Are there any common calibers that are made by opening up a 5.56 mm cartage. The same way that a .35 Whelen is a opened up 30-06?
this is difficult to believe. with 32 gr water case capacity vs. 58 gr with the 7x57 Mauser it really isn't a fair comparison. the 6.5 TCU gives slightly more energy at the muzzle than the 7 TCU, but even so it is 800 ft-lbs less than the 100 gr 7mm Mauser at 2200+ ft-lbs ME. The 6mm-223 with 80 gr. bullets can go over 1400 ft-lbs ME, and the 223 itself goes into the 1200+ ft-lbs area with maybe the odd hot load getting over 1300, so the improvement is at best 20% with these wildcats. The 6.8 SPC can go over 1700 ft-lbs so it shows a significant improvement over the 223, and AR uppers are available. Not to rain on the parade, but even this relatively simple change has not been accepted and a lot of research went into its design to make as much use as possible of the AR platform.... From what I've heard the lighter bullets can rival the 7mm mauser in the right length barrel, out of a 20" though, not sure.
Look up the 6mm TCU. Wes Ugalde did quite a few things with the 223, and although simple it's still all very interesting. I love the 7mm TCU and loading for it has taught me a lot about wild catting and even more about reloading.Great read guys !! Very informative. A 6x45 sounds interesting.