hers some more info on wire vs etched reticles and ffp vs sfp
"There are wire reticles, and there are wire reticles, most often mounted in the rear focal plane because they can be small enough to prevent blocking out the target with their subtense. Some "wire" reticles are etched chrome foil, with thickness only around .001-inch, others are constantan wire up to .003-.005-inch diameter, some are flattened to give duplex(wide outer wires and fine intersection.) In most cases, wire forms are located in the second focus, where they are hard-mounted to the body tube, subject to shocks suffered by the outside envelope.
Glass substrata contain (floating, seemingly unsupported) sight patterns of very small vacuum-deposited chrome lines or diamond-scribed and pigment-filled (for illuminated, low-light sighting) details, usually in the front focus. Front-focus glass is much more shock resistant than rear-focus wire, because the glass sits in the movable erector tube that cushions and softens recoil or field handling shocks with the bias spring installed to eliminate thread backlash and looseness in the E&W adjust screws. Also, the wire reticle is unsupported over a length up to .5-inch across the reticle frame (field stop in optical terms,) where the glass reticle with its substrate and cover plate is supported in the smaller diameter front focus by a sandwich that is nearly .25-inch thick."
