buckey said:
Can't find a thing wrong with this batch of FMJ's ( Not bent,bases look good, and weight is with in spec)
Those factors aren’t the big players in FMJ grouping precision problems. The main problem is uneven jacket wall thickness, which you can't see from the outside and that unbalances the bullet. It's harder to form an FMJ jacket easily than it is to form a hollow point easily because you are forming the bottom of the jacket cup into a point rather than a flat base or a boattail.
That imbalance causes the center of mass (CM) to circle the bore axis as the bullet goes down the tube, analogous to a weight being swung around your head on a string. Then, when it clears the muzzle, the bullet’s CM is thrown tangent to the side of the barrel it was closest to at exit and away from the mean trajectory. The velocity of this radial toss is equal to the CM's angular velocity of rotation at the exit.
That drift fattens groups up nicely when the CM offset and location in the chamber are random. In funky 30 cal 147 grain NATO ball type FMJ's, I've seen that imbalance result in up to about 2.5 fps of drift away from the normal trajectory, resulting in an 8.6" group at 100 yards. More often, in cheap FMJ’s it is about a third of that. Drift at such low velocity doesn’t cause enough drag for air to slow it appreciably, so the result is group widening in proportion to time of flight to different ranges.
To find out if your bullets have the problem, there are four methods I am aware of. One is a bullet spinner based on Harold Vaughn's air bearing design that takes a couple of minutes to come up to speed with each bullet and uses a microphone transducer to detect vibration from the imbalance, though a guitar pickup and other methods should work fine. Another was used by Mid Tompkins as described on this board by member Bart B., which required a custom bullet holding collet used in a Dremel tool to spin the bullet fast while watching the Dremel’s current demand. Eccentric rotation of an unbalanced bullet increased the load on the Dremel tool, so current increased with it. The third method is the
Vernon Juenke type device that uses a small inductive proximity detector to detect uneven eddy current loading by the jacket thickness while the bullet is rotated. The fourth is a horizontal torsion pendulum. This is a small v-trough suspended between two fine music wires under tension and coupled close to the balance point of the trough with a bullet lying in it. You rotate the bullet part of a turn and let it settle, watching for the angle of twist of the trough between the wires to change. It takes a little understanding to build one of these.
A less common issue, if the bullet looks good, is boattail eccentricity, but you can measure that easily. You can check boattail concentricity on a case or cartridge runout concentricity checker. If you have neither or yours lacks a suitable setup, place the cylindrical bearing surface portion of the bullet in a v-block with the tip up against a flat vertical plate or stop that you clamp to one end of the v-groove. Set the indicator anywhere along the boattail and rotate the bullet in the groove with your finger. You don’t want to see any runout.
