Based on this post at The Firearm Blog, which lists the dimensions of the bullet and its weight, AND this calculator for drag, twist, and ballistic coefficient, the G7 BC for the M80A1 bullet is about 0.190 at velocities from mach 1.1 to mach 2.5 (mach 1.0 is the speed of sound).
Bullet overall length: 1.1865″
Nose length: 0.640″
Meplat diameter: 0.033″
Boattail diameter 1: 0.220″
Boattail diameter 2: 0.265″
Boattail length: 0.170″
with a bullet weight measured at 130.87 grains.
As needed for the calculator below, dimensions were converted to calibers by dividing by 0.308
Is a G7 BC of 0.190 good enough? Well, it is less than the poor BC of the M80 147 grain lead round (0.200). But the M80A1 has a higher velocity, and so it should have a better trajectory. It’s an OK ballistic coefficient helped out by a higher velocity.
The M80A1 is a lighter projectile, launched at a higher velocity in order to approximate the trajectory of the older M80 round.
2750 fps at 78 ft.
G7 BC of .200
3050 fps at 78 ft.
G7 BC ??
At these velocities, to match the M80 at 900 meters, the M80A1 would need a G7 BC of .1792 for wind drift, a BC of .1602 for drop, a BC of .1633 for time of flight at 900 meters, and a BC of .17692 for velocity. So the G7 BC of the M80A1 might be between .16 and .18, based on this comparison.
Based on a small photo of the M80A1 next to the M80 (1.12″), the former appears to be about 10% longer, making it 1.23″.
What might the BC be for this bullet? Well, the M80 has a G7 of .200 and the M80A1 is a lighter bullet, so it could be less than that figure. By comparison, the Berger .270 caliber 130 grain (lead) VLD has a G7 of .231. There is no possibility that the M80A1, as a fatter and less dense round, gets close to that value. It would overly-generous to suppose the G7 BC of the M80A1 to be even .210. But let’s do some math to find a reasonable estimate of the value.
We don’t know the BC, but we know the caliber, length, and weight. That should be sufficient to estimate the G7 BC, given a set of BCs from bullets of the same caliber and length. BC is directly proportional to weight, given the same basic shape, the same caliber, and the same length.
The 155 gr HBC .308 bullet has a G7 of .236 and a length of 1.237 (per JBM Ballistics). Dividing 130 by 155 gives us a reduction in BC to 83.87% for a G7 of .198.
The 168 gr Hornady HPBT .308 bullet has a length of 1.232 and a G7 of .222. The same math gives us a G7 of .172 for the M80A1. But let’s do a few more bullets and then average the results.
The 167 gr Lapua Scenar has a length of 1.236 and a G7 of .216. This gives us a G7 of .168.
The 150 grain Nosler Ballistic Tip has a length of 1.228 and a G7 of .203. The result is a G7 of .176 for the M80A1.
The 155.5 grain Berger “fullbore” has a length of 1.250 and a G7 of .237. The same math gives us a G7 of .198.
Based on the above considerations, I would put the M80A1 ballistic coefficient (G7) at between .168 and .198, with a likely value (from the average BC) of .1824.
Given the above estimates, the G7 BC of the M80A1 is almost certainly less than .200 and perhaps as low as .160. The best estimate is about .180 for the M80A1 G7 ballistic coefficient.