Strings,
I don't consider the M16 a DMR with the 5.56x45. Yes, you can make holes in paper at 600 yards, but there's little or no energy left.
While the 5.56x45 is a great close combat cartridge, I have been thinking in terms of a universal cartidge that could replace both the 5.56x45 and the 7.62x51. Looking at what is out there, the 6.8 SPC doesn't have the long range performance, and the 6.5 grendel case isn't really ideal for a military weapon, having too little body taper and too sharp a shoulder while ballistics are fairly good.
There's much talk about an imtermediate cartridge, so in thinking of one, I tried to come up with something between 5.56 and 7.62 - literally. But the case had to keep the weight down to not lose the advantage of weight vs the 7.62x51
I started with a 0.452 case head, the same one used for the 7.62x39 and 6.5 grendel. However, those two cases are short and the 6.5 grendel only has reasonable case capacity if blow out, resulting in the disadvantages already mention.
In any case, starting with the two current service cases, the 5.56 and 7.62 (0.22 and 0.30 caliber) something halfway in between is basically 6.5mm. Looking at weights, a bullet of about 106gn is again halfway in between. There is a large selection of 6.5 bullets in that approximate weight, from 100-110, including a number of Hague convention permissible FMJ bullets.
Again, noting the lack of body taper and sharp shoulder of the 6.5 Grendel, particularly compared to the sloping body of the M43 7.62x39, I sought a compromise. The M43 has a body taper of about 1 degree 20 minutes and a shoulder angle of 16 degrees. The 5.56 and 7.62x51 have body tapers of about one third of a dregree and shoulders of around 20 degrees. I gave my cartridge a 1 degree body taper, and a 20 degree shoulder.
Finally, I elected to not worry about fitting in the M16 magazine. The new round could fit in the M16 magazine with som small loss of capacity, but it would preclude the use of longer, high BC bullets by DMR and MGs if desired.
I based the final case on the Czech 7.62x45, resulting in the 6.5x45mm Comparing it to the 6.5 grendel, it gains about 200 fps with every bullet, and is about 200 fps slower than the 6.5x55 Swede. Bullets in this weight range tend to have BC of aroound 0.400+ in 6.5, exceeding the best 223 and 308 bullets in their respective military weight ranges. Even the much vaunted 0.224 77gn SMK only has a BC of 0.362, while the 0.308 175gn SMK used in the M118LR has a BC of 0.505. By comparison, the 0.264 107gn SK has a BC of 0.421, and the 142gn has a bc of 0.559.
Finally, if one wishes to fit the round in the existing M16, it is possible to modify the case so that it is slightly shorter (6.5x42mm) with the body taper and shoulder of the 6.8 SPC and still beat the 6.5 Grendel by 150 fps, but one loses the ability to shoot the heaviest rounds.
