The TRS rail from Saber Tactical is a "must-have" for long range airgun shooters.

This rail replaces the original accessory rail and gives (besides a slick appearance) a number of interesting features:

20 MOA Rail

Like the original rails of the M3 (and the later MK2), the TRS rails have a built-in angle of 20 MOA (minute of angle).
Due to this slight tilt, the scope has a larger vertical adjustment range.

Integrated Bubble Level

The bubble level at the rear of the rail allows you to check your alignment of your rifle and scope.
Experienced shooters know how essential it is to consistently keep the rifle level for that long-range precision.

Delrin Shroud clamp

The clamp at the end of the barrel ensures that the barrel is fixed in place.

This prevents barrel droop, reinforces the position of the barrel in the housing, and prevents POI shift when changing moderators.

Extended Picatinny Rails

Quite obvious, but not unimportant is the extra amount of picatinny rails.
These extra rails allow optical aids or accessories to be mounted.

An interesting added benefit to this extra length of barrel is the ability to use an "over-bore" style bipod such as the UTG Over Bore or similar options.

M-Lok Slots

On the sides of the rails are M-Lok slots for mounting accessories such as extra rails, lamps, lasers, etc.