Part of the issue for Police forces is a finite amount of officers at any one time that they can deploy. How they deploy them is based on the expected issues at that time/day of the week and any planned events that may be taking place.
Therefore if no one reports crimes/ incidents then they do not show up on the radar, so officers are not deployed to that area. Therefor if something then happens, you have longer to wait until a suitable unit is available.
In the days of health and safety, it is irresponsible to deploy an unarmed officer to try and stop an armed offender.. (Having gone up against an person armed with a pistol, with just a baton and cuffs - it does loosen the bowels slightly

) As to asking how you know it is a rifle, there have been countless calls where guns have been seen or heard even by people that are used to them (shooters/soldiers/police officers) that turn out to be nothing of the sort.... fireworks, cars backfiring, table legs in a bag. some of these calls have ended up in people being shot, so asking questions to gain intelligence/information/evidence is not a bad thing.
The more you report, the more likely that there may be someone available and reasonably nearby.