Bolt Catch Lever

[ar15556]

Why is the bolt catch lever knurled, or grooved?
It doesn't move in the direction of the finish.

[GTifosi]

The angular pressure against the bolt release lever moves at 90 degrees to the grooves which allows greater thumb grip traction in dry, wet or muddy conditions, which is the whole purpose of the grooves to begin with.

'Knurling' as a term is generally reserved for round/cylindrical objects as it represents the process of application, not the end result.
Were the grooves individually machined like in the old style releases that had very many very narrow pitch grooves instead of the relatively sprawled out very few of today, it would be done through the process of broaching not knurling.
But, as wide as they are spaced out today, its obvious they are the result on no machine process but forged or MIM as part of the item construction.