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Also confirm muzzle velocity, Preferable with a chronograph (buy or borrow). Measure EACH shot of at least a 5 shot carefully aimed group at 100 yards and note the point of impact and velocity of each of those shots. Preferably post a photo of the target with each shot and it's velocity marked. Take the velocities with the chronograph set about 10 feet in front of the muzzle. Most bullets will be more stable at 1000 fps than in the range of 1050 to 1300. So far no one as mentioned bullet velocity, velocity dispersion, or the shape of the groups. That's not to say faster velocities can't be used. Some bullets can have suffieient stablity across the entire transoinc range.
Before even doing shootnig test I'd have made sure my bullets are seated on or within .01"- of the lands, then worked up the the loads to give 1000 fps before doing any accuracy tests. I'd also have loaded ammo with its neck diameter between 0.002 and 0.005 of my chamber's neck.
If there is a lot of velocity variation and vertical stringing then you've probably got a problem with bullet seating depth and/or the fit of your cartridge to the chamber. If you loads are faster than 1050 fps but have uniform velocities then I'd expect a bullet stability problem. Groups could be any shape. Barrel twist and the specific bullet designs matter. Bullet weight an shape aren't the whole story. The interall mass distribution in jacketed bullets matters too. You can't determine a bullet's stability by looking at it ( though it is an indicator).
I agree on checking the barrel twist rate. (use a cleaning rod with a brush or tight patch. Mark the rod. measure to the mark, pull the rod for 1 turn of the mark, measure again to the mark and subtract. Be careful the brush or patch isn't slipping over the lands.
Also note the elevation, temperature, and barometric pressure when you do the tests. (air density is calculated from that those. Along with air density spin rate and velocity determines the stability for a particular bullet.
Please give the name and model of each bullet you use. Bullet weight and nose shape affect stability but they don't define it. Even the internal mass ditribution affects bullet stabity. The suggestion of using 220 grain Sierra matchkings isn't that they're more stable than other bullets, just that they're well known to give good performance. Other bullets may be more stable but not seat near the lands.
8 inch groups at 100 yards is terrible. It's not a subtle problem and the answer will be a head slapper when you find it.
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