The following from the Wilson Combat website: "The 6.8 Special Purpose Cartridge (6.8x43mm) was initially developed by MSG Steve Holland and Chris Murray, a United States Army Marksmanship Unit gunsmith to offer superior downrange lethality over the 5.56 NATO/.223...

Another of the Lazzeroni range without its own headstamp. This one uses the Firehawk case.

Apparently only about 2000 of these cases were manufactured during the late 90’s, but it could not be used because of the fact that “SPITFIRE” was already registered as a trademark. Uncertain how many got into circulation but not a lot.

Developed as a 1,000 yard match rifle, if they can get the barrels to last more than a few hundred shots!   

This is the 300 Weatherby necked to 6.5mm with no other changes, which makes it the same as the 6.5/300 Weatherby-Wright Magnum that was first listed in the 1962 edition of P.O. Ackley’s Handbook for Shooters and Reloaders. It was developed by Lt. Col Paul Wright of...

Very similar to the 6.5 – 300 Weatherby but with a higher shoulder/shorter neck.   

   Match cartridge based on the 7 x 57 Improved necked to 6.5mm  

   Introduced around 1994 by Layne Simpson, 3 years before the 260 Remington was standardised    

This is another attempt to fit the next best design on the AR platform. Based on the 5,56 case with a 6,5mm bullet   

   Based on the 5.56 NATO round loaded to the same OAL, it is an attempt to give an improved cartridge on the AR15 platform.

   Developed by Fred Smith of Bulberry Barrel Works, Hurricane, UT. It is based on a slightly shortened 30-30 Win case necked down and given some Ackley treatment

The 6.5mm-06 A-Square originated as a wildcat, based on the popular .30-06 Springfield.  A-Square standardized the dimensions of the cartridge and submitted them to SAAMI in 1997. Currently produced by Nosler, Quality Cartridge and A-Square. No. 1 produced by OPM...

This is a new development by Hornady and is considered the big brother of the 6.5 Creedmoor cartridge. It is a flat shooting cartridge that gives extended range because of the increased case capacity. It is based on the Ruger Compact Magnum case and gives about 28%...

The 6,5 Grendel dates from around 2002 and was officially launched in 2003, five years before the 6,5 Creedmoor. It is a compact cartridge that was designed to compete with the 5,56 NATO in battle conditions and more specifically to operate from a standard AR15...

From OUTDOOR LIFE, this is the story about the development of the 6.5 Creedmoor.  

This is one of the LAZZERONI range of cartridges. It is one of two that does not have its own headstamp, but the 6.17 Flash uses the SCRAMJET case

Relatively old wildcat cartridge based on a shortened 6mm Remington   

Match cartridge designed by Robert Whitley in 2007. Not to be confused with the 6x47 Federal, which is the 222 Magnum case opened up to 6mm, the 6 x 47 Lapua is based on the 6.5x47 Lapua case necked down to 6mm.             ...

   At one stage this development formed part of the US SAW (Squad Automatic Weapon) program, but tests with the 222 Magnum case opened to 6mm with commercial bullets, different propellants and various over-all lengths did not show sufficient progress to...

Also known as the 644 SUPER, this is a match cartridge similar to the 6mm BRX, but with a longer case   

Based on the 250 Savage necked to 6mm. Comes in different shoulder angles   

Can almost be classified as one of the "first" attempts by American wildcatters with the 6mm. This is the standard 250 Savage case necked down to 6mm with no other changes. Various versions of 6mm experiments exist based on the 250 Savage case, as well as the 243...

   Based on a shortened 243 Winchester case

222 Remington necked to 6mm with no other changes   

The no h/s version is an early Western prototype

   Developed by White Oak Armaments, this is an improved version of the 6mm HAGAR.