I'd be pushing for a 12mm base diameter. Basically as wide as a. 45. Then you add the extreme 45mm OAL, and it's essential my pistol has a specially designed grip because that's huge.
The exact amount of taper would be carefully decided. also it might have a polymer coating to aid feeding. Just. Like the 5. 7x28
much of the length is taken up by the very long VLD bullet, but case capacity would be maybe 70% of 556.
Okay about 21 grains. So 45 case head or not and what is the length of the cartridge, sorry if I missed it? You said basically a 45 or 10mm necked down, which one? Might be handy to push a concept that involves existing production tooling? Can you put together a case diagram ot basic concept for us to look at ? I know what the 224 boz etc looks like but there is a difference between 45acp and 10mm case length. Which relates best to the long VLD and COAL you have in mind?
Energy is 1100, from a 20. 7 inch barrel.Range would easily exceed an M4, the caliber and performance are identical to the 4. 7mm HK case less...... or the 4. 85 british. Which was a 500m round. The 4. 7HK had no issues with barrel wear.
Keep in mind battle rifle performance is not just about maximum range engagements. What range does your VLD start to outrun a 5.56 energy wise?
The 4.6 HK case lost in trials against the 5.7x28 due to less target effectiveness. Just personal opinion only the 5.56 is already weak enough sauce. again my personal opinion only but the majority of 1st world warfare these days is policing 3rd world insurgents. I think this space race for 'logistics' at some point took a back seat to killing power, especially in this era where we never face down large amounts of enemy our our own parity. We are shooting at people running for the hills because we have a a multi-platform war machine behind us. I was a contractor in the middle east after I left the army for several years, and though it is 10 year old anecdote I remember there was a big push to get more 7.62 x 51. A lot of the Euro guys had them, everyone drooling over their g3's.
Thats just my personal opinion mind you, my army had the FAL before they went to 5.56 so I am part of 'that old crowd'. Good luck with your venture, anything new is interesting.