6.5x55 is outdated  newer 6.5 cartriges more than emulate its performance in oal suitable for existing 308 magazines. 6.8x51 stuffed magnum performance in a case that fits the .308 mags. But high pressure is hardly free lunch.

'' This basicaly what we get when we are limited to magazine oal and case rim dimensions.

This is the slightly old table from Litz , its basicaly .308win ,7-08 ,260Rem  ,234Win  , all just necked down .308win  .

I am somewhat surprised SOCOM picked 6.5creedmoor over old .260Rem , as 6.5creedmoor is a relatively poor design with internal ballistics issues (its extremely hard to load to low ES and SDs) and worse feeding candidate than .260Rem that would be perfect for use in full auto guns.

A number of recent calibers out of the US are quite poorly designed jet still gained some popularity. 6.5 Creedmoor, 6ARC, 22Nosler  each one with a bunch of issues .

Constraining your ammo to magazine length can really compromise performance. Oftentimes you have to use a stubby, low BC bullet just so it fits. You can see substantial performance gains at the same magazine length if your neck down to a smaller caliber (re-barrel). This allows your bullet to have a proportionally longer nose (higher BC) and a higher powder-to-bullet weight ratio which makes more velocity. Consider this example between a 308 Win vs 243 Win. The 308 will push a 175-grain bullet around 2650 fps. The 243 will push a 105-grain hybrid at 3000 fps, plus the 105 has a higher BC!

“US SOCOM, after seeing from the insight all the greatness of the 6.8x51 mm, decided to resume its 6.5 mm Creedmoor program“

This one is quite surprising. As a replacement for 5.56, 6.8 seems pretty nuts.

But as a replacement for 7.62 or 6.5C,it seems much more compelling. There weapon and ammo weight is essentially identical but with much better ballistic performance for 6.8.

Ii wonder what SF saw that made them want to go back to 6.5?

Mr. T (MrT4) said:

I am somewhat surprised SOCOM picked 6.5creedmoor over old .260Rem , as 6.5creedmoor is a relatively poor design with internal ballistics issues (its extremely hard to load to low ES and SDs) and worse feeding candidate than .260Rem that would be perfect for use in full auto guns.

I know that the 6.5 CM case has little taper, but what are the other design deficiencies.   It seems to follow the pattern used by most recently introduced cartridges: Mainly a relatively long freebore ( 0.199"/5.055mm, or 0.75 cal) with a tight clearance (0.0005"/0.012mm).  Is that itself a problem, or is it something else?  I have read that "it is not as accurate as it should be" but have not seen an explanation.  Thank you.

Upon looking at the SAAMI specs of the .308 Win family, the 260 Rem has the relatively longest freebore of 0.45cal with a fairly tight (0.001"/ 0.025mm) clearance.  Going by today's thinking, it should have more accuracy potential than the 308.

gatnerd said:

But as a replacement for 7.62 or 6.5C,it seems much more compelling. There weapon and ammo weight is essentially identical but with much better ballistic performance for 6.8. Ii wonder what SF saw that made them want to go back to 6.5?

I really doubt that a "standard pressure" 6.8 mm will do anything the 6.5 mm Creed can't, and you can find now "tactical / precision" loads for the 6.5 mm that you could only expect to see after 2025 for the 6.8 mm. Seems interesting that every time there is a demo with the 6.8 mm, the bullet loaded is a Nosler Accubond, which is neither a FMJ, nor and OTM, but a truly hunting bullet with an impressive C7 of 0.298 for the 150 gr LR version.

The "+P++" 6.8 mm have much better ballistics than the 6.5 mm Creed, but also have drawbacks in terms of recoil, safety, cost, availability & weapon service life.

Can't put a finger on it where it went wrong but 6.5CM is quite temperamental. Calibers like .223Rem ,308win are superb designs in their own right , accurate more than folks can dream of , have good burn so can be loaded to very low ES SD numbers , all the popular 'competitive' calibers like 6BR , 6.5x47 , 284win , 6XC can easily be loaded for superb accuracy and consistency (ES SD), 6.5creedmoor is not even close  which is kinda surprising considering how popular it is now , 6.5CM cases are availibile in both small and large primer variants and it doesn't seems to cure the lack of consistency even with absolute premium components. Not to mention there are plenty of instances where 6.5CM has issues in some semi autos like FN SCAR that has issues with pierced primers and failures to  feed a cartridge that was supposedly drop-in fit with only barrel change.

260 rem issue over 6.5creedmoor is that the longest bullets like  147g Honady ELD-M need to be seated deep to fit magazine oal. But in many ways 147 an 150g class bullets are too heavy for these cartridges to achieve high MV. You are limited to 2750 FPS or something out of 26' barrels , if you cut barrel down to 20 or 16  (16'' at 2350fps with 147eld)its far to slow for practical use . 260 rem like 264 USA is well shaped for feeding in full auto  guns.

.260rem  and 6.5CM  , bullet in between is extreme heavy weight in 6.5mm caliber  156g Berger Eol ,that is best left to long action cartridges.

stancrist said:

And if one wants to go for an existing cartridge, 6.5x55 seems to me like a bad choice. As shown in the pic below, 6.5x55 is too long to be used in rifles designed for 7.62x51.

True. I did not look at the dimension at all. Just the performance.
If the continuous use of the same weapons is desiered its obviously not the first choice.
The Swedish military used 6,5x55 in  the FN Mag wich they later converted to 7,62x51. The ballistics of the 6,5x55 are really good and its an old cartidge. Applying some modern tech its performance should get even better.
7,62x51 also can be hot loaded.
I just don't get what 6,8 does that non of the alternatives does.

schnuersi said:

I just don't get what 6,8 does that non of the alternatives does.

Money and PR for persons and companies involved

American defense business at its best, where money rule over everything. Fruitless and endless R&D on budget money is a preferred sport of most big- and medium-size contractors who can buy themselves enough pawns from Congress for support

Mr. T (MrT4) said:

Not to mention there are plenty of instances where 6.5CM has issues in some semi autos like FN SCAR that has issues with pierced primers and failures to  feed a cartridge that was supposedly drop-in fit with only barrel change.

It could also happen to .308 / 7.62 x 51 mm weapon, where 168 gr match loads developped for bolt-action rifle suffered the same fate (pierced primers & FtF)  when used in the HK417... so I won't take this kind of bad experience as a case against the 6.5 mm CM.

I understand your point, but in FN SCAR 20, there are surprisingly folk reporting many problems with these in 6.5 Creedmoor on factory ammo no reloads. In the end given that 6.5CM is not operating at any higher pressures both .308 and 6.5CM use the same primers and same case head, so can't quite see why the gun would need a specific 6.5CM bolt. 

at 12:00 they actually say they measured all the specs on firing pin and bolt hole etc and its all in specs.

https://www.youtube.com/watch?v=eUTLPwoZSdE

Interesting digital daysight ,this is definetly where we are heading if the energy consumption can be tackled.

https://www.youtube.com/watch?v=FWmy7Bi25lY

EmericD said:

I really doubt that a "standard pressure" 6.8 mm will do anything the 6.5 mm Creed can't, and you can find now "tactical / precision" loads for the 6.5 mm that you could only expect to see after 2025 for the 6.8 mm.

I just did a brief search, and I could not find any 6.5 CM ammo now available that looks like it would meet military requirements.

There is no EPR, lead free FMJ, AP, Tracer, Blank.  All still have to be developed, so 6.5 CM has no advantage over 6.8 in this regard.

Lead core FMJ is marketed, but it lacks a crimped primer, and appears to not have waterproof sealing of the primer and case neck.