Funny. I've driven to the range many times with the windows down. I don't hear gunfire from the highway. The noise isn't noticeable until I turn into the crater near the archery range.
I believe the shape of the crater already dampens the sound plenty. If you park near the restrooms in the center, you can easily hold a conversation and not need hearing protection even with heavy rifle range use going on.
Since good quality ear protection takes gunfire noise down to about 30dB, it's very difficult for me to believe anyone in a HK home hears enough gunfire to be bothered.
Therefore, planting any sort of trees or shrubs to absorb noise would not be cost effective, since the level of noise is already at acceptable levels, and any additional sound absorbed by trees would be negligible.
I can speak to this, as I live in the area. I live about 1 mile, as the crow flies, from the range and family members that we visit regularly live on Anapalau St., the road that takes you to Koko Head District Park.
You can definitely hear the gunfire from my house, and the closer you get, the louder it gets. It becomes more or less noticeable depending on atmospheric conditions as well. Sometimes it's barely noticeable pops, but when the wind is right it gets much louder. Having lived here for 20 years, I've grown accustomed to it. As have the relatives.
The bottom line is, if you bought a house next to a gun range, hoping for peace and quiet, you made a grave error and you should move.