im loading for a .243 and trying to load with the bullet seated into the lands. I notice that if i eject the round out there is only marks about 1/3 around the bullet. is this normal or are my bullts not being seated straight?
Most likely you are not seeing rifling marks, but instead when you open your bolt to eject the loaded casing, the bullet is being scraped along the side of the chamber and that is why you are only seeing marks on one side of the bullet.
Try using a case like this. I make a sample round (with a slotted neck) to hold the bullet, and I use it like you described to locate the rifling. The chamber pushes the bullet back when it contacts the rifling.
After carefully extracting the sample round, I set it on our Digital Headspace Gauge. It's designed to locate chamber clearance (at the shoulder), but it can also measure your bullet "jump" to the rifling.
Just "zero" the gauge, and it's calibrated to your rifle. When you set your handloads on the gauge, it displays how far behind the rifling your bullet is seated. This example shows the bullet is seated -.002" behind the rifling.
Have you tried using a candle to ''smoke mark'' your bullett before pushing it tight to the lands? Sometimes its easier to see the shiny marks thru the soot. But like the others posted, your marks are probably from extraction.
The explanation you're looking for is simple if your rifle is of the plunger ejector style. While extracting the round, the ejector forces the bullet hard into the rifling on one side only, if, as stated by royinidaho, you push your finger on the round keeping it square as you extract it, it should show up all the way around the bullet.
I smoke my bullets so the rifling shows up easily, but I doubt you're getting full contact with the rifling if it's only showing up on one side.
It should be the same depth all the way around the bullet if you're getting full contact.
I've got another explanation that you might want to take a look at, but hopefully this isn't the answer; the chamber is reamed off axis with the bore/barrel. I've seen this before in factory chambered rifles, really badly in one case, involving five (5) separate rifles that came off the same run. These had rifling that extended all the way to the chamber mouth on one side of the barrel. Rotate the bore scope 180 degrees, and the rifling stopped so far up the throat that they looked like Weatherby's with extra freebore. Naturally, the only way any of these were ever going to be tackdrivers was to grab them by the barrel and start using them as a hammer. I doubt, that's the issue here, but I have seen it, and it IS a possibility. Worth taking a look . . .