An acre is 43,560 sq ft. It can be 1' wide x 43560' long up to 208.71' x 208.71' and it can be configured in any shape... square, trapezoid, pie,L, Zig-Zag etc. take x 20 for 20Ac.
One way to measure the distance would be to use an arc from the closest corner of the neighbor's occupied structure and that would define the safe setback zone.
Now, I interpret the the law as meaning you can't be shooting from within that safe zone but as long as you are outside of that (and I believe your impact area should also be outside it), then I don't think there would be an issue if the bullet travels across part of the safe area arc, especially if traveling away. A strict reading of the law seems to only indicate the spot where the firearm is discharged from - not the path of the bullet nor the POI.
Up until now I've only read in the Fish & Game regulations that indicate the required distance is 1/4 mile (440yd) from an OCCUPIED structure. IIRC I also read in the G&F regs that even if you have written consent from a neighbor, they can rescind it at any time and don't have to tell you. I'm not a lawyer so from a layman's point of view and for discussion sake...
I looked into this law further and I find various problematic contradictions that could be an issue depending on who is interpreting it... here is my opinion on how I read it.
Looking at section 13-3107 (copied below), it seems to be the specific section of law that governs this situation under the title of Unlawful Discharge of Firearms... which is probably what you would be charged with...
Item B. indicates it is a class 6 felony, and will probably be charged as such, however Section 13-604 is specified as typically applying to this offense, which allows for a slightly less serious misdemeanor charge. However it is court discretionary and depending on circumstances/additional charges,prosecutor, it could be charged under 13-704 which is more serious and carries some hefty penalties.
13-3107, Item C (a) indicates the definition of "occupied structure" as... "(a) "Occupied structure" means any building in which, at the time of the firearm's discharge, a reasonable person from the location where a firearm is discharged would expect a person to be present."
This seems to also imply that if the shooter doesn't expect anyone to be there then he could conclude that this restriction is not applicable, based on his 'expectations'. However the arresting officer will likely be considered a "more reasonable" person.
Another consideration is referenced section 13-3101 (Definitions) which defines 'occupied Structure' as...
"6. “Occupied structure” means any building, object, vehicle, watercraft, aircraft or place with sides and a floor that is separately securable from any other structure attached to it, that is used for lodging, business, transportation, recreation or storage and in which one or more human beings either are or are likely to be present or so near as to be in equivalent danger at the time the discharge of a firearm occurs. Occupied structure includes any dwelling house, whether occupied, unoccupied or vacant."
This definition expands the scope and meaning of the term 'Occupied structure' considerably over what 13-3107 seems to say.
Another troubling item is 13-3107 C.8. "More than one mile from any occupied structure as defined in section 13-3101."
This seems to imply that the 1/4 mile rule mentioned in (C) 3 only applies if a city,town or county implemented such and if not then the state standard is 1 mile instead of 1/4 mile (except when hunting because Game & Fish adopted the 1/4 mile rule)???
So if you buy property outside a municipal boundary and the county has no shooting distance ordnance, then do you have to follow a state mandated 1 mile safe zone??
Since vehicles are included in the definitions, and they usually need a road, what if you are shooting within a mile of a road, even in the opposite direction (as in your property may have a dirt access road off a paved road but it is less than 1 mile from the road to the back of your property where you are shooting). ||--------[_________X}------>
Lets say your outside the 1 mile compliance zone and someone is camping in their tent or trailer over the next hill- is that an occupied structure that now requires 1/4 mile separation? Do we measure as the crow flies?
Section D.2(c) seems to allow you to build an underground range...
This is getting more complicated as I read further... this is why it is important to
1. keep liberals out of government and
2 support AZCDL as they get changes made to laws that can bite us....
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13-3107. Unlawful discharge of firearms; exceptions; classification; definitions
A. A person who with criminal negligence discharges a firearm within or into the limits of any municipality is guilty of a class 6 felony.
B. Notwithstanding the fact that the offense involves the discharge of a deadly weapon, unless a dangerous offense is alleged and proven pursuant to section 13-704, subsection L, section 13-604 applies to this offense.
C. This section does not apply if the firearm is discharged:
1. As allowed pursuant to chapter 4 of this title.
2. On a properly supervised range.
3. To lawfully take wildlife during an open season established by the Arizona game and fish commission and subject to the limitations prescribed by title 17 and Arizona game and fish commission rules and orders.
This paragraph does not prevent a city, town or county from adopting an ordinance or rule restricting the discharge of a firearm within one-fourth mile of an occupied structure without the consent of the owner or occupant of the structure.
For the purposes of this paragraph:
(a) "Occupied structure" means any building in which, at the time of the firearm's discharge, a reasonable person from the location where a firearm is discharged would expect a person to be present.
(b) "Take" has the same meaning prescribed in section 17-101.
4. For the control of nuisance wildlife by permit from the Arizona game and fish department or the United States fish and wildlife service.
5. By special permit of the chief of police of the municipality.
6. As required by an animal control officer in the performance of duties as specified in section 9-499.04.
7. Using blanks.
8. More than one mile from any occupied structure as defined in section 13-3101.
9. In self-defense or defense of another person against an animal attack if a reasonable person would believe that deadly physical force against the animal is immediately necessary and reasonable under the circumstances to protect oneself or the other person.
D. For the purposes of this section:
1. "Municipality" means any city or town and includes any property that is fully enclosed within the city or town.
2. "Properly supervised range" means a range that is any of the following:
(a) Operated by a club affiliated with the national rifle association of America, the amateur trapshooting association, the national skeet association or any other nationally recognized shooting organization, or by any public or private school.
(b) Approved by any agency of the federal government, this state or a county or city within which the range is located.
(c) Operated with adult supervision for shooting air or carbon dioxide gas operated guns, or for shooting in underground ranges on private or public property.