One of the operations necessary to reload a round of ammunition is opening up the case mouth to accept the base of the bullet at the bullet seating station. This is typically called “flaring” – though occasionally you’ll hear it called “belling” – the case mouth.
This is accomplished by, as you operate the handle to raise the ram (the big piece of metal that moves up and down as the machine operates) having a funnel-shaped piece of metal – round on the bottom then angling out higher up – called an expanding die enter the casing. The lower, round portion, which is only slightly undersized for the diameter of the cartridge casing, enters and centers the expanding die in the casing. Then, as upward movement of the ram, and therefore the casing, continues, the case mouth butts up against the angled portion of the expanding die; the casing continues even further upward, at which point the expanding die deforms and expands the soft brass case mouth, flaring it outward.
This is important because another operation that must be completed during handloading is to place a bullet on top of the casing, then force it, base first, down inside the casing at the bullet seating station. The case mouth must be flared in order to allow that to happen smoothly, otherwise the sharp case mouth will cut into the bullet, and you’ll wind up with a “smear” of lead, or jacketing, or plating, or polymer coating, coming up and over the case mouth. This would make the overall shape of the loaded round too fat at that point, leading to a failure to feed.
When adjusting the expanding die, it’s important not to overdo it. You want enough case flare to easily accept a bullet, without cutting into its surface. However, in pursuit of that, you don’t want to flare the case mouth TOO much, or you’ll stretch and weaken it so much you’ll severely impact and shorten case life, causing the case mouth to split during expansion after only a few reloadings.
When setting up your reloading machine, and adjusting the amount of case mouth flare, for that particular cartridge, insert an empty casing into the machine, operate the handle, run the casing up into the expanding die, operate the handle in the opposite direction, lowering the ram, remove the casing from the machine, examine its mouth. Not enough flare? Turn the expanding die down a bit. Too much flare? Back the expanding die out. Keep doing that until you have exactly the amount of case mouth flare needed. While the casing is out of the machine, you’re going to have to dump the powder back into the powder measure, since all Dillon reloading machines flare the case mouth and drop a powder charge at the same station.
Adjusting the expanding die, to the point you have enough case mouth flare but not too much, it’s jussssst right, is an exacting, somewhat fiddly process. But it’s absolutely necessary.
If you measure the case mouth on a sized, unflared case mouth first, you want to flare the case mouth to .010” – .020” larger than this dimension. The lower number is fine for solid copper and jacketed projectiles. The higher number is for softer projectiles, including lead, polymer-coated lead, and copper plated projectiles.
