Flamethrowers are primarily a tool, and accordingly, are not regulated under federal law as a weapon. As they do not fire a projectile from a fixed cartridge, they are not restricted under BATFE regulations.
The FlamethrowerPlans.com flamethrower will be recognized by some as kitbashed from various industrial components. This, however, does not make it any less effective. You can buy the plans (as the site indicates), components, or a finished unit. I opted for the finished unit.
The package is surprisingly compact considering the contents. There is the fuel tank, projector, hose and pressure hardware. The instructions are very clear and simple. Also included is a modified propane torch as an igniter. All that is required is to charge the CO2 tank, fill with fuel and attach a propane bottle.
Safety is obviously paramount with something that projects burning fuel. The instructions recommend a wet test with water first, to verify function safely. We did this, and measured a range of right at 50 feet, and a firing time of right at 1 minute. This is a shorter than historical military models, but with a longer burn time.
Fuel can be kerosene or diesel, or gasoline, and thickeners can be added to increase range and effectiveness. Diesel/kerosene are recommended both for safety, due to a higher flash point than gasoline, and because they will offer a bit more range. Thickened with left over motor oil and such, the mix approximates the Russian napalm from WWII. I've tested it successfully with motor oil and vegetable oil. It's important to agitate the mix for good blending, and I had best results with about 3:1 fuel to thickener.
3:1 diesel to gasoline gives more range and a hotter flame, without undue risk.
I have not tested a thickened gasoline mix, but it can be made by using grated soap (not detergent. Ivory is best) that is oven dried, then blended and agitated with the fuel.
CO2 is available at many sporting good shops that handle Airsoft or paintball, for about $5 locally. If you have access to nitrogen, it will offer a bit more pressure and range.
Once charged live, we tested it by burning off brush around a drainage pond. There is actual recoil, or more accurately, thrust, from firing this. The operator remains relatively cool, but the flame emits radiant heat to the sides that is palpable at quite some distance.
The pond was about 50 feet by 20, it was surrounded by heavy greenery and weeds. It took less than two minutes (two charges) of both working and playing about to clear a five foot swath all the way around.
I then tested it on an abandoned animal run on my property, about 20' square. It was full of trash trees, weeds and brush. One tank, about 60 seconds, cleared it down to ash and stalks, making it much easier to finish clearing with shears and loppers.
We kept a fire extinguisher handy, and I recommend doing so for safety. It wasn't needed, but if it becomes so, it would be critical. As fun as it is to operate, this is not a toy.
For brush removal around a position to clear a field of fire, it would be much faster than hand tools, assuming the fuel is available. For igniting debris, even when wet and cold, it is hard to beat. CAUTION: Do not inhale fumes from burning debris if it might contain toxic chemicals, or if it contains toxic plant matter such as poison ivy.
For a defensive weapon, it would be hindered by the prep and loading time, and the limited burn time and range. However, for protecting an emplaced position, it would work very well to deny large swaths of ground to any attackers, for firing existing burn piles for cover or concealment, and for direct distraction and concealment behind a huge ball of flame and smoke. It could be used to corral attackers into a chosen area of effect. It would not instantly stop a vehicle, but any unhardened vehicle would be rendered inoperable in seconds. In extremis, it would be instantly incapacitating and lethal within its range, causing traumatic, contaminated burns. In enclosed spaces such as bunkers, sheds or entryways, it consumes most of the oxygen, leaving any occupants with a lungful of smoke and carbon monoxide. I caution that this would have to be during a siege or mass attack when law and order has broken down. Any use as a weapon with existing legal infrastructure would probably constitute pre-meditation, would definitely attract unfavorable government and media attention, and least of all but certainly, an EPA bill for cleanup. But if it was to hand when your life was threatened, it's a trump card of psychological magnitude and terror.
We found occasional ignition issues with pure diesel, the fuel blowing right across the igniter without catching. Releasing the trigger and re-firing usually corrected this at once, and the projected fuel then lit from the second shot. With mixed fuel and oil, there were no ignition problems at all.
There are improvements that could be made. The igniter would benefit from a fabricated attachment rather than hose clamps. While the pressure bottle was secure enough, a clamp assembly would be sturdier than relying on the fittings. However, I've had no trouble with reliability.
Photo links below show the short range and effectiveness of straight diesel.
I rate it excellent for brush removal. It's unexcelled for fun, if you like controlled fire. As a weapon, limited in utility but devastatingly psychologically effective.
There are other models out there, but I have found this to be the most cost effective, range effective, and capacity effective model.
To the best of my research, flamethrowers are unrestricted in 48 states, prohibited in Maryland, and require fire marshall approval to use in CA, which, given the risk of brush fires, isn't entirely unreasonable in this case.
NOTE: I purchased my own model and use it on my property for brush removal. I was not compensated in any way for this review.