased 140 grain .270 , 7mm or 6.5 that would be near square around 3K FPS with M E like wise . Ideas ?.
Recoil in a automatic or semi cyclic fire rate is very manageable even with a 140 grain projectile !.

Shoot Straight Know Your Target . ...

Shoot Straight Know Your Target . ...

quote:

Originally posted by Edmond:


maybe on your side of the pond..

Edmond et al: This is what I was referring to:
"The weapon currently issued to our soldiers expends 15 rounds in 9/10 of a second. I don't know what regulations call for these days, but when you're headed for a fight, you want to have lots of magazines for your rifle and plenty of cartridges. Half a dozen magazines would be nice and a dozen even better. The advertised purpose in the 60s of downsizing from the .30 caliber M14 to the .22 caliber M16 was so that soldiers could carry more of the tiny -- some say ineffectual -- cartridges it shoots. Apparently they're needed: it's calculated that our forces fired a million rounds per enemy casualty in Viet Nam. it's calculated that our forces fired a million rounds per enemy casualty in Viet Nam. "

Taken from "For Want of a Nail", by L. Neil Smith

!,000,000:1 equals one hell of a ratio, no?

Looks like the candidate for new ( application ) cartridge would be going to 270 WSM or 7mm SA Ultra mag .

They would fit in my overall design criteria .

Shoot Straight Know Your Target . ...

One of the Naval forensic doctor and ballistic experts I know on two other forums would be to fashion a 7mm cartridge derived from the Czech 7.62x45 for the M16. I've seen pretty good performance in 6mm cartridges in the AR platform that were not based on the 5.56 case. It's too small.

Have any of you thought that the 222 Rem might make a better case for the M16 because by using longer heavier bullets today in the 5.56 ammo the bullet encroaches on the powder capacity. By using the 222 case the bullet can be loaded out much longer and all the powder capacity of the case utilized.

I'm firmly convinced that the M14 would have made no difference in Viet Nam except to bring out and show all the defects that it has and of course the soldiers would have been complaining abot what a pos it was instead of the M16. I also believe that if the current M16 and ammo were used back in Nam then there would have been far less complaints and problems.

I believe the only reason they brought the M14's out in the sand box is because that's the only other rifle our military has in storage and can get their hands on. For reliability, not accuracy, I think the FN FAL beats the M14. I believe in the tests for a new battle rifle the FAL actually beat the M14 but this country at that time didn't want a rifle made by a foreign country.

quote:

would be to fashion a 7mm cartridge derived from the Czech 7.62x45 for the M16

quote:

A short cased 140 grain .270 , 7mm or 6.5 that would be near square around 3K FPS with M E like wise . Ideas ?.

Already exists, it is called 7mm BR.

I had chambered some of my rifles in .308 x 1,5". I like the idea of a slightly downsized AR10 with piston system using this ammo.

Thanks Edmond ;

I was looking up some cartridges when I came across the last two I mentioned . Seem to be near square . I would think they should work just fine in a platform of the H&K 416 Design .

If the US military does and I personally hope they do . Move away from the Stoner Design or at least refine the gas system along with feeding problems , adopt a new more powerful cartridge with the weight around 120-140 grains .

Is anybody else having trouble with connection time outs ?.

I type pretty dam fast , and the last week or so I'm timing out or being cut off regularly . I'm on satellite up and down !. So what the ??.

Shoot Straight Know Your Target . ...

quote:

Originally posted by Edmond:

quote:

would be to fashion a 7mm cartridge derived from the Czech 7.62x45 for the M16

quote:

A short cased 140 grain .270 , 7mm or 6.5 that would be near square around 3K FPS with M E like wise . Ideas ?.

Already exists, it is called 7mm BR.

I had chambered some of my rifles in .308 x 1,5". I like the idea of a slightly downsized AR10 with piston system using this ammo.

Edmond,

With the CURRENT AR15/M16 bolts the 7mmBR has too large head diameter for the bolt. That is changing though, a company called MGI is working on a super bolt that will handle 308 pressures and more in an AR15.

Dr K

I'm having all sorts of problems connection with the Accurate forum. You are not alone.

I wasn't interested in using a 5.56 case or even staying with the bolt Dia. . I was looking for a cartridge that would deliver near square performance of 3000 FPS and Muzzle Energy .

In a Short Case acceptable for an AR Platform .
There are less than a hand full of cartridges that will do this in a 7mm or 6.5mm carrying a 140 grain projectile .

It's just my opinion and I'll be the first to admit I've been wrong on several occasions .

The time has come for US Military to step up to a heavier projectile and more powerful cartridge case to achieve this goal . IMO .

I base this on actual communications with reliable soldiers as well as Commanders field experience and feed back from Iraq . The lighter 62 grainer's aren't cutting cover and don't have the necessary energy to complete their mission .

Dust and dirt are a MAJOR PROBLEM in the M-4 , I can't believe that so many don't even have cleaning supplies !. Which is also A FACT . Which I would assume then they lack lubricant also . Dumb ass Private contractors have assumed so much of our military support roles , it's down right pathetic !!!!!!!!!!.

Shoot Straight Know Your Target . ...

quote:

Originally posted by Dr.K:
I wasn't interested in using a 5.56 case or even staying with the bolt Dia. . I was looking for a cartridge that would deliver near square performance of 3000 FPS and Muzzle Energy .

In a Short Case acceptable for an AR Platform .
There are less than a hand full of cartridges that will do this in a 7mm or 6.5mm carrying a 140 grain projectile .

It's just my opinion and I'll be the first to admit I've been wrong on several occasions .

The time has come for US Military to step up to a heavier projectile and more powerful cartridge case to achieve this goal . IMO .

I base this on actual communications with reliable soldiers as well as Commanders field experience and feed back from Iraq . The lighter 62 grainer's aren't cutting cover and don't have the necessary energy to complete their mission .

Dust and dirt are a MAJOR PROBLEM in the M-4 , I can't believe that so many don't even have cleaning supplies !. Which is also A FACT . Which I would assume then they lack lubricant also . Dumb ass Private contractors have assumed so much of our military support roles , it's down right pathetic !!!!!!!!!!.

Shoot Straight Know Your Target . ...

Dr K,

Here are some of the current rounds chambed for the AR15: 5.56, 6.5 Grendel, 6.8 SPC Rem, 204 Ruger, and the 7.62x39. That's not the entire list but lets talk about them. The 5.56 specs call for it to run about 52,000 psi in the AR15/M16. This is more then the 7.62x39, but guess what? The 7.62 will break bolts much faster then the 5.56 because it has more backthrust on the bolt. Now look at the 6.5 Grendel and 6.8 SPC...the Grendel has the same head size as the 7.62x39 and the 6.8 only slightly smaller, but both of these can/do run at a higher psi then the 5.56 so there has been some bolt problems with them too. The 6.5 Grendel is especially limited in the AR15 because it's pressure is limited to 50k to 52K. In a bolt rifle this excellent little round will blows the doors off the same round in the AR15.

This is what happens when you use a bigger head diameter cartridge in the AR15 then the 5.56. Have no fear they have wised up and new bolts of better steel are not just starting to be employed. This I believe will open a whole new realm for the AR15.

Joe

I'm not qualified to speak on rifle design for such a round but it seems to me that the 280/30 Enfield round from the early 50s that the Brits designed for their abortive EM2 rifle is not too far from what you're talking about. Any cartridge design is a compromise on a heap of things, weight & fire power having been mentioned. Space is obviously another & I can't see short fat rounds based on the WSM cases having much appeal in this sense. They may also compromise feed reliability & it is these things as well as projectile weight as much as cartridge weight & size that the 6.8 Rem SPC has done a better than average job of addressing. If a round like this cannot persuade the vested interests in the US military to step outside their comfort zone, I doubt that something even further outside these parameters can do it.
Steve.

The French designed an intermediate round in 1946, compared here with the german 7.92 x 33 mm

These weapons were developped between 1946 and 1948 for the

US Carbine 30 M1 round
German 8mm Kurz
French 7.5mm intermediate round

The Swiss developped a 7.5mm intermediate round in 1947 and a rifle in 1952

The W+F and the SIG 541 prototypes (later adoprted as StGw 90 and commercialy known as SIG 550) were built both in 5.56mm later adopted as GP 90 but the GP 80 was first adopted. Surprise! it was a 6.35mm.
then the decison was taken to switch to 5.56mm for political and commercail reason.

The requirement was that the swiss 5.56mm had to be more accurate than the 7.5mm and the newly adopted 5.6mm 63 grains GP 90 is, indeed.

Some interesting rounds to think about.

The 5.45 x 39mm is a very interesting one.

The 4.6 x 36mm H&K was designed for a precursor of the G 11.

quote:

Originally posted by DaMan:
I think the best way to go if you want an AR-Type rifle that will throw 120-140gr. bullets with authority..... is to go with the larger AR-10 sized action.

Cartridges like the 6mm x .223 (5.56x45mm) work OK in an AR-15 platform, but you're not really gaining anything with the larger diameter bullets in the small .223 case when the rounds are loaded to magazine length.

8X33mm, 308x1,5" and 7.62x51mm

I shot wonderful groups with 308x1,5", below 1/2 @ 300 meters using 130 grains Speer HP and 147 grains FMJ BT

another path


From left to right, 5.56 (SS109), 5.45mm Soviet, H&K's 4.3x45mm and 4.6x36mm
note the tip of the 4.6mm...

That 4.6 looks like a heck of cartridge for varmint hunting...

even the soviet 5.5 x 39 is better than the 223 in military guise..

Soldiers should be issued AR 10s, and then uppers in both the 223 ( which can be made) and also an upper in a 6.5 or 7mm bore for longer range needs...

and definitely using the H & K pistol system...

Still give me a good old M 14 any day, when the lead starts flying...I kick myself for not picking up an M14 when they were available by Springfield Armory in 243, and 7/08.. one in 260 or 6.5 x 274 would be a slick item...

Edmond ; Lets say on the .308 X 1.5 , 46mm X 36mm . What weight projectiles do they carry and velocities ?.Do you have any Stats ?. Or is there a site which post them I could look at ?.

Remember my original thought was 130-150 projectiles with 3000 FPS with near 3000 Ft lb.Energy .

Ideal would be a near Square MV X ME with a 140 grain projectile in a case size to fit the AR platform , without a whole bunch of magazine work . Bore bolt & Carrier barrel and such can be changed easily enough .

I was looking at a heavier projectile slightly larger bore with more energy that the .223 delivers . As I originally thought something like a 270 WSM would fit the ticket !.

However I'm unfamiliar with any ballistics on those cartridges you've listed .

Shoot Straight Know Your Target . ...

308 x 1.5"

http://www.reloadbench.com/cartridges/w308barnes.html

Thanks Edmond .

I'll have a look see .

Shoot Straight Know Your Target . ...

It seems to me this is like rediscovering the wheel. The Russian 7.62x39 does all the things that they are seeking to do with a 6.8 round. I guess I don't get it. Kudude

It doesn't come close to filling the bill !.
A 7.62 X 39 is a dismal performer in comparison to almost every other cartridge .

I was simply entertaining an Idea as how to get more " Power " for a cartridge that could be utilized in a M-16 or AR-15 platform . Lower and magazine well . That kind of thing .

I wanted a cartridge that is near square in MV & ME. Some where around 3000 FPS with that for Energy at the muzzle . All the while carrying a 130-150 grain projectile .

It has far more " Effective Stopping Power " than does a 5.56mm or .223 at 3200 FPS with a 62 grain projectile .

A 7.62 X 39 mm doesn't come close !.

From Craig Boddington . Seeing as he has Hunted far more than I have as well as in more places , I tend to generally buy into what he has to say .


FAST, FLAT, SWEET
The .270 remains a great cartridge, but the .270 WSM is even better. Velocities for .270 factory loads vary considerably from brand to brand, but if you look at the average "standard" load, the .270 WSM is about 200 fps faster, bullet weight for bullet weight. This is significant. Obviously, it flattens trajectory, but in real terms I'm not sure this is the important factor. With an aerodynamic bullet like the 130-grain Ballistic Silvertip and a 200-yard zero, at 400 yards the difference between the .270 (muzzle velocity of 3,050 fps) and the .270 WSM (muzzle velocity of 3,275 fps) is just 23?4 inches. More important to me is the difference in energy: 2,685 ft-lbs for the .270 with this load, 3,096 ft-lbs for the .270 WSM. An energy dividend exceeding 400 ft-lbs is significant, provided you can obtain it without unacceptable recoil.

This is one of the great beauties of the short magnums. The burning efficiency of their short, fat cases that allows them to generate their velocity while burning less powder than longer-cased cartridges with similar performance also keeps felt recoil surprisingly mild. I'm still not certain exactly why this is, but light recoil has been a consistent attribute of every single short magnum I've used. I won't advertise that a .270 WSM kicks less than a .270 Winchester, but I find it pretty hard to tell the difference.

Shoot Straight Know Your Target . ...

Dr. K

Well if you like Craig Boddington then he just did an article about the 264 Win Mag saying what a great round it was and he had one in his very young years and recently just acquired another one. No, I'm not saying that would fit in an AR, but Craig did say the 6.5 bullet is one of the most ballistically coefficient ones out there. With that said the afore mentioned 6.5 Grendel, for right now, I feel one of the best. I have one by the way and it's awesome for it's size.

Joe

The first swiss prototypes were directly derived from the FG 42and the StG 44, including the 7.92 x 33mm round.

Later, they used the roller delayed blowback system à la StG 45.

quote:

Originally posted by kudude:
It seems to me this is like rediscovering the wheel. The Russian 7.62x39 does all the things that they are seeking to do with a 6.8 round. I guess I don't get it. Kudude

There was an even better round, the Czech 7.62 x 45mm.

Some Navy forensic doctor who study lies in wounds and ballistics says the best round for the M16 and AR15 lies in the Czech 7.62x45, but in 7mm and maybe shortened just a tad.

Dr. K

You can work your problem backwards. According to my ballistic program a 308Win shooting 165gr bullets and 2700fps (factory) produces 2670ftlbs. A 30-06 with 165gr bullet and 2942fps produces 3171ftlbs.

I think you can put aside the idea of a cartridge that will fit the bolt and length of action of the M-4 that will produce 3000fps and 3000ftlbs. It just won't work.

From the aforementioned data, one also can see that your "standard" of 3k fps and 3k ftlbs as it relates to killing power for man or beast is nonsensical. You can kill any man on the planet with a .308 165gr bullet with much less velocity and energy.


I don't know what you are looking for, but the military needs a round that can stop people at very close range who are wearing bullet proof vests. The old M-16 with 62grain bullets developed enough muzzle velocity to penetrate most vest. (1200ftlbs @ muzzle) The M-4's 14.5"barrel doesn't. Contrary to your assertion about the 7.62x39, it produces 1461ftlbs at the muzzle, around 900ftlbs at 200yds and 650ftlbs at 300yds.

It is a ballistic fact that the smaller the hole, the less energy can be imparted to the projectile in a given length of barrel. The only way to get a .223 round to go faster is to have a longer barrel. One can increase the bore size as a means of imparting more energy to the projo, this is why the 6.5, .270 and 7mm bullets fair much better.

The experiences of WWII let the Germans to adopt a shorter, less powerful round for combat in areas which were increasingly "closer."
The Russians rapidly copied it and rifle tailored to it, the SKS. It was rapidly supplanted by the AK. The US adopted the M-14 which was an improved M-1 using new powders to give 30-06 performance in a smaller, lighter round, the 7.62x51. There was really nothing wrong with the M-14 except the M-16 caught the fancy of Sec Def McNamara who want all the services to use the same weapons, uniform, vehicles, etc.

Today, to further enhance its handiness, the M-16's 20" barrel has been shortened and then shortened again to make the M-4. This wasn't a problem when our enemies weren't wearing body armor, but today more and more frequently they are. The M-4's loss of velocity has diminished it ability to take out enemy in close quarter combat and in the desert at extended ranges.

This is the primary beef of grunts in the sandbox and Afghanistan. The secondary gripe is about the vulnerability of the weapon to sand and fouling. This has been "cured" in several modified versions that have gone to a gas system more like the AK's and eliminated the gas tube of the M-4/M-16.

Looking at the terrain we are fighting in, it appears that we need (1) a rifle that is effective at long range for desert and mountain fighting and (2) one that is short for combat in built up areas. Although one cartridge could be designed for both roles, you are going to change barrels or adopted a bullpup design. If you look at the myriad of bullpups being fielded around the world, most have a rifle, carbine and designated marksman model finding that one size fits all doesn't work.

Back to the ballistic conundrum, my 7.62x39 load shoots a 130gr bullet at 2250fps(SKS). I believe that in an 18" tube, I can obtain 2400fps with a 120gr 7mm(.284) bullet with a blown out version of the 39mm case. This would deliver 1660ftlbs at muzzle and just under a 1000ftlbs at 300yds. The .277 and 6.5mm(.264) rounds in 120gr would be slightly slower because of the physics involved (smaller hole), but there would not be a significant difference.

The 7.62x39 Russian case blown out and necked down to 7mm, .270 or 6.5mm would meet the requirements of a cartridge that be deadly at close quarters and reach out 300yds with man-killing energy. There is no need to achieve 3k + 3k. It would defeat much of the rationale of maintain the "mouse" guns on which non-shooters and females particularly can be trained to deliver effective fire in a minimum amount of time.

This is probably more info than you wanted, but I hope it explains the ballistic problem you pose and the futility of pulling criteria for ballistic performance out of thin air. Kudude

PS: Regarding the fielding of the M-14's, I understand that one of the wonderful things about the 7.62x51 (308Win) round is that it just blows sun dried bricks apart and nails whoever is behind them; whereas the .223 just bounces off.