The impact of this on walkers should be zero. The Warcop Range is a very
active live firing range which uses the fells behind as a back stop. It is only open to the public on pre-published open access days. I can vouch for how active it is because I used to live within earshot of it - imagine your neighbour slamming his car door repeatedly all day. None of it, including the overshoot area is Open Access. A lot of it is an SSSI and it has a much greater biodiversity then the surrounding areas which are managed for grouse.
The range is subdivided into areas, some more accessible than others, but walkers are only allowed onto the fells on the open access days and are restricted to the public footpaths across the range. There is a very real chance of encountering unexploded ordnance if you stray from the paths and even on the paths you may encounter the odd "don't touch" item. The Peninnes AONB also arrange escorted walks onto parts of the range not normally open to the public - very popular with the hill baggers who want to claim Little Fell.
Some parts of the overshoot area such as Mickle Fell are not open to the public and have no public rights of way on them. However you can walk there with prior permission which is managed by the Range Office. They are always very helpful and will do the necessary to get clearance from the surrounding estates that manage that part of the moor for the MoD.
I have been up there with the right bit of paperwork and have met walkers who could not be bothered to make the call or send the e-mail. Some have even walked in across the danger area and have expressed astonishing arrogance and/or indifference when I have pointed out that by doing so they have contravened the access laws. They did not seem to appreciate by doing so, they risked the MoD putting a blanket ban on all access to the fells, It just needs one accident to spoil it for everybody.
If you get the chance to walk the range it is well worth going. The very fact it is a range means it is incredibly wild and unspoiled. Try this to whet your appetite
http://www.haroldstreet.org.uk/routes/download/?walk=2586