Getting better base fill out


Staff member
When I get a used mold or a new mold that doesn't want to offer good bullet base fill out, it is a common practice for me to " break the top edge of the blocks " . I use this technique on Steel, Brass, and Aluminum moulds. 1 cavity, 2 cavity, 4 cavity, and 6 cavity.

This 1st photo will show the correct angle to hold the file. Be certain that the file doesn't enter the mould cavity. It may work out best for you to put the mould in a padded vise so that it cannot move. Then you can focus your efforts on the angle of the file and making short smooth strokes with the file.

I use a very fine cut flat Swiss file. ( The file I used in the photo above is too coarse, I have a very fine cut file, I just didn't have it handy for the photo session ) Some use a fine cut flat stone to accomplish this. It is important not to remove too much metal as you'll end up with " fins " on the base of your bullets. The amount of metal that you'll remove should offer you about the same gap as a vent line on the top surface of your mold blocks.

It is best to go a little at a time and stop and make about 40 bullets to see if the metal you've removed has made a difference in your problem. Remember , once metal is removed - it can't be put back on ! Only remove enough metal to offer good bullet base fill out and then STOP !

Look closely at the tops of the blocks.
You can see the " shiney " areas where I've removed a tiny bit of metal on both halves of the mould .

This technique that I'm describing makes a BIG DIFFERENCE in base fill out on any brand of mold, steel, brass, aluminum, etc !

The mold that you see in the photos below didn't want to offer bullets with decent base fill out regardless of the mold temp, alloy temp, etc. After the vent lines were added at the very top edge of the blocks ( see the last photo ) , the mold base fill out was near 99%.

Be certain that the sprue plate is not too tight. It should swing from right to left with very little effort. This will aid in the venting of air as the mold cavities fill.

I once had an inexpensive Lee , 148 Gr. 38/357 Wadcutter Mould.
The bases did not want to fill out very well.
Notice the shiny top edge of the mould in the next photo. This was done to both mould halves. That shiny top edge was created with a fine cut Swiss
file held at the correct angle.

Now look at the difference in base fill out after the venting process
is completed :

Here is an older Lee 6 cavity , plain base , .30 cal. mould.
Regardless of how clean the mould was, how hot the alloy temp was, it wanted to give about 40% of the as cast bullets with slightly rounded bases.

Same treatment outlined above.........He is the " AFTER PHOTO ".
A great casting mould now :

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S Mac

SW Mo Hillbilly
Thanks for sharing Ben. One thing I do differently is to file across the top instead of the way you show, maybe I should try your way.


Active Member
Thanks Ben. I hadn't thought of that but did it to my 358477 mould yesterday. Haven't got to try it yet but maybe today.


Active Member
Ben, I wasn't sure I had beveled enough but I ran 50 with a one hole mould with no culls and said "time to quit before I get too much of a good thing".


Staff member
"time to quit before I get too much of a good thing".
You speak with MUCH wisdom.
My old grandaddy always said.........." If it isn't broke, don't be trying to fix it. "


Official forum enigma
Subtleties play big in all of my dealings with mated metal tooling like mould blocks. Less is more, for certain. On my moulds that required this step (about a dozen of them, all Lymans) one delicate & light pass with the fine-grit side of an emery board stolen from SWMBO did the job.


Well-Known Member
I generally have trouble with rifle moulds and break the top of all the Al. moulds. Tom (and Lee?) started putting vent lines in the top but I still break them. I don't use tin in alloy. I get good bases when I fill the cavity quickly. Watching the pour carefully to make sure I get good flow and centered in the sprue hole. Off center or a dribble and bad base. Don't pour a particularly large sprue puddle and let the mould drop on bench immediately. Impulse kinda helps base fill out. Modified the Lee so it opens and closes quickly.