Re: need help designing a case
Great topic. You've hit on one of the conundrums of cartridge design: case capacity, projectile construction, barrel life, and maximum velocity. Here's my two cents based on what I've learned from personal experience over the last 15 or so years of plinking:
1) Accuracy/Velocity Nodes: in my experience, a given bullet will show accuracy at certain velocities; for me, these tend to be about 150 to 200 fps apart. My favorite cartridge of all time is the 300 WinMag loaded with a 240 grain Sierra. Depending on the barrel, I can run MOA out to 1000 yards at 2450 fps, but I can get a max velocity of 2850 to 2900 fps. Best accuracy tends to occur at 2600, 2750, and 2900, although at 2900 I get a lot of unexplained fliers. Perhaps a slower twist barrel would help, but I'm short of both the time and $$ to test that theory.
Why the WinMag and not a larger cartridge? When I used the 300 WBY or a 300 WBY Improved variant, I observed that I had the same accuracy nodes as the Win Mag, but I could not push the bullet any faster before I ran into pressure problems. With the powders I was using, I still had 10% case volume left unused. I like a full case, so I said to my self, "Self, why are you using these larger cartridges?" and went back to the Win Mag. I flirted briefly with the 30-378 cartridge, but could not get accuracy at the increased velocities I could get.
Part of this is undoubtedly barrel twist, but the other part is that air density increases as the cube of the velocity; twice as fast gives you 8 times the air resistance, and at veolocities approaching 3200 fps, the bullets didn't seem to fly any better than out of a WinMag at 2900. In fact, the only "high" BC bullet I've been able to get accuracy out of is a 6mm 105 grain AMax out of my 6mm 40X at 3200 fps, which is a 1/2 moa shooter for whatever life the barrel is going to give me - I guess about 600 rounds.
The name of the game is hi bc, stable flight projectiles that minimize time of flight and thus the dwell time of external vectors such as wind and gravity. The CheyTac cartridges are the best thing I've seen hit the shooting world since I've been shooting; John Taylor managed to balance cartridge size with and enormously efficient and well constructed bullet.
We're always trying to reinvent the wheel, but a good cartridge is made first around the best projectile available for that caliber, and then brass that will show the most load density for a that bullet at its most accurate velocity node in a given barrel twist. There are numerous formulae that minds greater than my own use to pre-plan this, but in my experience, here are my picks:
.224: 55grain Ballistic Tip @ 3250 fps, 1/8 or 1/9 twist
.243: 105grain AMax @ 3200 fps, 1/10 twist
.270: 130grain MatchKing @ 2950 fps 1/10 twist
.308: 175grain MatchKing @ 2750 fps from a 308/7.62 cartridge, 1 in 12 twist
.308: 240grain MatchKing @ 2750 fps, 1/10 twist.
The Future: getting ready to explore the 375 CT using Sierra MatchKings and a 1/12 twist Lawton Barrel.
Remember, just my $.02.
bviously there was no wind.
