Ok, This is going to depend on how much "fact" you want in your "fantasy".
First, some base line terms and principles of guns.
When you look at a gun, you are seeing 3 basic parts, (and this is VERY basic terms); The Lock, The Stock, and The Barrel.
The "Lock" refers to the the actual firing chamber and the associated mechanics of firing and loading the chamber. (this includes the trigger, the firing pin, ejection, and reloading of rounds, etc).
The "Stock" refers to the ergonomics of the weapon, meaning everything from the fore grip to the hand rest/grip, to the shoulder/hip rest. If it touches the body, its a part of the "stock"
The "Barrel" refers to the how the round is delivered. be this a crossbow bolt track, to the smooth bore tube of a shot gun, to the rifled tube of a pistol. if the projectile travels down it, its a barrel.
These 3 parts are usually broken down into many more parts, such as the breech, chamber, port, assembly, etc, but for what we will talk about, these 3 parts are enough.
When you look at the main characteristics of firearms AND what we need in Shadowrun, we can break weapons down to 6 major terms,
Accuracy
Damage
Penetration
Limit
Range
Ammo Capacity
Now why did I discuss those terms above? Because they play a direct partnership with the things we care about in SR, and is also the reason by there is a multitude of guns even within the same caliber.
The way and the materials that the "Lock" is created determines the amount of pressure that can be generated to act against the round be shot (and thus the damage). Sealed Locks - such as found in cylinder and bolt action-
generate more pressure (DAMAGE!), then blow back chambers - such as semi-automatics and automatics as part of that pressure is used to eject the spend round and load the next.
The Stock pays a big part in the accuracy of the weapon. After all, if the weapon is cumbersome, or awkward to use, then it will not be precise. However, with an implanted weapon, this matters little I admit, but does help to explain one weapon's accuracy over an other.
The Barrel plays a part in both accuracy and damage as well as range. As a general rule of thumb, the longer the barrel, the more accurate the round will be, and the amount of pressure built up behind the round. Now THIS does matter for implant weapons due to limited space. You won't get the same length of barrel in an arm that you would on a weapon. (you could get MORE!) Also, the more pressure behind the projectile, the farther it goes!
SR terms like "limit" is a reflection of the overall quality of both the weapon, and the design of the weapon. Even the best marksmen can't do much with a rusted up, warped weapon! And even among same caliber weapons, the quality of design can play a part in this. I get much better groupings out of my S&W .44 mag then I do out of my Remington .44 mag despite the Remmy having a 3" longer barrel!
So, what you have to do is, compare each weapon to the base Cyber weapon, compare the length of the weapon to where it is going on the character, see what you can easily remove from the weapon (the Stock components), determine if that will change the stats and do so, compare the length of the barrels to determine if a change is needed, and if so, in what way. And so on....
OR
You could just say "Fuck it" and just implant the desired weapon, as is, for the costs listed. I do. (minus the stock components naturally - but not a change in game stats)
As to the ammo capacity: Once again, you get what the weapon says, and that's it. After all, that is what you are buying when you buy the weapon which makes reloading some cyber weapons more interesting I admit. For revolvers, It should be noted that many of them can be easily modified to actually quick change the entire cylinder, so maybe a larger port that allows the spent cylinder to be removed and a full cylinder dropped in?
As to your Smartlink question: If the weapon purchased doesn't come with one, then no, you don't get them for free. However I believe a smartgun system can be added to the weapon for a cost (see the books if I am right, I am away from them ATM)