I'm trying to make some ammo decisions.
I've been shooting all six of these over the past several years and trying to pair down to one primary ammo for each gun. The actual cartridges are:
Double Tap 155gr (sierra bullet #2165)
FGMM 7.62 175gr
Hornady 123 ELD-M
Federal 130gr Berger
I used several of the formulae - ideas in Brian Litz's “Applied Ballistics for Long Range Shooting”, Third Edition, 2015 .
For those unfamiliar with these ideas, briefly:
Danger Space - a measure of the distance in front of and past a given size target at a given distance at which the in flight bullet "covers" the target in the vertical dimension. The larger the danger space the higher the probability of a hit.
Lag Time - The total flight time minus the flight time in a vacuum. We can control for this by selection bullets/cartridges which minimize the lag time. A smaller lag time results in less wind deflection.
Predicted MV. This is a handy lilttle formula that allows us to estimate the possible MV of a bullet/cartridge given a known MV for a known bullet and the weight of a second bullet. Since I had real MV numbers for all six of these, I ran the calculations in both directions. This gave me an idea about which cartridge was the "hotter" load. The prediction is based on the idea that predicted MV will have the same pressure as the known cartridge.
So the biggest surprise was the 155gr SMK versus the 175gr SMK. BL makes it clear that the heavier, higher BC bullet will usually "beat" the lighter faster lower BC bullet, unless the lower BC bullet is going at a much faster (possibly unsafe) velocity. Well, in this particular case, the 155gr wins on both danger space and lag time at the distances I compared (between 500yds and 1,050yds ... where I can get to on my land). And more surprisingly the MV predictions indicate the 175gr load is the hotter of the two. I have some HSM 155gr 7.62 load ammo inbound and the nominal MV for that cartridge is 2900. For the Double Tap load the nominal MV is 2850, though the observed MV is 2774 out of my 22 inch barrel. So we will see what the HSM load can do.
With the 123gr vs the 130gr ... I keep wanting the 130gr to be better ... the BC is higher ... but out of my gun anyway ... an 18 inch stoner in this case ... the observed MV over two 20 rd sessions has been 2252 and yesterday 2244. With that average MV of 2248 ... the 130gr is not worse, but it is not much better. And not worth the extra cost . BUT, the MV predictions indicate the 130gr could be loaded hotter. If I was going to continue with the 6.5G I would drill into this and possibly reload trhe 130gr if I could show that a load could exist which would beat that 123 by a sufficient margin. But right now, I'm thinking of putting the gun on the shelf, keeping the 1200rds of 130gr and 800rds of 123 I have and deactivating this line for my purposes.
As to the 190gr vs the 220gr ... well the MV prediction indicates the 190gr load could be hotter ... if I can find one, then the 190gr will win out to 1200yds. I was a bit surprised here. But the numbers are the numbers.