Misfire,
I would recommend getting one that you can see when your in a solid shooting position. It will amaze you how much cant just a simple torque by the wrist will add to the rifle.
You should be able to see the level when your ready to release the trigger for the shot.
As for effectiveness, most of my customers that come out to my range think they are very effective and consistant shooters at 500 yards. Many of them are pretty good no doubt.
Most will also say that they do not need one of these ACDs on their rifle, there just do not make that big of a difference.
Well, I have then shoot at 500 yards and see what groups they get. This is off a solid bench, generally with long range varmint rifles or big game sporter rifles. Groups will on average with a good rifle be in the 3.5" to 4.5" range.
I then install a Scopelevel and have them shoot again and generally groups are reduce by at least 30% and often by up to 50%.
I have yet to see a shooter not tighten up their groups at 500 yards and out using an ACD and generally the difference is dramatic.
For big game hunting under 500 yards I would say in a quality rifle you really do not need one. Also shots inside this range can tend to offer less preperation for the shot. For these I do not recommend using one as you could miss a shot opportunity setting up.
FOr shooting or hunting outside 500 yards, I highly recommend them. For shots at this range, generally you have plenty of time to set up for the shot and they can be used to full effect.
For shooting past the 1/2 mile mark, they are a nessesity in my opinion. Just take a look at a 1000 yard BR firing line and see what they are using. I am much more familiar with the 50 BMG rifles for 1000 yard BR and they all use some ACD. Most use the Scopelevel.
Good Shooting!!
Kirby Allen(50)