The Access Code says:
The Ministry of Defence (MoD) has a presumption in favour of safe public enjoyment of its estate wherever this is compatible with operational and military training needs, public safety and security. The MoD needs to carefully manage access when active military training is underway, and where there are unexploded munitions.
Always take note of advice from range staff, troops and from warning signs. If in doubt, look for an alternative route or turn back. Red flags (in daytime) and red lamps (at night) indicate live firing areas, which might not be fenced. Do not enter a range if flags are raised or lamps lit. Be careful when crossing the land as there could be trenches or voids, and never pick up objects as they could be harmful. Be prepared for sudden noises that can startle people and horses.