Gunblast article

[Spook]

Originally Posted by LouBoyd

I'm not sure why your asking me. Calculating that is just a matter of running the two bullets in most any exterior ballistics program with their muzzle velocities, appropriate BCs, and weight as inputs, then checking energy vs distance to see where they cross.
The gun, cartridge, or even the caliber doesn't matter (within the limitations of the G() tables)

Lou I figured if anyone could reverse engineer the crossover point it would be you.... and I trust your calculations more than I do mine

[rsilvers]

Lapua (16.5 inch barrel):

http://www.midwayusa.com/product/481...cket-box-of-30

http://www.lapua.com/en/products/spo...rfire-rifle/19

2045 J at 0 M (1508 ft-lbs)
1487 J at 100 M (1097 ft-lbs)
1045 J at 200 M (771 ft-lbs)
712 J at 300 M (525 ft-bs).

300 AAC BLACKOUT 125 grain Match:
2215 fps muzzle velocity, 1360 ft-lbs, from a 16 inch barrel. QuickLoad thinks this will change to 2227 fps from a 16.5 inch barrel.
BC is 0.338.

| 95 606 0.1477 1489 +0.3 3.5 1.25 10.3 -0.3 -0.11 1.48
X 100 602 0.1558 1470 0.0 3.8 1.30 11.4 0.0 0.00 1.56
| 105 599 0.1639 1452 -0.4 4.1 1.35 12.6 +0.4 +0.13 1.64
| 110 595 0.1722 1434 -0.8 4.5 1.42 13.9 +0.8 +0.26 1.72
| 115 591 0.1805 1416 -1.4 5.0 1.48 15.3 +1.2 +0.41 1.81
| 120 588 0.1889 1399 -2.0 5.4 1.55 16.7 +1.6 +0.56 1.89
| 125 584 0.1974 1382 -2.6 5.9 1.63 18.2 +2.1 +0.72 1.97
| 130 580 0.2060 1364 -3.4 6.5 1.71 19.8 +2.6 +0.89 2.06
| 135 577 0.2146 1348 -4.2 7.0 1.79 21.4 +3.1 +1.07 2.15
| 140 573 0.2233 1331 -5.1 7.6 1.87 23.1 +3.6 +1.25 2.23
| 145 570 0.2321 1314 -6.1 8.2 1.96 24.9 +4.2 +1.44 2.32
| 150 566 0.2409 1298 -7.2 8.9 2.04 26.8 +4.8 +1.64 2.41
| 155 563 0.2498 1282 -8.3 9.6 2.13 28.8 +5.3 +1.84 2.50
P 157 561 0.2534 1275 -8.8 9.9 2.17 29.6 +5.6 +1.92 2.53
| 160 559 0.2588 1266 -9.5 10.3 2.22 30.8 +5.9 +2.04 2.59
| 165 556 0.2679 1250 -10.8 11.1 2.31 32.9 +6.6 +2.25 2.68
| 170 552 0.2770 1235 -12.2 11.9 2.40 35.2 +7.2 +2.47 2.77
| 175 549 0.2862 1219 -13.7 12.7 2.49 37.4 +7.8 +2.69 2.86
| 180 545 0.2954 1204 -15.2 13.5 2.58 39.8 +8.5 +2.91 2.95
| 185 542 0.3047 1189 -16.9 14.4 2.67 42.3 +9.1 +3.14 3.05
| 190 538 0.3141 1174 -18.6 15.3 2.77 44.8 +9.8 +3.37 3.14
| 195 535 0.3236 1159 -20.4 16.2 2.86 47.5 +10.5 +3.60 3.24
| 200 532 0.3331 1145 -22.3 17.2 2.95 50.2 +11.2 +3.84 3.33
| 205 528 0.3426 1130 -24.3 18.2 3.04 53.0 +11.9 +4.08 3.43
| 210 525 0.3522 1116 -26.4 19.2 3.14 55.9 +12.6 +4.32 3.52
| 215 522 0.3619 1102 -28.6 20.2 3.23 58.9 +13.3 +4.57 3.62
| 220 518 0.3716 1088 -30.9 21.2 3.32 62.0 +14.0 +4.82 3.72
| 225 515 0.3814 1074 -33.2 22.3 3.41 65.2 +14.7 +5.07 3.81
| 230 512 0.3913 1061 -35.7 23.4 3.50 68.5 +15.5 +5.33 3.91
| 235 509 0.4012 1047 -38.2 24.6 3.59 71.8 +16.2 +5.59 4.01
| 240 505 0.4111 1034 -40.8 25.7 3.69 75.3 +17.0 +5.85 4.11
-------------------------------------------------------------------------------------

The energy that the Lapua has at 200 meters, the AAC has at 235 meters.
The energy that the Lapua has at 100 meters, the AAC has at 95 meters.

So the crossover point is around 110 meters / 120 yards.

[LouBoyd]

Since we're talking about the 300 BLK and the tradeoffs of bullet selection for energy versus distance this may be of interest. It's why I'm only interested in the 300 BLK with heavy VLD bullets.

Using rsilvers BC of .338 and a MV of 2216 fps for 125 grain 300 BLK ammo
and a BC of .673 and a MV of 1025 fps* for a 240 grain SMK in a 300 BLK case
Quickload shows the 240 grain subsonic will have more energy beyond 500 yards.

At 520 yards the 240 grain has 41 feet of drop , 1.6 seconds time of flight, about 1.9 " per mph crosswind deflection. and 425 ft-lbs energy.

At 520 yards the 125 grain 300 BLK only has 13.5 feet of drop and 1 second time of flight but it has 4.5" per mph crosswind deflection and 417 ft-lbs energy.

The drop may seem like a lot for the 240 but it's only a function of time of flight and it's repeatable. Wind velocity is difficult to measure (downrange matters too) but the high BC bullet subsonic bullet has less than half the crosswind deflection.

Additionally the 240 from a 16" bolt action with a suppressor will be very quiet.

Of course a 240 SMK can be shot from a 30-06, 300 Win Mag or 300 Norma Mag and get better performance in all categories except quiet and recoil. For quiet and recoil the 300 BLK is a superior cartridge. There are larger subsonic cartridges with better ballistics but none as easy to obtain.

*1025 fps is not the fastest the 240 SMK can be driven by the 300 BLK cartridge. In a bolt action where it's not magazine limited the 240 SMK can be driven to over 1200 fps with Accurate 2460 powder giving over 800 ft-lbs energy. However this will put the bullet at the high end of the transonic range with much higher drag. It would lose the virtue of the 240 BLK of being quiet with a suppressor.

[rsilvers]

We had the 220 SMK radar tested and the G1 BC was 0.720. So the 240 should be even higher. Sierra has never tested it at subsonic speeds which is why their number is lower.

[LouBoyd]

Originally Posted by rsilvers

We had the 220 SMK radar tested and the G1 BC was 0.720. So the 240 should be even higher. Sierra has never tested it at subsonic speeds which is why their number is lower.

Thanks, that's useful. Do you have the specific radar velocity where the 220 SMK's G1 BC was 0.720?

I'd also like to know radar measured subsonic BCs for the 240 Sierra, 210 & 230 grain Bergers, and the 208 Amax in the range of 800 to 1000 fps if anyone has those.

[rsilvers]

First, at the subsonic speed, a BC based on a sphere was the most accurate (lowest error). G1 was second. Then I think it was G8. And G7 was dead last.

The G1 BC would change if I used 800-1000 fps vs if I used 950-1000 fps. I don't remember the exact velocity range that came out to 0.720, but it was probably 1000 fps down to the velocity that would typically occur at 300 meters.

Whatever the velocity range was, it was what I decided at the time was most typical.

[rsilvers]

By the way - the BC for the 125 grain is Sierra's value for the target velocity range of 300 BLK. We got 0.320 on radar. For a ballistic program, I would personally use 0.320.