100 for it to be only visible in first person, using a 0 on a group makes it visible in third person only.
Interesting fact about LODs, Milkshape will only use the highest LODs provided and work down from there. If you had a model with LODs 1 and 2, groups with an LOD of 2 would be visible in first person only, while groups with an LOD of 1 (or lower!) would be visible in third person only.
Also, you can add a number at the end of a group in Milkshape, and it will automatically have that LOD upon export. For example, if you named your group "gun2", the group would already have an LOD of 2 in the export window.