From memory, there are only two ways for a Ranged (Light) weapon to reach out to Long or Extreme range: The first is that there are two Long-ranged pistols, the X-30 and DE-10, and the second is Sniper Shot, which upgrades the roll’s Difficulty once per range band. I don’t think that additional rule is necessary because it’s so niche, but I think it actually makes a lot of sense. I was initially going to disagree, but as I thought about it more and went through my arguments, I realized that they cut in favor of your point. The X-30 lacks a proper optic, and while the DE-10 has a native Telescopic Optical Sight, that doesn’t help with weapon stability, only target acquisition. The +1 Difficulty would make the X-30 Daunting at Long, and the DE-10 would be Hard at Long, just like a rifle without a TOS. I don’t intend to implement that change in my games, but for those niche scenarios, it makes a lot of sense.
The Pistol Stock is basically the effect of the DR-45 Dragoon, which I don’t particularly like. On one hand, you have the convenience of skill-switching, but on the other, how often is it actually beneficial to make a Ranged (Light) weapon Ranged (Heavy)? Why not just use Ranged (Heavy)? Couple thoughts for tweaks or changes: Increase weapon encumbrance by 1 and change the effect.
An obvious option to change the effect would be to make it stay Ranged (Light), but remove the penalty for firing at Long range, but that would only affect a very small number of weapons, or the rare character who builds a pistol-reliant sniper. I would instead go with the same approach I used for the DR-45 here P-47’s Fix-It Shop (DR-45) and make the conversion give a Ranged (Light) weapon Ranged (Heavy) and Accurate 1.
Clip-On Barrel, I can’t get behind. Pistols are naturally very short-ranged, and increasing their range can get goofy quickly. Together with your +1 Difficulty houserule, it works okay, but the maneuver to swap feels way too easy to me. I also think it’s too cheap. Encumbrance should probably only increase by 1. Narratively, I think it works, given how blasters work in Star Wars. I’m not sure whether the better approach is to cap the range increase at Medium or at Long, but given that we have precedence for Long-ranged pistols, I think that capping it at Long makes sense.
For the DR-45, just see the link I sent. I prefer it because it makes more of a trade-off.
I think the A280-CFE is good as-written.
A note on encumbrance: Changing the encumbrance of the weapon between different versions is a bit finicky because you still have to have the attachments on your person in order to swap them out, so most of the time it’s better to just wrap the encumbrance together—fewer things to keep track of.
I think you should write up a doc that’s basically a mini-houserule set for convertible pistols. Add in the +1 Difficulty houserule and the mods that let you convert the pistol into a rifle, and we can see where there may be places to expand and flesh it out. Currently, Long-ranged pistols are the nichest of the niche, but your approach will make them more common, which makes the houserule more important. I expect a lot of interdependency between the conversion attachments and the houserule.