stealthsmiler - I both agree & disagree with what you say. On one hand, no, I don't really want full airport style screening to get into a theme park - if we go down this route, then the terror organisations have won. I also fully agree that most people take way too much crap with them on a day out - but even the basics (say keys / cash & a mobile phone) will all activate a metal detector. This is before you throw belts & watches in for good measure.
On the flip side, look at the huge impact The Smiler incident had on Alton Towers. Four serious injuries including two partial amputations of limbs led to a huge drop in attendance - long before Alton Towers admitted human error was the cause. The general public perceived that the rides / place was unsafe (which we know is untrue, but it's what Joe Public thinks that counts).
Now I am unfamiliar with weapons such as firearms & have no idea how many rounds you'd get in a magazine on a small firearm - but guess that 6-10 is not far off the mark. If someone was to get such a weapon into a theme park (any theme park - this is not Alton specific) and discharge the said rounds, then you are looking at a number of potential fatalities + injuries caused by the panic that would ensue in the aftermath. Given what four serious injuries did to Alton Towers attendance, think what this would do to a parks attendance?
I ski in the USA most years and you invariably get talking to people on ski lifts. Even now, some 14 years down the line from 9/11, you'd be amazed how many Americans you talk to who have driven for over a day (or two) to get to Colorado as they won't fly after the events of 9/11. People are fickle - if they perceive something as unsafe / a danger area, they stay away.
I cannot stress enough that I hope there are no incidents of any kind at any theme park, or indeed anywhere in world. That said, I think most would agree that we live in a very different world to what we did 20 years ago. There are lots of people (or groups) out there who do not like various other countries & seek to let them know this. Most recently, you cannot ignore the events in California or Paris to name but two.