Okay, so we've danced all around this topic for literally months now. We've covered various aspects of firearms, from myths to components to how to select the right one, and it's about time we walk through how to make your own submission.
Let me start off with a shameless plug: though there are several Chaos businesses that produce weapons, Rusty's Custom Firearms and Cutlery is mine, and near as I can tell, it's the only active one that takes custom orders for just about anything. If you can dream it, we can build it.
However, if you want to make your own, here's how I would recommend you do it.
Firstly, determine what role you need the weapon to fulfill. If you're a smuggler, you might choose something small and compact that you can comfortably carry, but still packs enough firepower to get the job done. For this role, blaster pistols are ideal. If you're a soldier, you want something with a little more ass behind it, usually a rifle or a carbine. Remember that, if written correctly, the weapon's weight, size, and recoil will impact how it performs in a fight, so choose wisely.
If you're going to primarily be fighting Force Users, slugthrowers are preferable to blasters. If you're going to be fighting NFUs, however, a blaster packs a far greater punch, though it won't have anything close to the same range. Long range snipers will almost always want to go with a slugthrower, and if stealth is an option, you don't necessarily want traditional firearms. Rail guns, coil guns, and other electromagnetically propulsion systems are generally stealthier than chemical combustion, but you have to manage your power as well as your ammo consumption. Remember that there's nothing wrong with choosing a canon weapon if you find one that suits your needs. Unmodified canon weapons require no submission, and can save you a whole lot of work.
Once you've figured out what you want your weapon to do, find a good picture for it. Google Image Search is your friend here. If there's a particular weapon you want to base yours off of (say an M4 or Tavor), that's a good place to start. Any common production weapon will have a ton of pictures to choose from, and if you want to spice things up a bit, you can always use modifiers in the search. I like to search for "badass" or "tactical" versions. It sounds cheesy, but you find a lot of really neat stuff. If you want something a little further from reality, try searching for sci fi or futuristic [insert weapon type here]. There's a lot of really beautiful artwork out there, just remember to credit the artist whenever possible. Failing that, link to where you found the image.
A good picture is important, because you're not only making the submission look cool, you're giving everyone a general idea of what your weapon looks like. When you're interacting with people in a story, it makes it a lot easier for them to write their responses if they have a visual to go off of. I'm not exaggerating when I say that a good picture can make or break a submission's effectiveness in a fight. A picture is worth a thousand words, and a gnarly looking one impresses folks a lot more than facts and figures.
Of course, facts and figures are important as well. The submission will ask for things like the weapon's range, magazine size, and so on. For most sections, you don't have to go into excruciating detail. The weapon's materials, for instance, will usually be durasteel, plastoid, and firearm or blaster components. You don't have to list every little screw and spring in the thing, just give a general idea what it's made from. If you want to use a restricted material such as beskar or phrik, make sure you include the link to the thread where the material was acquired under the dev thread portion, and make sure you've dotted all your i's and crossed all your t's when you use a thread.
Also, though it helps to list a maximum effective range, it's not necessary. As it says in the template, you can always say "Personal" or "Battlefield", or "Equivalent to [insert weapon here]". If you want to be specific, remember that you're dealing with the maximum effective range, not the maximum distance a projectile or bolt will travel when fired from the weapon. Pistols, be they blaster or slugthrower, are rarely effective beyond 50 meters, and that's pushing it. Most will fire much further, but hitting the target at that range is more a matter of luck than skill. Blaster rifles will rarely shoot further than 200 meters, while slugthrower assault rifles are usually good for 600 or so. Your average sniper rifle will be good for between 800-1500 meters, with certain monstrous anti-materiel (meaning designed primarily to take out things rather than people) going out to 2,500 or so.
You will need to be specific when it comes to weight, length, and magazine size. If you don't know, look up an IRL or canon weapon similar to what you're trying to make and go from there. Typically, pistols will be under 30cm, and usually between 13-30cm. Carbines will be between 50-70 cm, and a full length rifle will start around 90cm and work up from there, depending on the type. Pistols will weigh between .5kg to 2kg, carbines between 1.5 to 3kg, and rifles from 2.5 on up, and the more powerful the round used, the heavier the weapon will be.
Magazine size is a little tricky. Note that I say magazine rather than clip. Generally, a clip is a device used to hold rounds together in such a way that they can be loaded into a magazine. A magazine holds shells under spring pressure until they can be loaded into the firing chamber. The two terms are not interchangeable, and should not be used as such.
Moving on.
Determining how many rounds your weapon can fire depends on a couple of factors. First and foremost, the type of weapon must be taken into consideration. Blasters don't rely on magazines, barring a few exceptions. They use separate gas cartridges and power cells, both of which must be monitored and reloaded as necessary. How many shots you get out of each varies, but a good rule of thumb is your gas cartridge should have five to six times the capacity of your power cell. Pistol power cells will rarely contain more than 50 shots, and rifles may go up to 100 or even higher, depending on the circumstances.
Slugthrower pistols will rarely contain more than 20 rounds in a magazine. This is largely because they're designed to fit flush with the end of the grip, for ease of carry and concealment. Extended magazines are available for pretty much every pistol out there, but they're rarely used. Revolvers will have between 5-7 rounds, with some such as the LeMat or .22 caliber revolvers having more. Assault rifles and carbines will typically store between 20-30 rounds in a magazine, and likewise for submachine guns. Shotguns typically carry 4-8, though models that use detachable box magazines rather than tubular magazines can hold more. Sniper rifles will usually hold 5-10 rounds, depending on the size of the round and the rifle.
Some people consider the Special Features section to be fluff, and with the recent addition of the strengths and weaknesses section, it's not as important as it used to be, but one still must fill it out. A good rule of thumb is to give a 1-3 sentence highlight reel of what the weapon is designed to do and any sort of tricks it might have up its sleeve.
The Strengths and Weaknesses portion of the submission is far and away the most important to get right. Get it right and your submission will pass easily. Screw it up and you may end up having to nerf your weapon, or end up doing dev. There's nothing wrong with doing a little development, but it is extra work, and dev threads can be dry as hell to write. Balance is the name of the game here. Generally speaking, for every strength, you want a corresponding weakness, and the greater the strengths, the greater the weaknesses. Remember that weaknesses need to be real, tangible things that will affect how the weapon handles in combat. The judges are quite good at picking out the real ones from fluff, and you'd do well not to waste their time by trying to fudge things. Play it straight. Easy weaknesses to add are weight and recoil; a powerful weapon will have both in spades. Reload time also works. If a weapon might be difficult to use well or has some flaw that could lead to catastrophic failure, throw that in as well.
The description section basically covers how the weapon functions. You don't need to get super detailed here unless you really, really want to, but you do need to provide a general overview of how the weapon works and how it might be used. This is also a good place to include some backstory where applicable. Backstory really only adds flavor to the submission, but if it's entertaining enough, it might impress the judge and that never hurts. If your weapon has a neat trick, describe how it works. If you've listed flaws in the weaknesses section, cover them in greater depth. A good description can make or break a submission.
Well, now you know how I make submissions. There's no one right way to do it, and while there are plenty of wrong ways, so long as you approach your submission with an open mind and are ready and willing to work with the Factory Judge assigned to it, nearly any idea can be approved. Just remember to treat them with respect, and if you disagree with something, try to voice your disagreement in a constructive manner and back it up with facts, figures, and logic. They're here to help you, after all, and being a dick does not make them more inclined to do so.
Anyway, if you have any questions or comments, by inbox and the comments section are open.