MarcSpaz wrote:I have heard that does make a difference. The barrel is a 18” 1 in 10 and I have CBC FMJ’s which I believe are 147 grain. The box is not labeled, so I am not sure. If I try 300 or further this week, I am going to try to find some 155 grain. 168 might be too high a drop rate at 1000 yards. I am going to try to hunt down those accubond 165's Gunderwood mentioned too. For out to 350-400 yards, I bet that would be a kick butt round.
If you want to learn about external ballistics, Bryan Litz's book is one of the best. It's well written and simple.
http://www.appliedballisticsllc.com
A bullets stability is estimated by the Miller Stability formula.
http://en.wikipedia.org/wiki/Miller_twist_rule
Basically, if the center of pressure (point where the net aero drag is being applied) is in front of the center of gravity, there is an overturning torque; the bullet wants to fly backwards which wouldn't be very aerodynamic. Short bullets have a small CP/CG lever arm and don't require much spin at all, long bullet (e.g. rifle) require more. For a given caliber, heavier typically means longer. Spinning a bullet makes it act like a gyroscope so that as the overturning torque attempts to flip the bullet around, the gyroscopic effect counters it. The actual imparted spin rate varies on twist rate (which is constant in the barrel) and velocity. Your barrel is a 1:10" twist if it still has the original DPMS on it (it completes one revolution per 10"), but the faster you push the bullet through the barrel will impart a faster twist rate.
We generalize this for specific cartridges since a .308Win is only capable of pushing a bullet of a given weight so fast. Thus, a heavier bullet in the caliber equals slower linear velocity, also mean less rotational velocity. It get's worse because heavier bullets in a caliber are typically also longer so the CP/CG lever arm is longer and the required rotational velocity to keep it flying straight is higher. Most people don't bother with calculations unless they are outside the norm. A 1:10" twist is fairly fast for a .308Win, but it's what most people prefer for shooting up to ~175s-190s. It will shoot lighter bullets as well, but will spin them a bit faster than is considered ideal. Generally not a problem until you spin it so fast that the bullet can't handle it. Stay away from the handful of varmint 30cal bullets and you'll be fine. 175's are the sweet spot for a 1:10" and many guns love the 175 SMKs. 18" is on the short end for 175s getting enough velocity to be useful IMHO. Your barrel should stabilize up to about 190s, but that's really just too short(18") and too heavy for a .308Win. Stick with the NATO round on the low end ~147gr and 175s on the high and you'll be very happy.
The Noslers I recommended for SHTF/hunting will not be as accurate as match bullets like the SMK. However, the terminal performance is excellent and if you ever have to take a glass shot, AccuBonds are your friend even if they aren't great long range bullets (due to low BC which is really a poor choice of boat tail angles/lengths). Although, they do have new AccuBond-LRs, but given those expand as low as 1300fps, I don't think they are going to do as well in glass shots.
Shooting 175gr match bullets you should hit 1k no problem and have fun stretching it out that far. I've shot a 16" barreled AR10 to 700 yards on slightly smaller than man sized steel using 147gr NATO reloads and a 3.5x ACOG. Groups aren't pretty due to the terrible wind drift and low magnification, but a 147gr on target is still a 147g on target in a fight. However, it's really a ~500 yard gun (IMHO) when shooting bullets with real terminal performance and considering danger space (the real trajectory comparison).
sweet looking rifle you have there
