Concealing large weapons has always been difficult and slightly confusing.
Strictly speaking, if you have an item on your person, all it takes is a single hit on a Perception + Intuition Test to notice it. If you're "actively" attempting to hide an item from notice, then it becomes an Opposed roll of Perception + Intuition against your Palming + Agility as the defender.
A weapon's Concealability directly modifies the perceiver's dice pool. A claymore or katana has a Concealability of +6, which means when facing someone with an Inuition of 3 and
0 Perception (and thus defaulting), they get 3 + 0 + 6 - 1 = 8 dice to detect your big ol' sword. If you want to have reasonably good odds of avoiding detection
reliably, you probably need to have about 3 more dice than the enemy, but preferably as many as you can manage to improve your chances, so you'd want 12 dice minimum, and preferably more like 15.
That's for someone with
0 Perception and average Intuition. If you're against someone with 3 or 4 ranks of Perception, you need 4 or 5 more dice yourself, meaning to hide a katana from a professional is going to take anywhere from 15 to 21 dice in Palming. For a sense of scale, in 4E, you couldn't really get past 14 dice (soft-capped 6 Agility + maximum 6 Skill + 2 Specialization) without Augmentations, Adept powers, or taking the Aptitude positive quality.
Of course, 4E had a few tricks to help even the playing field. Among other things, Arsenal's weapon mods gave access to Chameleon Coating, which reduced Concealability by 4 - essentially a must have for a concealable katana. You could also wear certain garments, like the corebooks' Lined Coat or Arsenal's various "Fashion" Greatcoats, which offered an additional reduction of 2 point for items concealed beneath it. This meant the opposition doesn't get all those free dice, and it's easier to keep your items unseen.
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As for your suggestion of Physical Mask, I see no reason why it shouldn't work perfectly well on any object. You can also use other spells like Invisibility and Camouflage, and even other forms of Magic like a Spirit's Concealment Power.
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Personally, though, I've always ruled that anything which is fully out of line of sight counts as instantly "Concealed" unless it is somehow visibly deforming the container it is inside. If you carry a Tommy Gun in a guitar case, not even the most Perceptive person in the world can actually see that weapon - they'll see the case, and they might suspect something is up if the case seems out of place (like carrying it into a Corporate office building, for example), but they won't actually be able to detect anything inside it. In contrast, if you stick a pistol in your pocket or down the back of your pants, it creates a gun-shaped bulge which is pretty noticeable and which many people would instantly recognize.
Consequently, if your GM agrees with that line of thinking and you want to be able to carry a sword around without having to make Palming tests all the time, I recommend having some sort of non-descript container to carry it in that won't draw much attention. You can find various kinds of "luggage tubes" or "travel tubes" (I'm not familiar with a commonly accepted term for this sort of container) for a variety of different hobbies that involve long, thin objects - like
fishing rods,
camera tripods,
art supplies and more.
There is even such a thing as a "sword bag", coming in all sorts of varieties. Kendo style bags can be quite
modern, or
more traditional. Or maybe you've got more of a
European style going on? They make bags for
claymores. They make bags for
fencers' rapiers. They make bags in
strange shapes, to accomodate curved blades.
So if you want to carry around a claymore? Get a bag, or a tube, and sling it over your shoulder. Disguise it a little, maybe, to make it stand out less. Stick a fishing pole in the bag with the sword and leave it poking out of the top. Or get a bag with a Kodak logo on it, and wear a camera around your neck. Or maybe fleck the bag in colorful paint and leave a bunch of paint brushes visible in the small outer pockets. Or just have
a plain, simple bag that could be holding just about anything. Or go with
a sleek, professional tool case of some sort.
These are all useful options - in fact, you might want several or even all of them and more, because each one is going to work best in a certain sort of situation. The point is less about making the sword invisible to detection, but rather making it blend in. For day-to-day carry, you probably just want a plain bag. For situational smuggling, you'll want a specific container that suits the situation at hand. If you're wandering around in a touristy location, the camera bag cover is going to hold up well, but it won't work terribly well in a neighborhood of warehouses and factories. The paint supplies cover will work great in areas of natural beauty and around places of creativity like Universities or art studios, but it isn't going to make much sense down at the local gun range or at your local auto mechanic's shop - you'd be better off with the tool case, then.
Also remember that not every situation is going to be "winnable". Sometimes, you just
have to leave the claymore home, because smuggling it somewhere is going to be damn near impossible. Times like those, you have to be flexible - maybe make do with a smaller sword or a knife, or have a disposeable weapon smuggled in for you by someone on the inside. In 4E, I was always a fan of the Victorinox Memory Blade as a reasonably powerful and immensly Concealable blade - but remember that sometimes it's less important how visually concealable something is, and more important how easily it can be detected via MAD Detectors or Millimeter Wave Scanners.
~Umi