For 1200 yard shooting, I shoot groups with bullets that have no more than .002" variation from the bullet length "x" that I started the group with. This means that if I measure bullets and put a length of .?32 in the first row of my ammo box, I will put all the bullets that measure .?33 in the next row and .?34 in the next row and so on. When shooting a given bullet for group (lets say the .?33" row) I will grab a bullet only from that row or from the row above it or the row below it if there are any of those left over. Usually, I shoot the shortest bullets first and then move up to the next row. So in this scenario, I would have most likely already shot the .?32" and now I have started with the .?33" row. Now I am down to only two of those bullets left and I need three shots for the group or whatever. I simply grab ammo from the
.?34" row and shoot. So I will go up or down .001" of the "x" bullet. I would never ever grab ammo from the fifth row in my box and shoot it together with ammo from the third row for 1200 or 1000 yard shooting. For anything under 800 yards, I don't think bearing surface makes enough difference to worry about. Now when I shoot at 2000 yards, I make kill attempts or group attempts only with ammo from one row of lenghts. If I get to the end of that row of ammo and need another shot or two, I will fire the new row as a sighter and then make corrections to account for the difference in ballistic coefficient. At 2k, we found that .002" difference in bearing surface length made dang near a full MOA difference in point of impact! That is over 20" just from shape of bullet-nevermind the differences in speeds and all the other stuff we have to take into account.
I have measured more bullets than anyone else I know of and I would say that Lapua Scenars are the most consistent. Then Nosler Accubonds, Nosler Ballistic tips, Barnes TSX of good lot number, Sierra MK's(except for the 338 300 grain) and Hornaday AMAX tie, JLK's, Bergers, and then all the flat base tough hunting bullets come in last. Nosler Partitions are probably the worst overall or maybe Failsafes. Basically, the longer the bullet, or the harder and thicker the jacket, or the more materials used in one bullet, the more inconsistent the bullet will tend to be. I measure some of my 66 grain 6mm custom bullets from Ronnie Cheek that have #8 ogives and there have been none that aren't perfect!! But of course, these are J4 (soft) jacketed, short bullets that are made by hand so there should not be too much difficulty in manufacturing these compared to Bergers tough job of trying to make a needle-nosed 115 grain 6mm vld bullet from the same soft j4 jacket material.
In chronographing the different bearing surface lengths, I have not seen them make too much difference in speed variations. At least, nothing that would have been distinguishable from normal extreme spreads for any given load. I may have seen a little faster velocities from the shorter bearing surface 300 grain MK's but not much. A couple of feet per second if any.
Hope this helps. /ubbthreads/images/graemlins/wink.gif
