Precision rifle shooters know how to avoid accuracy problems caused by canting (or tilting sideways) their weapon. Normally this issue becomes a "hunting accuracy" problem only out past several hundred yards. But it is important to realize that the "problem distance" is related to the number of inches of drop, not so much the distance. And even a fast 406 feet per second crossbow has about 100 inches of drop at 100 yards. About the same as a flat shooting rifle at, say, 700 hundred yards.
The following example is slightly over-simplified but the numbers are pretty close to actual.
If you sight in your TAC 15/15i at 20 or 30 yards, there is a drop at 100 yards of approximately 100 inches
. Picture this. Place an 8 foot 2X4 out at 20 or 30 yards and sight in so that when you aim at the bottom of it, you hit the bottom of it.
Now move the almost-100-inch 2X4 out to 100 yards. If you hold your TAC 15/15i perfectly level and aim at the top of the 2X4 you will hit the bottom of the 2X4.
Next tilt the 2X4 slightly to one side, say 6 degrees, which is the same as one minute on a clock face. Not very much, is it.
Now take a string and attach it to the top of the 2X4 and let it fall to the ground. You'll see that it is about 12 inches to one side of the bottom of the 2X4. When you aim at the top, you will hit about 12 inches off to the side
To emphasize, this means your arrow will impact 12 inches off to the side and your deer is badly wounded, or missed if you are lucky.
Interesting aside -- if you are not hunting but simply shooting groups at 100 yards -- and if you don't control your cant, even a slight cant off a level bench can through your groups off by an inch or more. You'll maybe assume it was wind, but it wasn't. One degree of cant at 100 yards is two inches of horizontal error. By the way, in all these crossbow cases the vertical error is negligible.
So, what to do. It's easy to keep level if you are shooting from a bench that is level. Or shooting prone from level ground. But what about shooting sitting on a hillside or from a treestand, while in the heat of the moment.
So if you plan to hunt out to 100 yards, you need to use a scope level on your crossbow.
BUY SCOPE LEVEL AT LRH GEAR SHOP
GO HERE FOR A MORE LENGTHY DISCUSSION OF CANT