I'm interested in collecting the range of cartridges of various calibres which have been specifically designed to be used in a standard AR-15 action (i.e. not ones which were developed for another weapon but have since been chambered in the AR-15). I know about the following:

.25 Winchester (military experimental)
6.5mm Grendel
6.8x43 Remington SPC
.30 Whisper
.458 SOCOM
.50 Beowulf

Can anyone add to this list?

Tony Williams: Military gun and ammunition website and Discussion forum

Don't forget the experimental 4.32mm cartridge from Frankford Arsenal and Remington which predated the .17 Remington.

FWIW:  There are more Whispers than just the .300.  To the best of my knowledge, there were 6mm, 6.5mm, 7mm, and a .338 Whisper Mk II.  (The latter used the .221 Fireball case instead of the original .338 Whisper's 7mm BR case.)

I believe I havent seen any mention of the 5.56x45 yet.  Does that one count?

Quoted: FWIW:  There are more Whispers than just the .300.  To the best of my knowledge, there were 6mm, 6.5mm, 7mm, and a .338 Whisper Mk II.  (The latter used the .221 Fireball case instead of the original .338 Whisper's 7mm BR case.)

Yep: see the article on 'Cartridges for Silent Sniping Rifles'on my website, which has a group photo of the Whisper range.

Tony Williams: Military gun and ammunition website and Discussion forum

Quoted: Quoted: I believe I havent seen any mention of the 5.56x45 yet.  Does that one count? Never heard of it.  New wildcat? How about the 6x45?

Somehow I managed to overlook the 5.56x45....

Was the 6x45 specifically designed for use in the AR-15?

Tony Williams: Military gun and ammunition website and Discussion forum

I forgot a couple:

.45 Professional
.502 Thunder Sabre

Thanks to everyone, very helpful

Tony Williams: Military gun and ammunition website and discussion
forum

The 6x45, aka 6mm-.223, is often used in AR-15's for hunting in states, where .22 cal centerfires are not legal for deer or larger.  Whether it was originally "designed" for this or not does not matter.  

handgunhunt.com/tech/t13/ says it was developed for benchrest and varmint.  The 6mm TCU is very similar, but is fireformed to a different chamber and shoulder angle.  "The 6X45 only requires the handloader to run cases through the sizing die to expand the neck; shoulder angle and body taper remain the same."

It IS used for AR-15's, and is probably the easiest calibre conversion.  Only the barrel is changed.  The same upper, with no mods, bolt and extractor, and even magazines as 5.56 are used.  6x45 uppers are available from Olympic Arms, Model 1 Sales and others.

Easy to do, buy some .223  brass (www.gibrass.com and get their prepped LC brass), run through 6x45 dies, check the length and trim neck if needed.  Load with a variety of .243 dia bullets.  There is loading data available.  

Check www.huntingnut.com/cgi-bin/reloads/loads.pl?mode=LoadList&dbase=DBASE87 for data.  There must certainly be more reliable load data around, but I found this with a quick search.

Case dimensions www.stevespages.com/jpg/cd6x45.jpg

Serious consideration was given to adopting the 6x45 for the SAW and to a lesser extent, to replace the 5.56 NATO in the M16's.

Quoted: Serious consideration was given to adopting the 6x45 for the SAW and to a lesser extent, to replace the 5.56 NATO in the M16's.

Various different cartridges were experimentally developed for the Squad Automatic Weapon, including some based on the 5.56x45 case, but the definitive 6x45 SAW actually had a larger case diameter.

Tony Williams: Military gun and ammunition website and discussion
forum

I stand corrected.  Thank you.

Was the .440 Corbon designed for the AR?  I remember seeing an upper available in it.

Quoted: Was the .440 Corbon designed for the AR?  I remember seeing an upper available in it.

Nope, it was originally meant for the MRI Desert Eagle.