Oh, that's a horse of a different colour. I still think you'll run into issues making it sturdy enough for melee combat, while light enough for accurate (safe) ranged attacks. Many of the spears I've seen at Dag or Bel are bamboo or something equally heavy/flexible and six to seven feet long. You can throw 'em, but you'll get crap distance before it flies off course; and folks can easily just knock it, or grab it, out of the air
You might be have better luck with something like a Roman thrusting spear (¿hasta?). Shorter than a pilum (under 6 foot, close to 5) and designed for dirty, close-combat. At that length, you can work a sturdy core for stabbing, but it would be light enough to reverse and chuck (though, then you're boned for a weapon)
Though, the real issue you'll run into is weight distribution. Most javelins are front-heavy and tapered to course-correct - this would almost be the reverse and that's gonna make it aerodynamically perverse. You can copper-tap the javelin head, but it'll take some reckoning. ¿Any reason you want to make two in one?
So my only concern with making a shorter spear for close range (5 ft or so?), is that it could defeat the purpose of having a spear in the first place - primarily for range. Typically, I fight with a shield, spear, and sword so that I can draw the sword if I throw the spear. But if the spear has roughly the same reach as the sword (3 ft blade), there's less point in having the spear in the first place, you know. Though ya, I also don't imagine this guy being very long (8,9 ft) because I can't imagine throwing something that long would work very well at all. Regarding the throwing distance though, I'm not planning on throwing this guy too far. If I can get an accurate and quick, 10-20 feet, I think that's alright. I plan to use the javelin mostly to give me options in a close quarters fight, rather than sniping someone from a long distance (though hey, if it does that too, that'd be neat).
And yaaaa, weight distribution is what I'm mostly trying to figure out. I'm mostly throwing aerodynamics out the window with the big ol' nerf football head, but I don't think I have much of a choice there because I definitely want the throwing end to be nice and squishy safety-wise. Most of my focus then is on the comparable weights of the two heads. If my understanding is right (definitely correct me if I'm wrong), a javelin wants to be weighted heavier to the front so - as you said - the weight pulls the tip down and course corrects the throw. On the other hand, a spear should be more weighted towards the back with some sort of counterweight so that leverage can assist in faster thrusting and cutting. If all that is correct - a javelin should be front-weighted and a spear should be back-weighted - it could be possible to make a combined weapon where heavier javelin head allows for better flight when thrown while also acting as a counterweight when flipped around for better leverage for the spear when thrusting and cutting. But I have no idea what those sweet spots are which is what I'm trying figure out - how front heavy does a javelin need to be? Does that range of weight also actually fall into the correct weight range for what a spear counterweight should be? Though this also brings up the question of how will I make the nerf football (which is very light) heavier - I'm thinking I might have to add something extra to that end specifically for weight once I know what that correct balance of weight should be.
As for why I'm trying to combine the weapons, well, mostly just to give me options. I used a spear/javelin combo like I'm describing a few years back and it worked lovely. It had the range of a long spear, I could flip it up and throw it very quickly, the possibility of throwing it kept people I was fighting on their toes. If I threw it, I would draw my sword real quickly - so the ability to use it as both a spear and javelin meant I could size up who I was fighting, figure out whether a sword or spear would be better, and if the answer was a sword, I could get a good throw in (maybe hit their legs) before going in with the sword. And since I'm using a large shield, spear, and sword, I'm already holding a bunch of weapons - carrying both a spear and javelin separately in that mix would be quite a lot. And if the purpose is to use it as both a spear and javelin, the double heads work way better than a single head. With a single head, I have to shift it from underhand when using as spear to overhand when using as javelin. It can be a quick transition, but there's definitely room to drop it when flipping it around - especially since I'm flipping it with one hand as my other hand has the shield. With two heads, I just have to rotate my arm to switch between an underhanded and overhanded grip - a way more stable and quicker way to switch. When I originally made the weapon a few years back, I very haphazardly threw it together. Now that I'm getting back into fighting, I want to make a new one and put a decent amount of thought into how I can make it as effective as possible.
Also, what do you mean by copper-tapping the head? I haven't heard of that before
