reloading 222 questions

ReloadKY said:
Why is their a noticeable difference between the flat base and the boat tail bullets?
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There are several reasons. Hgmeyer's results are contrary to most, but it points out that you really have to try things to be sure what your gun likes.

The reasons flat base are usually found to be best are several. Bryan Litz puts it to the fact manufacturing a perfectly symmetrical profile is easier to do with a flat base. That is, it is easier to get a flat base square than to keep a gradual taper perfectly coaxial with the rest of the bullet. I have seen photos of some cheap Eastern European boattail bullets on which you could actually see the boattail meeting the bearing surface higher on one side than on the other even in the low res ad photo. Any slight misalignment of the formed boattail axis with the bearing surface axis of the bullet results in two drift components that gradually move the bullet off the mean point of impact. A flat base has nothing that needs aligning, so its an extra factor in a boattail. A boattail that is anything less than perfectly centered means the boattail meets the bearing surface higher on one side, as I described, and as the bullet emerges from the muzzle gas starts escaping on the high side first, and that's the source of a radial deflection that becomes drift. The drift velocity is low—typically on the order of less than a foot per second—so it doesn't cause enough drag to slow itself, and thus it pretty much stays with the bullet over the whole time of flight, causing the error in MOA to get bigger as the range increases becaise the TOF for each successive hundred yards becomes greater and greater as the bullet loses velocity. The same asymmetry will also cause drift by moving the bullet center of mass to the side of the bore axis during spin. That results in the rifling lobbing the bullet tangential to the muzzle at the angular velocity of that center of mass, which becomes a drift component as it exits the muzzle. It further proceeds to wobble slightly in flight due to eccentric spin from that off-center mass.

Another factor is that a flat base clears the muzzle all at once and with gas blowing off its base perpendicular to the bore axis, while a boattail has gas directed at an angle off the boattail as it emerges from the muzzle and a little beyond. That means any small imperfection in the muzzle crown or just uneven distribution of powder particles have a chance to contribute a little drift by deflecting off the taper which acts as a wedge for it to drive against.

Another factor is that, when a FB and BT bullet are the same weight and type of construction, the BT will usually be the longer of the two. Length is a bigger factor than weight in bullet stability, with the longer bullet being less stable for the same spin rate at the same velocity. When you have a gradual rifling twist that is optimal for a short bullet, you can get groups opening more for the longer bullet.

All that said, the makers of match bullets turn out some pretty awesome product these days and essentially perfectly symmetric boattails are much more common than they used to be. So if you have a really good crown and if you are shooting at longer ranges, and if your gun's rifling twist is on the short side of tolerance, they may still do best in your particular gun. You can but try!
 
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