ADL stands for A model, Deluxe and features a blind magazine (no floorplate to drop the rounds out from the bottom).
BDL stands for B model, Deluxe and you've guessed it, it has a different trigger guard which has a floorplate which the ADL lacks. Otherwise, they're the same rifle.
If you're going for customizing into a sniper, ahem, precision marksman rifle, you may be better off with the ADL and buying a steel triggerguard (about $200). Steel will take the compression better.
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