Nick,
Measuring the throat diameter is easy. The easiest way to measure a .303 or Krag chamber is to take a factory .38 Special soft-swaged lead, round-nosed bullet, drop it into the chamber, and then use a section of squared off, chamfered and polished 3/8" brass or aluminum rod to pound it into the chamber neck until the drive rod comes to a stop against the shoulder of the chamber. Then just tap the slug back out from the muzzle and measure it with a micrometer. This lets you capture the chamber neck diameter, case mouth transition, and throat diameter, which are all you need to know to order your mold from Accurate or NOE.
Attachment 228963