It sounds as if the consensus (and Bushy)
is right on: either an overly long cartridge case, or a bullet not seated deeply enough. Maybe a combination of the two? A tightly compressed load with max. seating length could be enough to force the bullet forward.
Cartridge cases are easy: just size, trim & chamfer.
Can you look at the sides of the bullet and see engraving marks on it, just behind the ogive, that match the rifling?
They are tricky little devils to spot, until you have done it once, are spaced evenly around the circumference of the bullet, and look square, or slightly rectangular, and contrast with the bullet's polished appearance.
If you can see those, it means your bullet is being forced into the rifling, and more seating depth is indicated.
Try this: Using a caliper, measure the length of the marks. They're little, and eyestrain is guaranteed.
Do more than one, and average them. Then, measure the overall length of your round. Subtract the length of the marks from the round length, which will give you ~ the cartridge length with the bullet just touching the rifling, then shorten that by whatever bullet to rifling gap is best for your rifle. .020 is a good place to start. Adjust your seating die, and Fear Not! Then go shooting, you've earned it.
Don't be afraid of seating the bullet with the base well into the powder space. It may be necessary for safe operation of your gun, and the velocity lost by doing that is not enough to worry about. Or use a shorter (lighter) bullet .
Good luck with your search.