Default is whatever you have set in your dashboard preferences. Did you know you can set default controls/colors/etc in the dashboard for racing games and shooters? If you have it set to high sensitivity, Reach defaults to 8.
I think I heard on a majornelson podcast though that dashboard defaults only kick in the first time you load up a game.
Default sensitivity in all Halo games has been 3 (unless of course you change your profile preferences - which almost no one knows about).
I think to be competitive you should use 4 or 5, which is what the majority of MLG players use, whether it be in Reach or Halo 3.
The highest I've seen people using on the MLG forums is 6, never seen anyone anywhere say they use 10 except my friend (and he is good but I'm not sure how much sens has to do with it).
Halo is a bit strange that way, unlike COD and other shooters were the pro players always play at higher sensitivities.
I actually got worried and tried using 5 for a time and thought I was doing better but then switched back to 7 and wasn't doing any worse so it was likely just the placebo effect.
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Also not many people know this about the grenade launcher:
The little indicator on the launcher that normally shows ammo etc, when you secondary fire (hold down the trigger) and release a manually triggered grenade the indicator changes to show distance the grenade has travelled (and the fact that it is in 'linked' mode)
Now say you're trying to take out a camper and get the grenade into his camping spot:
Then the indicator will show a skull and crossbones with a 1 next to it, telling you that if you released the trigger and detonated the grenade you will kill 1 enemy (the number can go up accordingly), this makes it extremely useful and much easier to use at range or say if you're setting a trap grenade near an entrance somewhere and you don't have line of sight to the area.
It also has a tracking system much like the needler or the plasma pistol (overcharged shot) at closer ranges, the reticle will turn red when you have it over an enemy or vehicle and if you then fire a grenade (normal fire) and keep tracking the opponent/vehicle with the reticule the grenade will track and result in an instant kill.