How a Gore cannon looks depends on whether they are sentient or just newly created. For those Necanthropes who are freshly out of the White, the Gore Cannon tends to look like a ball of ectoplasm surrounded by an exo skeleton that mimics the appearance of the Necanthrope itself. If the Nec looks like a demon with black scaly skin, then the Gore cannons exo skeleton will look similar. If the Nec looks like a diseased corpse with translucent skin, then the gore cannon reflects this.
However as the Gore cannon reaches and surpasses its sentient stage it starts to reflect more of its on personality. A Gore Cannon that likes causing physical pain will start sprouting small blades or hooks. A Gore cannon that likes being sneaky will have a body that appears to fade into shadows. The one thing that remains constant is the bulbous ball of ectoplasm, although this can represent a different body part for the cannon (in some it is the central bodymass and head, in others it appears like a swollen abdomen of a spider or ant.)
The Gore Cannon doesn't have a PEN amount. If it had PEN 0 then I'd recommend applying the combat rules as is and take the 'natural' PV into account. But as it doesn't I have it passing through any armour and affecting the target directly. If it hit a hardened Vevaphon I'd still have it ignoring the armour. And doing damage directly to the Vevaphon.
My reasoning is that Gore Cannon is not a 'Physical' attack. It is an ability that does not get influenced through knowledge of Physics or man-made barriers between the Gore Cannon and the target. It is a direct manifestation of 'Something Else' (tm) and as such doesn't care what it gets through to get to you. The only exception to this is DeathSuits or other Science Friction devices, which are designed specifically to resist Ebb attacks.
Gore Cannon damage is internal damage; weakening of internal organs, restricting blood flow, disrupting electrical impulses. Skin isn't affected, with any wounds being internal bleeding that requires more than just Kick Start to recover.
The design of a sentient Gore Cannon is reasonably limited besides the wide choice of abilities availavble for each cannon. The reason for this is that each ability costs flux and this has to be spent before the Cannon is created. Depending on how you reflect Experience expenditure in the game, will reflect how you handle the creation of the Gore Cannon.
I use the approach that once the player has gained the requesit amount of XP to take Gore Cannon to its appropriate level, it is not simply a case of bingo out pops a sentient cannon. Instead the player must careful design the abilities that they want and draw on flux in a ritual of sentience. This event should be a significant event in the players eyes as they are making something living through the Ebb (only otherwise available through a Sentient DeathSuit and EbB Medical Resurrection.) Not wanting to fail this the player will want everything just right prior to making his cannon sentient. At this stage any Flux that the Necanthrope can gain his hands on can be used. Be it through flux in his DeathSuit, Burn Gems or an Ebb Medi-Kit.
Once everything is prepared (and the planned Ebb cost of the Sentient Gore Cannon known) the player rolls his formulae skill at -1 per every ten Flux that is being expended on the Cannon. If this is succesful then the Gore Cannon comes out as desired. If it fails then the GM gets to subtract abilities until the Cannon is at a cost that would have passed the test (ie: if the final roll was a 10, then the Gore Cannon loses ten flux points of abilities to reflect the -1 penalty that caused it to fail). If the cost of the Cannon is reduced to zero flux points or a double one is rolled, then the Necanthrope has failed, losing all of the stored flux and reducing his formulae by one point to reflect the fact that although he thought he knew the Ebb well, he actually hasn't understood part of it. The Necanthrope is then allowed to try again for an amount of days equal to the number failed by. (in this example after 24 hours).
At each additional level of Gore Cannon, the Necanthrope can upgrade his Gore Cannon by an amount of flux equal to his new rank in the ability.
Gore Cannons tend to be more sentient and self-reliant than DeathSuits, allowing them to intimidate their victims and stalk them without direct control from the Necanthrope. They will also act in the Necanthropes best interests. Problems can arise when a Necanthrope is incapacitated or killed as the Gore Cannon can survive without him, however, there are a few limitations.
Firstly the Gore Cannon requires a certain amount of Flux and Blood each 24 hours or it dies. This it would have to gain through using its own abilities or being provided them by someone else. Additionally it would continue to act out whatever the last thought it had from the Necanthrope was. For example if the Necanthrope was thinking about revenge on those who had just incapacitated them, then the Gore Cannon will hunt down the individuals responsible. If instead the Necanthrope was thinking 'Aargh, shot, help' the Gore Cannon would probably remain and protect its owner.
If a Gore Cannon is Widowed then it is possible for it to connect to another being. However I think this should be limited to either one of the Union (only likely if they have lost their own cannon) or possibly to a Brain Waster or Ebon who is the Necanthropes Vassal or family relation. In the last two cases it is likely that the Ebon or Brain Waster will soon find themselves entering the White and joining the Union.