It started with Murray using the 7.62x45 Czech as a base and was called the 7x46mm UIAC. The AMU took his idea and started working on the 264 and 277 USA and I believe changed to Carcano cases since the 7.62x45 blanks were hard to find. The AMU turned it over to FN and FN swapped it to a 308 diameter case and shortened in to 43mm. That's how things get messed up from the original designers idea.
Now The mags will be the same thickness and length as 308 mags and only be .2" shorter front to rear. When it was based on the Carcano we could get 2 extra cases in a 20round mag.
Murrays main goal was to make a GPMG that was lighter than a M240. He converted a MG42 to 7x46 and used an AR10 to test the cartridge.
moral of the story-don't waste your time trying to do something for the military because someone down the line is going to change it or cancel it.
I constantly wonder about the terminal performance of anything that is not 7mm and 6.8 since those rated the highest in original testing, of course if we could approve better bullets some arguments would be moot. Also you have to wonder about recoil and follow up shots, yes first round hits are best but everything is getting shot multiple times, you need to be able to stay on target.
Brass fetcher did some interesting testing by putting load cells under near gel blocks. They recorded the pressure wave imparted to the gel block as the bullet penetrated to see the KE transfer to the block. Guess what? 6.5mm didn't do very well, it was only marginally better than 5.56mm, while 6.8mm did substantially better. Now if the goal is have marginally better terminal performance than 5.56 but extend the range, 6.5mm fits the bill.
If the goal is to significantly improve the terminal performance at 5.56 ranges, 6.8 fits the bill. I can see where one might do better than the other, but on average, 6.8 is far better suited to actual ranges that most infantry fight at. Even Jim Schatz's reports noted that 75% of fighting in Afghanistan, one of the longest range theaters of combat we have seen in our history and a very unique environment, typically took place under 500 yards.
So why not go back to what France, Russia etc. is doing now (mixture of weapons, big and small caliber) and what our own forces used to do prior to the implementation of 5.56? I'm not sure if you guys have read through these, but I attached them because they have a really good history lesson within. Particularly the document 1027721. The MCX Spear is trying to be the "Ghost Gun" that doesn't exist. I agree with Jeff Gurwitch, it's a great Afghan gun for the mountains, it's not so hot as a replacement for 5.56 and typical infantry roles.
6.8mm version would be better for terminal performance especially barrier blind, but it seems like their goal is to now provide 5.56-like terminal performance but at much longer ranges in an M4 sized or close to an M4 sized package. Not a bad concept and the 6.5mm low drag bullets are a good fit for that task, but one of the big issues with long form factor bullets is poor barrier performance, which is important for CQC, a primary role of the M4 or any carbine.
That's why 6.5 G bullets did poorly against barriers in the JSWB-IPT (REDCOM) ES-1A-9001 testing from 2002-2006. If they shortened a 6.5mm bullet, then they lose their high BC's and provided no advantage at range over 6.8 SPC (actually worse) while also being lighter / lower mass than their 6.8mm counterparts. The length is the key to their high BC's, but that length works against them on barriers. Barrier blind is about stability, longer projectiles are inherently more unstable.
It seems they are now all obsessed with the idea of trying to counter 7.62x54R belt fed PKM machine guns with M4's or something similar in size which wasn't the original goal or issue and is really dumb...like Mr. Gurwitch noted, you should counter a belt fed with a better belt fed, which the M250 certainly is.
The original issue was enhancing the terminal performance of 5.56 within the 0-500yd envelope, which either 6.8 SPC or an enhanced newer version at higher pressures would do while preserving the barrier performance. Not trying to make an M4 shoot a 1,000 yards (well outside the capability of the shooters and optics on these rifles anyway). The M7 6.8x51mm can do that and certainly outclasses the resurrected 6.5x43mm.
Physics don't lie:
The energy transfer to the target is nearly 40% higher with 6.8mm vs. 6.5mm, identical TSX bullets in each caliber (except the .50 which used a Gold Dot). 6.5mm ELD bullets are just marginally better than 5.56mm. Aside from extending the effective range, they don't solve some of the primary issues of 5.56: damage to target, especially after barriers. That is the very reason why the NGSW project specified 6.8mm bullets.
Looks to me like a great concept for a very specific role, a kind of 5.56 DMR type role (MK12's, Recce's) but with extended range, but not one well suited to a variety of tasks and just how much more "effective range" would you get out of that 6.5 USA version vs. a 6.8mm USA using the same case technology and gun? At the practical ranges it could shoot, the 6.8mm would still be the better choice.
Building on the 6.8x51 advances to field a higher pressure, optimized 6.8 MRC (Medium Range Cartridge) good to 500 that works in current M4s with a bolt, mag and barrel change seems like a no brainer to me.
A couple of thoughts on long slippery bullets besides barrier penetration. FMJ versions slip through flesh with ease, just like the air, and they don't do transonic particularly well either.
What do I know though, I thought giving Bagram to the Chinese, and $80 Billion in top of the line weapons systems to terrorists was a bad idea.
BTW, this is what long skinny 6.5mm bullets get you against auto glass a 100 yards (bottom, from Roberts 2008 small arms symposium presentation):
That's one of things I liked about 110gr 6.8 SPC bullets, they strike a good balance between size, mass, length and achievable velocity to trajectory on par with 77gr 5.56 bullets while producing considerably more damage and also performing well against a variety of barriers all without resorting to uber specialized bullets.
Altough I'm sure a 6.8mm MK318 or A1 type bullet would work even better.
It does equally well though heavy concrete blocks. There was some testing from arfcom some guys did with 7.62x39, 6.8 SPC, 5.56 and an old M1 carbine .30 cal a while back and the 110gr OTM was the only bullet they tested that blew through both blocks and destroyed the water jug behind it.
No other caliber or load tested worked that well consistently, which is why 110gr OTM is probably one of the best general-purpose loads out there for SHTF, Military and LE. It's a very accurate bullet with very good barrier blind performance.
It has a good BC of .360, you can safely get 2650~2700 FPS from 16" barrels giving it MK262-like trajectory and it performs reasonably well in a variety of applications. Sure, the OTM load isn't specifically designed for barrier blind, but 110gr OTM in 6.8 SPC sure does perform like a dedicated blind load, while also matching or exceeding all the properties of 75-77gr 5.56 loads.
Fun fact: the non-uniformity of the open tip aids in reliable fragmentation. I know people complained about reloading 110gr OMT's due to the variation (trying to get accurate OAL's), but it's by design and helps with inducing fragmentation on impact.