According to Lee's story, Sarge/Blaze was gifted with unnatural long life due to exposure to a secret Nazi reanimation serum used in some of Dr. Schabbs' experiments on the dead. As a result he lived all the way up until the Doom era, and was taken (as per the Q3 intro video) by the Vadrigar. The period of time between now (modern) and the Doom Wars has not been written, so weapons such as napalm launchers very feasibly could be used irrespective of any current international treaties. Speaking of which, let's look at a few facts:
Many of Blaze's WWII era weapons are of German origin, despite the fact that he's an American.
The USA used shotguns as trench weapons in WWI despite German objection that they considered shotguns to be "Weapons causing unnecessary suffering" according to Article 23 of the Hague Convention, and shotguns are still used by modern militaries and civilian police forces.
Link to info on the Hague Convention.
Thermobaric weapons, or "fuel vapor" bombs are recently devised incendiary weapons that are currently used in combat.
Napalm was used extensively in Vietnam.
Flamethrowers were also used in Vietnam, quite often they were mounted on tanks.
Definition of flamethrower. I like how civilians are allowed to own them. 
Info link on How Flamethrowers Work
On this page you can see a photo of a flamethrower being used in Vietnam.
A quote from this source in regards to Napalm:
The use of napalm and other incendiaries against civilian populations was banned by a United Nations convention in 1980 [1]. The United States didn't sign the agreement, but claimed to have destroyed its arsenal in 2001.
As you can see the US did not sign this particular agreement, it is referring to use on
CIVILIANS as opposed to use against combatants. Napalm was indeed used in combat by the US as recently as 2003 in Iraq, despite the US claim that its napalm arsenal was destroyed in 2001.
Full texts of all four
Geneva Conventions. (Link to first Geneva Convention is currently broken)
To my knowledge most weapons considered "banned under the Geneva Convention" or "banned by international treaties" actually refers to Article 23 of the Hague Convention as "Weapons causing unnecessary suffering". None of the texts of any of the Geneva Conventions nor the Hague Convention specifically prohibit the use of flamethrowers and/or incendiary agents in combat. What exactly qualifies as a "weapon causing unnecessary suffering" is often disputed. Since the Hague Convention predates the combat use of backpack and tank-mounted flamethrowers, and since flamethrowers have been used extensively in WWII, Korea, and Vietnam, Article 23 does not appear to apply to the tactical application flamethrowers and other incendiaries.
As for hollow point bullets, most rifle ammunition used by militaries are "full metal jacket" rounds for three reasons. First, they cycle reliably in the weapon. Second, they're cheap to mass produce. The third reason involves the balistics of center-fire rifle bullets. Rifle bullets need to be aerodynamically as perfect as possible for long-range accuracy. Also, the shape, velocity, and energy transfer of rifle-caliber bullets defeats all personal body armor except a hardened 3/8" steel trauma plate supported by class III kevlar. Such heavy armor is cumbersome and impractical for most combat duty troops, and the protection area of the trauma plate is only about 12" x 6" over the heart and lungs. Rifle bullets impart hydro-static shock on any soft tissue impacts due to their high velocity. This is capable of stopping the heart and causing other internal organ injuries. This, not bleeding, tends to be the killer, assuming the round did not pass directly through the heart, lungs, or brain. Hollow point bullets typically are used in pistol-caliber weapons where the velocities involved are incapable of imparting hydrostatic shock.
Hollow point bullets maximize the size of both the temporary and permanent wound cavities to maximize energy delivery and tissue trauma in order to facilitate a quicker stoppage. Police and SWAT teams routinely employ hollow point rounds for their stopping ability since using rifle-caliber rounds in civilian settings can result in unintended casualties from either a missed shot penetrating a building or overpenetration of a round through the intended target striking someone behind them. Pistol-caliber rounds, shot (from shotshells), and 12 gauge slugs can be stopped effectively with class II kevlar as they lack the velocity and profile needed to penetrate body armor.
To clear up any further possibility of confusion, there ARE hollow-point rifle cartridges, and they do not work the same way as hollow-point pistol cartridges. Hollow point pistol rounds are usually copper-clad lead bullets with a large cavity. They are meant to deform and expand when soft tissue is impacted. Some hunting rifle bullets also work like this, but these are typically used in muzzle-loading rifles. Hollow point military caliber rounds (like 7.62x39 HP) are usually marked as "hunting" cartridges but are typically sold on the civilian market for owners of military-pattern firearms. These are almost always copper-clad, steel-jacketed lead core bullets. Unlike pistol hollow points, these are "hard" bullets that do not deform, and unstead of having a large hollow point they have a small section of the nose cut off, with a hollow cavity inside the nose of the steel jacket that makes them tumble more during a soft tissue impact than a FMJ round would. This design maximizes wound potential while keeping roughly the same accuracy as a FMJ round. These also will go through kevlar body armor the same as a FMJ round.
For military purposes, hollow point pistol-caliber rounds are more expensive to manufacture than ball rounds, and pistol-caliber rounds are typically used only in side arms. Most combat forces wear some kind of kevlar and carry automatic rifles, making pistol rounds a bit of a joke, so the need for hollow point pistol rounds is non-existant in the military. Hollow-point rifle cartridges may or may not be banned by some kind of treaty for military use, but any tactical advantage gained by the use of such rounds is minimal when compared to the sheer volume of fire in a military firefight anyway. (Such treaties are usually only signed on a don't care/won't use it anyway basis) Civilians using semi-auto rifles for hunting or defense may see some limited benefit from HP rifle rounds. Rifles, machineguns, artillery, and explosives tend to be the primary ground weapons of a modern military force. Anyone who thinks a hollow point pistol or rifle round is "cruel" should see the effects of a hand grenade or mortar shell, both of which are not banned by any treaty.
Since flamethrowers have been used as recently as Vietnam by US forces, and Napalm as recently as 2003, I would say the possibility of Blaze's Napalm Launcher being designed for some future military application is very much viable.[/color]
