If you plan on tapping the front trunion with out removing the
barrel,
Then you will need to grind the points down on
a few 10-32 taps so you can get threads all the way to the bottom.
(less than 1/4" deep hole )
Do not drill too deep or you will be sorry. I use 3 taps
to get the job done. A regular tap to get
the threads started, a second that goes almost gets good threads to
the bottom and a third
which has the tip ground to the point of being gone but there just
enough to get the tap started in
the almost fully threaded hole.
These are known as Taper, Plug and Bottoming Taps.