I was poring over some old ballistic tables and noticed round nose bullets seemed to be fairly aerodynamic compared to what many current-day companies advertise for their round-nose soft point and FMJ loads.
A lot of companies have round-nosed bullets with low ballistic coefficients. For example Remington's 220-gr .30-06 Core-Lokt is advertised at 0.294, the 180-gr is 0.248. Federal loads a 220-gr Speer Hot-Core in .30-06 with an advertised BC of 0.293. Norma says their 156-gr 6.5mm Alaska has a BC of 0.276, their .308" 180-gr Alaska has a BC of 0.257. Others like Hornady have similar figures, but Woodleigh gives noticeably higher BCs for its round nose bullets.
Round nose advertised BCs (Norma is ICAO standard)
I think many companies just estimate, or make a very rough calculation or inadequate test, because very few people actually care about the BCs of round-nosed bullets for obvious reasons. They probably spend much more effort on the BCs of their longer-ranged bullets (although those may be inflated...)
So the form factors for the bullets above would be:
220 Core-Lokt = 1.127
180 Core-Lokt = 1.093
220 Hot-Core = 1.131
156 Alaska = 1.138 (METRO)
180 Alaska = 1.036 (METRO)
Average = 1.105
Based on the old trajectory tables, the round-nosed .303 MK VI had a ballistic coefficient of about 0.385. The manual for the 1898 Krag has tables which suggest a ballistic coefficient of 0.346-0.347 out to 500 yards. Wikipedia has a small chart on 8mm Lebel ballistics which gives the flat-nosed "Balle M" a ballistic coefficient of about 0.354 increasing to 0.411 as it hits the sound barrier (French must have used a much different drag function).
.303 Mk VI
.30 Army (.30-40 Krag)
Commercial 8mm "Balle M"
So I checked Sierra's site because I have more faith they have properly tested their bullets' BCs. They claim to conduct firing tests on their bullets and they provide multiple G1 BCs over a range of velocities. They also have a few round nosed bullets. According to Sierra their 220-gr, .308" round-nose has:
.310 @ 2600 fps and above
.335 between 2600 and 2200 fps
.378 between 2200 and 1600 fps
.410 @ 1600 fps and below
Sierra #2180
The BC averages about 0.360 over the first 400 m, 0.365 for 500 m. Using the BCs for this Sierra bullet the drop results were a decent match for the column "angle of elevation found by firing with the service sight" in the Krag manual out to 1500 yards. It was closer than the "computed angle of departure" column on the same page.
Form factor, 220-gr Sierra RN:
.310 = 1.069
.335 = 0.989
.378 = 0.876
.410 = 0.808
Average = 0.936
400 m = 220/7000/.308^2/.360 = 0.920
500 m = 220/7000/.308^2/.365 = 0.908
I also checked Sierra's 180-gr RN, which is advertised with the following BCs:
.240 @ 2800 fps and above
.280 between 2800 and 2200 fps
.330 between 2200 and 1500 fps
.355 @ 1500 fps and below
Average 400 m BC: 0.309
Average 500 m BC: 0.316
And this gives the following form factors:
.240 = 1.129
.280 = 0.968
.330 = 0.821
.355 = 0.764
Average = 0.921
Average 400 m FF: 0.877
Average 500 m FF: 0.858
Last but not least I checked the discontinued Sierra 7mm 170-grain RN:
.280 @ 2500 fps and above
.317 between 2500 and 2000 fps
.360 @ 2000 fps and below
Average BC = 0.319
Average 500 yd BC = 0.333
With the following form factors:
Average FF = 0.944
Average 500 yd FF = 0.904
So based on Sierra's data, it would seem the BCs of generic round nosed bullets should be more like the following:
.308 220-gr, 2400 ft/s = 0.360-0.365
.308, 180-gr, 2700 ft/s = 0.310-0.315
.285, 173-gr, 2400 ft/s = 0.330-0.335
.264, 156-gr, 2550 ft/s = 0.340-0.350
Note that even though a round nose BC may be higher than some spitzer bullets, this is due to the greater weight. The form factor is much worse. For example .308" Core-Lokt 150-gr PSP has an advertised BC of 0.314. Let's assume the 220-gr Core-Lokt has a BC averaging about 0.360 over the first 500 yards (a lower FF is better).
150/7000/.308^2/.314 = 0.719
220/7000/.308^2/.360 = 0.920
If the 220-gr had the same FF as the 150-gr PSP, it would have a BC of 0.460.
Just some musings!
A lot of companies have round-nosed bullets with low ballistic coefficients. For example Remington's 220-gr .30-06 Core-Lokt is advertised at 0.294, the 180-gr is 0.248. Federal loads a 220-gr Speer Hot-Core in .30-06 with an advertised BC of 0.293. Norma says their 156-gr 6.5mm Alaska has a BC of 0.276, their .308" 180-gr Alaska has a BC of 0.257. Others like Hornady have similar figures, but Woodleigh gives noticeably higher BCs for its round nose bullets.
Round nose advertised BCs (Norma is ICAO standard)
I think many companies just estimate, or make a very rough calculation or inadequate test, because very few people actually care about the BCs of round-nosed bullets for obvious reasons. They probably spend much more effort on the BCs of their longer-ranged bullets (although those may be inflated...)
So the form factors for the bullets above would be:
220 Core-Lokt = 1.127
180 Core-Lokt = 1.093
220 Hot-Core = 1.131
156 Alaska = 1.138 (METRO)
180 Alaska = 1.036 (METRO)
Average = 1.105
Based on the old trajectory tables, the round-nosed .303 MK VI had a ballistic coefficient of about 0.385. The manual for the 1898 Krag has tables which suggest a ballistic coefficient of 0.346-0.347 out to 500 yards. Wikipedia has a small chart on 8mm Lebel ballistics which gives the flat-nosed "Balle M" a ballistic coefficient of about 0.354 increasing to 0.411 as it hits the sound barrier (French must have used a much different drag function).
.303 Mk VI
.30 Army (.30-40 Krag)
Commercial 8mm "Balle M"
So I checked Sierra's site because I have more faith they have properly tested their bullets' BCs. They claim to conduct firing tests on their bullets and they provide multiple G1 BCs over a range of velocities. They also have a few round nosed bullets. According to Sierra their 220-gr, .308" round-nose has:
.310 @ 2600 fps and above
.335 between 2600 and 2200 fps
.378 between 2200 and 1600 fps
.410 @ 1600 fps and below
Sierra #2180
The BC averages about 0.360 over the first 400 m, 0.365 for 500 m. Using the BCs for this Sierra bullet the drop results were a decent match for the column "angle of elevation found by firing with the service sight" in the Krag manual out to 1500 yards. It was closer than the "computed angle of departure" column on the same page.
Form factor, 220-gr Sierra RN:
.310 = 1.069
.335 = 0.989
.378 = 0.876
.410 = 0.808
Average = 0.936
400 m = 220/7000/.308^2/.360 = 0.920
500 m = 220/7000/.308^2/.365 = 0.908
I also checked Sierra's 180-gr RN, which is advertised with the following BCs:
.240 @ 2800 fps and above
.280 between 2800 and 2200 fps
.330 between 2200 and 1500 fps
.355 @ 1500 fps and below
Average 400 m BC: 0.309
Average 500 m BC: 0.316
And this gives the following form factors:
.240 = 1.129
.280 = 0.968
.330 = 0.821
.355 = 0.764
Average = 0.921
Average 400 m FF: 0.877
Average 500 m FF: 0.858
Last but not least I checked the discontinued Sierra 7mm 170-grain RN:
.280 @ 2500 fps and above
.317 between 2500 and 2000 fps
.360 @ 2000 fps and below
Average BC = 0.319
Average 500 yd BC = 0.333
With the following form factors:
Average FF = 0.944
Average 500 yd FF = 0.904
So based on Sierra's data, it would seem the BCs of generic round nosed bullets should be more like the following:
.308 220-gr, 2400 ft/s = 0.360-0.365
.308, 180-gr, 2700 ft/s = 0.310-0.315
.285, 173-gr, 2400 ft/s = 0.330-0.335
.264, 156-gr, 2550 ft/s = 0.340-0.350
Note that even though a round nose BC may be higher than some spitzer bullets, this is due to the greater weight. The form factor is much worse. For example .308" Core-Lokt 150-gr PSP has an advertised BC of 0.314. Let's assume the 220-gr Core-Lokt has a BC averaging about 0.360 over the first 500 yards (a lower FF is better).
150/7000/.308^2/.314 = 0.719
220/7000/.308^2/.360 = 0.920
If the 220-gr had the same FF as the 150-gr PSP, it would have a BC of 0.460.
Just some musings!
