Lapping a Mould: What to do if your mould is dropping small bullets

We've all had a mould that dropped small bullets.  It's expensive to send it out for machining, and the manufacturer may fix it how you want, but in many cases is a hit or miss proposition.  

You can fix it with a few tools. Carefully follow these steps and you can improve the bullets that you cast and shoot.  This translates to tighter and more consistent group sizes at the range.

First, here are a few 358-429 Keith's cast with a lapped aluminum mould, they are already sized and lubed:

There's various ways to lap a mould.  This is one technique that works well and is proven. This 358-429 mould is aluminum with 5 cavities.  The number of cavities doesn't matter, however other materials take longer and are more difficult to lap. 

The process, step-by-step:

1) Get some nuts, for 357 I use 1/2 inch, one per cavity
2) Heat the mould to casting temp                             
3) Open the sprue plate and rest a nut over each cavity

 

4) Cast through each nut, overfilling by a little bit.  If it runs a bunch,that's OK (if not full enough the nut may spin on the lead)
5) Drop'em out as usual, and let it all cool
6) Lightly coat the bullet with compound (note below)

The bullets in the picture below have already been used to lap.

7) Put each bullet back into the cavity it came from (twist off the lead if it keeps a wrench or socket from fitting the nut)
8) Either with a wrench or with a electric drill/socket start slowly lapping, while applying pressure on the handles.  Use a c-clamp (if handy) to tighten down the cavity being lapped, adding pressure slowly.  Lap each cavity for 1 minute (longer if by hand) per lapping session, and vary the speed and reverse direction if possible.   Keep it slow and you'll be good

9) Clean it up and go-to step 1

That's the process.  Use the cast bullets to actually do the lapping, in the same cavity they were cast in.  Measure each newly cast nut-bullet at a couple of points, like on the seam and then across the seam. Lap each cavity enough times to get the size you want, or just to polish it.  I lapped mine 3 times with 600 grit compound and 1 time with Rubbing Compound.

The lapping compound can be something like Wheeler 600 grit, auto rubbing compound (used this for the final-lap or polishing as it is very fine) or even toothpaste for polishing only.  Valve compound works too.

Because I use a lubrisizer, I wanted to get the minimum measurement to .0004 over my final size.  So with a .360 size as the goal, the minimum I wanted is .3604. If you don't size your bullets, just stop when each cavity is at the right size.

Benefits of a lapped mould:

1. The bullets just fall out of a lapped mould.  
2. Sizing  & lubing is easier as lube doesn't shoot-the-gaps and flood the die.
3. The best result is having fully sized bullets and not only shooting smaller groups but shooting them more consistently too

Lapping a mould is messy, so between sessions wash the mould at a sink with Dawn or something similar.  For a 5 cavity mould it takes 15-20 minutes for 2 lapping sessions.  So it doesn't take long.  Go slow and keep the stress down.
 

Good shooting!