Primary Arms SLx 3x MicroPrism

Primary Arms calls the diopter ring at the rear of the eyepiece the critical first step, and it comes before mounting, before illumination, and before any live fire. The MicroPrism uses an etched reticle rather than a projected dot, which is why it can be brought to a sharp focus for eyes that see a red dot as a starburst, severe astigmatism included.

The technique is the part that gets botched. Point the optic at a featureless light background, a clear blue sky or a blank wall, and check the reticle in quick glances rather than a steady stare. Your eye accommodates to a blurry reticle within a second or two of staring at it, which convinces you the focus is fine when it is not. The reticle has to look sharp at first glance.

This is a one-time adjustment tied to your eye, not to the rifle. The correct setting differs from person to person, so a MicroPrism handed to another shooter needs it redone.

The SLx 3x MicroPrism ships with a removable 1913 MIL-STD base and a set of spacers that produce eight different centerline heights. As assembled in the box the base puts the optical centerline 1.41 inches above the receiver rail. With no spacer the mount alone gives 1.1 inches; the cantilever low spacer gives 1.41 inches, the cantilever medium and the straight spacer both give 1.535 inches, and the cantilever high gives 1.64 inches. Stacking the straight spacer under a cantilever spacer yields 1.845, 1.97, or 2.075 inches. The tallest configuration has an extended overhang so the optic clears most rear iron sights on a standard AR.

Two different torque values apply and they are nowhere near each other. The 10-32 low-profile socket head screws that hold the base and spacers to the optic take 25 to 30 inch-pounds through a 3/32 hex. The T25 Torx crossbolt on the removable base, the fastener that actually clamps to the rail, takes 55 to 60 inch-pounds with blue thread locker on the threads.

With the base removed, the underside of the optic body is machined to the industry standard mini prism footprint, so third-party mini prism mounts bolt straight on.

The optic leaves the factory mechanically centered, so a squarely mounted base normally needs only small corrections. Every click moves the reticle 0.25 MOA and total travel exceeds 80 MOA in both windage and elevation, which is a quarter inch per click at 100 yards and an eighth of an inch at 50.

The turret directions are worth reading twice because elevation runs opposite to the habit most shooters build on a riflescope. Clockwise on the elevation adjustment screw moves point of impact UP and counterclockwise moves it down. Clockwise on the windage screw moves point of impact right and counterclockwise moves it left.

Work in a few clicks at a time rather than dialing a full correction in one pass.

The illumination knob carries thirteen brightness settings. NV1, NV2, and NV3 are night vision compatible and cannot be seen by the naked eye at all, so a knob sitting on one of those and appearing dead is working as designed. The highest settings are Daylight Bright and hold up against open sun. An indicator triangle on the knob shows which setting is live.

The knob rotates infinitely in either direction, so you always have the shortest path to the setting you want regardless of where the dial currently sits. Returning the dial to 0 switches illumination off.

AutoLive motion-sensing turns the illumination off when the optic sits still and brings it back when the rifle moves, which is what makes leaving it switched on in a safe viable. Primary Arms rates runtime at 29,000 hours on medium illumination from the single CR2032. Because the reticle is etched into the glass, a dead battery costs you the illumination and nothing else.

The 3x MicroPrism runs a single 3V CR2032 cell behind a screw-down cap on the left side of the optic, integrated with the illumination knob. No tool is required: the cap unscrews counterclockwise with a quarter or any similar coin.

Polarity is the step to get right. The positive side of the battery faces OUT toward the cap, not down into the housing. Primary Arms also advises against rechargeable cells in this optic, which can damage the electronics.