Without any context I am going to take a "shot" at this...
I believe the speaker is referring to a #12 shotgun shell, containing approximately 2400 pieces of soft shot. Usually used for killing snakes, it mostly has more bark than bite when fired from a 12 gauge shotgun at long range, and is ideal for scaring a prowler off without causing too much damage.
Phil Sweet has commented that the loose shell could have been kept handy for a purity test: evidently to test if the "run" is at 100 proof or higher, the powder from an opened casing could be mixed with the output and ignited: a bit of lore I was unaware of.
Clare has also commented that the #12 could refer to the Fox #12 shotgun, a high quality piece first produced in 1909, and thus relevant to the discussion.
The umprovided context (I guess)
Back in the day, clandestine liquor producers (moonshiners) would produce in a secluded spot in the woods during "season".
There were several points during production which were more dangerous, from a legal standpoint.
Approaching the site, it was possible there were "revenooers" waiting to arrest them. During the "run", if the wind changed or died off, it was still possible to be detected. Other dangers included thieves who would wait for the run to finish to steal it.
This is why most moonshiners kept a shotgun handy, to protect the still and "run".
Now the really dangerous part here is that it is generally a bad idea to mix firarms with alcohol. For most people producing "white lightning", there is always a temptation to taste the brew when it first is running. Apart from the fact the first "foreshots" are usually high in poisonous methanol, the ethanol produced at this point will be at its highest concentration: if they are using a "thumper" it can go as high as 88%.
The better way to test purity is always with an alcoholometer, but seasoned pros use a "shake test" to estimate the purity.
IDIOTS taste-test. And if you taste too much and get blind drunk and stupid and lay your 12 gauge shotgun shell down on the hot still, there is a danger of "cooking off" a round--with disastrous results.
You keep it up you're gonna put your number 12 down on a still some day an' git [sic] your head blowed [sic] off
So I figure that this is what an oldtimer was warning another (probably younger) partner about.