Does anyone know a good way to measure bearing surface on a bullet with a boat-tail? I need it for some loads I'm working on to test a theory of mine. The manufacturer doesn't mention the length of the bearing surface online but it may be something that I could call and ask about if measuring myself is too difficult. Thanks in advance.
I'm guessing the bearing surface would be the part of the bullet that goes inside the case? Should be easy to figure out. Oal minus desired case length subtracted from bullet length.
If you don't have a blade micrometer you can use the front edge of a good set of calipers. Just keep measuring from the boat tail on up until the diameter starts to decrease where the ogive starts. This will vary on most every make of bullet. Scribe a fine line on the bullet at that point. Then take a scale or a depth micrometer and measure by eyeball from the back of the bullet to the line you just scribed. Subtract the length of the boat tail and that is your bearing surface, with a bit extra to make up for the land diameter.
Thanks for the idea. I did forget about the land diameter do you think that will make much of a difference when calculating the rotational speed of the bullet? Edit: I do have a mic set but I only have ball and standard. I don't know anyone that has a blade mic either.
Ah so it would be the part starting at the diameter of the lands and going to the diameter of the grooves. I should have known that I'm terrible with terminology.
You can use a regular micrometer, just use one side of the spindle and anvil as you advance your measurements toward the ogive. I'm not sure about your question regarding measuring the rotational speed of the bullet? That is easily determined by velocity in feet per second against the rate of twist of your barrel. For example, to keep it simple hypothetically, if your bullet is loaded to say, 3,000 FPS, and your firing it out of a 1 in 12" twist barrel, the bullet would leave the muzzle rotating at 3,000 revolutions per second, or 180,000 RPM, (revolutions per minute). The depth of the lands and grooves of the barrel, along with the length of the bearing surface would have no effect on this. Only the velocity of the bullet itself, and the rate of twist of the barrel it was being shot out of.
There is a formula I'm playing with that calculates the speed with respect to slippage from material properties and land diameter. The formula that some bullet and barrel manufacturers use to calculate the maximum bullet weight that will stabilize.
If a bullet were to "slip" in the rifling, it would strip out, much like a thread. The only place I've seen this used was in calculating the speed of a boat in regard to engine RPM vs. prop pitch. Usually they factor in around a 6% slip factor because water is a movable liquid.
Old man with poor ways.... Put the bullet on a sheet of glass, not standing up. With a good light behind it, measure with calipers from contact points on both ends. Depending upon how good your eyes are, you'll get damn close. If the bullet touches the glass, it will touch the rifling. The crunch will add some, but the ogive of the bullet and depth of the rifling with be the determining factors. Getting that nit picky is a waste in my eyes. But what do I know.
