The process of lapping a mould is straight forward and serves two very vital purposes; either cleaning up mould cavities for better bullet release or enlarging mould cavities for larger diameter projectiles. Let’s look at the clean first.
Many moulds, especially those straight from the factory, suffer a frustrating but easily cured disorder that prevents newly cast bullets falling freely from the cavity. Ideally upon opening of the bullet mould the lead alloy projectile(s) should drop freely or at the very least require only a gentle tap on the handle hinge point to giggle them out. Freshly cut or neglected moulds can have microscopic burrs on the edges of the cavities invisible to the naked or aged eye that whilst very, very small are more than sufficient to retard the ejection of a cast bullet. Lapping the cavity with very fine lapping/grinding compound quickly removes these and produces a mould that drops bullets freely. This process is also very effective for removing surface rust or built up residue such as burnt on oil caused by a lack of proper cleaning prior to usage.
Bullet diameter increase. Cast bullet accuracy can often benefit from a bullet of a slightly greater girth than a given mould will produce. Often as little as 0.002 inch can make a substantial difference in the size of a target group and with careful lapping it is very possible to increase the diameter of lead alloy projectiles without specialised equipment.
The first step is to fill the cavity or cavities with lead alloy, allow to solidify and cut the sprues but leave the formed bullets in the mould. With the sprue plate in place drill a hole via the pouring holds into the base of the bullets. This hole need not be deep to begin with as it is a guide or starter. Finish the hold with the sprue plate out of the way.
Next, remove the bullets and place them in a soft jaw vice with careful attention not to deform the projectile and inhibit re insertion into the mould. Taking a long shank screw, wind the screw into the bullet to create shank to mount in a drill chuck.
Place the shank of the screw in a handheld power drill and coat the bullet evenly and sparingly with fine grinding/lapping compound. Holding the drill in one hand and the mould in the other close the mould over of the bullet and begin rotating the lap in the cavity slow allowing both hands to move freely so as not to bias the lapping motion.
Burr removal in achieved rather quickly so a minute of slow rotations is most likely enough. Cavity enlargement is a longer process and directionally proportional to the material from which the mould is constructed. Aluminium being softer than iron, steel, brass is much faster to enlarge.
The following photo shows a Lyman 225462 double cavity mould (one cavity converted for hollow point casting) after a quick lapping to remove burrs for easier bullet drop. Cleaning and degreasing is required on completion.
When enlarging a cavity it is important to set a baseline measurement in order to know how much progress is being made. I enlarged the nose section of a Lee aluminium mould and started by taken sulphur casts of the chambers and measuring them. As work progressed the mould was cleaned and more casts were taken so as to be able to measure progress. The benefit of using a sulphur cast is the mould does not have to be taken up to lead cast temperature then allowed to cool again all of which would be time punishing. The actual sulphur casting commencing measurement need not be exactly the same as the lead alloy bullet as what is needed is a base line to which the intended gain is added.
EG: a mould throws a lead alloy bullet at .311 inch however a .313 inch is required. Net gain required is .002 inch. Cast a sulphur impression and lap until a total of .002 inch is added to the girth.
The following photo shows the sulphur casting (with a little graphite added for colour) lying beside the cotton swabs. Sulphur is available at hardware and gardening stores and is easily melted with a butane torch. Care should be taken to melt the sulphur very slowly so as not to ignite the powder and extreme care should be taken not to inhale the sulphur dioxide emitted by the melting process.
Keep your powder dry.