I'm not sure how hetfield palm mutes,but there are two ways I palm mute...I either rest my hand on the brige lightly on the strings or I rest them right over the bridge pickup to where my pick strikes above the neck pickup.

Both give different sounds...resting on the lightly on the bridge will give you a more 'open ballsy' or 'breathy' power chord and the decay of the chord is slow.Resting right on the strings above the bridge pickup will give you a 'punchy' or 'thick chug' power chords and the decay of the chord is almost immediate.In conjuction with a distorted amp use both to find the sound you want.(also,picking above the neck pickup can make different A.H's available)