The null-space grip, stock, and gunner harness from Armory all have this effect:
As a move action, you can activate the grip to pull the weapon into an extradimensional space, leaving only a disk 1-1/2 inches in diameter. This disk is easy to hide on your person, granting you a +4 circumstance bonus to your Sleight of Hand check to do so. Additionally, the disk might not be recognized as a weapon without Engineering or Mysticism and detect magic used together to identify the item. As a move action, you can squeeze the disk to return the weapon to normal form.
A null-space gunner harness also is reduced down into the disk along with its attached weapon:
It functions like a null-space grip (see left) that turns the harness and its attached weapon into the disk. A null-space gunner harness otherwise functions as a heavy gunner harness.
Question 1: What is the bulk of such a disk? The weapon was pulled into an extradimensional space, which tends to reduce or eliminate bulk. But nothing in the item description actually says it reduces the bulk. Is this an oversight? Or does sending a null-space gunner harness with an attached reaction cannon into the extradimensional space cannon really leave you with a 1.5" disk with 5 bulk?
Question 2: Can weapon fusions be activated on a weapon that has been reduced down to a disk? For instance, can you use the called weapon fusion to teleport the disk to your hand? If you throw a returning weapon with a null-space grip, and an enemy then activates the grip to turn the weapon into a disk, does the disk fly back to your hand on your next turn?
Question 3: When turning a null-space gunner harness from a disk back to a harness+weapon, does it appear already donned on you, or does it appear in your hands, requiring that you spend a full action on your next turn to put it on? Or do you get a choice if it appears on you or in your hands?
Question 1: The rules are unclear. I would reduce the bulk to negligible while in disk form.
Question 2: I see no reason why not. Also, I've alwasy imagined the calling fusion to be more of a teleportation effect than a "fly back" effect.
Question 3: Again the rules are silent. For convenience sake, I'd say it appears donned in the proper fashion.
Q1: I agree it should be negligible since none is given.
Q2: I don't think the fusion works, the disk is not itself the weapon, and is part of the accessory. You don't put the fusion on the accessories.
Q3: I agree, the point of it is to have it already on you when you expand your harness to full size.
I agree with Xenocrat.
The main thing being that fusions probably don't work while in disk form.
question 1: What is the bulk of such a disk?
Negligable
Question 2: Can weapon fusions be activated on a weapon that has been reduced down to a disk? For instance, can you use the called weapon fusion to teleport the disk to your hand?
Should work.
If you throw a returning weapon with a null-space grip, and an enemy then activates the grip to turn the weapon into a disk, does the disk fly back to your hand on your next turn?
I have no idea WHY you'd throw a harmless poker chip at the enemy, but the returning weapon makes it sound like the thing is moving through the air as soon as it lands. As there's no "foes can make a reflex save or attack roll to snag it out of the air" clause I'd imagine its immune to being picked up.
Question 3: When turning a null-space gunner harness from a disk back to a harness+weapon, does it appear already donned on you, or does it appear in your hands, requiring that you spend a full action on your next turn to put it on? Or do you get a choice if it appears on you or in your hands?
Given the price I would let it pop onto you however you popped it into the null space. Otherwise why not just shove it into a much cheaper starfinder backpack?
Question 1: Yes
Question 2: No. The fusion is on the weapon. If an enemy uses the grip to activate the null space. The disk stays put.
Question 3: I would speak with your GM about that. A harness is something you have to don. It kinda makes sense. Kinda doesn't. Simply due to the fact that unless you are in the EXACT position you were in when you activated the null space disk, how would it know where the harness goes? I mean.. if your left arm is over ur head, would it know to loop it around that arm etc... Any change in the exact position you were in.. arms.. legs.. head looking left or right.. would alter how the harness is redeployed. At least, that's how I think of it. Now.. that being said, talk to your GM. For the price I would have to agree with the previous posters.. it's an EXPENSIVE item but it's also one of the ONLY items that lower the full attack penalty on weapons.