I built a house with those exact same braces using 8' 4x4s. You can go longer but heed a couple things:
1) Pay attention to the directions they come with. Make sure you use 2x6, not 2x4s for the sides of your floor. You'll need them to put lag bolts into the sides.
2) DEFINITELY use cross braces on your legs. Run them from the very top of one leg to the very bottom of the other leg. This is where the support of your legs come into play.
3) The taller the house, the more careful you'll need to be when pulling it up to keep it from falling on over. Take your time and go SLOW. There's plenty of videos on youtube of folks pulling them on over on accident. The best way to built is modular. Build your floor first with legs and then pull it up. After it's up, add your walls and ceiling.
4) Use some type of anchor system. I drilled holes in my legs large enough to run rebar through them at opposite angles of the legs. I then bent the rebar at the top to act as an anchor.
These are great suggestions and I’d add a coup,e of more.
Run a horizontal braces around the base of the stand along with diagonals. The 4X4s will twist over time and the braces at the bottom will help keep things in place.
Use cables for your bracing but use some wood braces as well.
Pad the inside of your shooting house with carpet samples because it will sound like a drum when you move around.
Add material for a shooting rest at the inside of the openings because a 2x4 wall doesn’t give you a lot of support. We use some sand bags for rests in the towers we have along a power line for long shots.
Lastly,
Don’t ask your framing carpenters to build 3 modular towers on a construction site. Because you will get everyone’s best ideas on what makes a great tower stand.... show up with a trailer and you get a 16 high 6x6x6 tower with treated wood and 3/4 in treat plywood with OSHA approved stairs etc... they are damn heavy and took about 8 people, one flat bed truck and loader to assemble. They will never move again....
If you are in areas where it gets cold, think about ventilation because it sucks big time to be sitting under a metal roof when the sun comes up and it rains all morning inside the tower from the frozen condensation that melts for two hours....
