It's the decision of that county on what to allow on parcels of land under 10 acres. I dug around and found this info.
I am not claiming it to be the last word on this. Just some ideas;
If it is part of a subdivision or development, there may be restrictions on a "gun range". You should be able to learn if there are any restrictions on it at your county property records office.
At no time can your ammo end up crossing your property line without permission of the owner to shoot on his/her land.
For City jurisdictions; Agriculture Code subsection 251.005 and Local Government Code subsections 43.002 and 229.002. As best I understand; Cities can not regulate parcels equal to or over 10 acres (firing at a distance exceeding 150 ft from residences/occupied bldgs) using handguns, shotguns, bow and arrow, air rifle, or BB gun if they were annexed after Sept 1, 1981. They can not regulate centerfire or rimfire rifles or pistols of any caliber on at least 50 acres more than 300 feet from an occupied bldg or residence. This is assuming the ammo is reasonably believed to be contained within that area (you are using a backstop, etc) Under 10 acres in a city limits, there are restrictions on discharging a weapon except for self defense.
For County jurisdictions (rural land): Local Government code subsection 240.021 A lot is up to the local Sheriff for what happens on less than 10 acres. The Commissioner's Court can prohibit shooting on 10 acres or less for safety reasons. You have to check with your local Sheriff and County Commissioner's Court to find out what they allow on less than 10 acres. I suspect it has to do with whether or not there is complaining from neighbors or an obvious safety concern. I have a friend who is building a private gun range on less than 10 acres in Tarrant County (not in any city limits). He asked the Sheriff's office if there was any problem with that and was told to go ahead as long as it met the safety protocols for a gun range.
Your land should not be located too close to COE land so if it's 2 acres adjacent to a COE lake or waterway, that's a problem.
I hope this helped and didn't just make it more confusing.
I am sure there are far more knowledgeable members who will chime in.