Quoted as is from a reddit post by MolinaroK.
I'm going to describe how to make a build.
Use this website to plan: https://underrail.info/build/
On the left side in the planner, set your level to the max of 30.
You said you want to use Guns so pick one as your main weapon and focus the build on it: Assault Rifles, Shotguns, Pistols, Sub Machine Guns, Sniper Rifles, Energy Pistols, Chemical Pistols. PICK ONE.
Now in the character planner, at the bottom of the Feats column, check the "Show All" box. And at the top of that column, type the weapon type you choose in the search box. Those Feats are going to be the most important part of your build.
Now, look at the requirements for them and adjust your Attributes accordingly to meet the minimum requirements. For skills, Max out Guns.
Now if you picked a light weapon, like Pistol, or Energy Pistol or Chemical Pistols and SMGs, you will want more Dex that with other Gun types since Dex reduces the Action Point cost to fire the Gun. With the other Gun types, you will want enough Dex for any Feats you decide you want that have a Dex requirement.
Once you have set your Attributes and Skills to the requirements and taken all the Feats you want related to your chosen weapon, delete what you put in the Search box. But don't hit the "X" beside the box, that actually removes the Feats column!
Now read through the all the Feats and decide what looks worth taking to you. As you do, select it after setting your Attributes and/or Skills to meet the requirement for that Feat.
If you run out of available Feat selections but find something further down the list you want, then review your current selections and decide if you should drop one for it. Keep doing this until you reach the end of the Feat list.
Now, whatever Attribute Points you have left to spend, put into Perception since that will scale up your Guns skills, and your Guns Skill scales up your Accuracy and your Damage. Playing on Normal, for your first try I would say the following about Perception: Low but ok (9 or 10), Pretty Good (11 or 12), Good (13 to 15), too much (16+).
Now for crafting, the good news is that those skills can be very useful, even when you don't max out the investment. So, considering your desired build I'll give you some tips. Your primary weapon type has 1 main Crafting skill tied to it. For Chemical Pistols it is Chemistry and you should want at least (150), or even as much as (170) depending on your luck with finding high quality components. I put the numbers in () to indicate it is your modified skill and not the number of points invested that I'm talking about. For Energy Pistols, it is Electronics. And for all other gun types, it is Mechanics, with the same range of skill investment needed as indicated for Chem Pistols.
For Tailoring, making your end game Stealth Gear will likely will need around (140) to (150) Tailoring, maybe less, maybe more, and maybe you will find a nice loot drop and be happy enough with it. What you end up wanting here can vary quite a bit and if you feel starved for points, deciding to go with loot drops for this is not a bad choice if playing on Normal.
Energy Shields are very useful to all builds and end game crafted ones are much better than loot drops. Crafting your own will require roughly the same investment as for making your Gun. So, (150) to (170) in Electronics.
Chemistry will be needed to make the Mines you will be using. The top tier, which is Tier 5, require (112) Chemistry. Tier 4 mines require (69) and is not a bad choice if you want to save points. Hand Grenades have the same point requirements as Mines.
For Biology, it is hard say what you are going to want. Absolute minimum would be (20) for making Gas Grenades, which are awesome. Going higher is needed for making all the various types of Medicines and Buffs. Some common choices would be (55) for the most common types. Or (100) for a wider variety. And the max you would consider is (130), yet I've never bothered to go that high. This is one choice you may want to make using the Wiki. The Biology page of the Wiki lists everything in order of skill requirement. Review them and decide how high up the list you want to go. It is only higher up the list that it gets harder and harder to find them on vendors and loot drops.
Now for your Traps skill here are the break points:
At (100) you will be able to Disarm 90% of the traps in the game, and recover many of them to use yourself.
At (130) you will be able to Disarm all but a handful.
At (165) you will be able to Disarm all of them.
At (185) you can Disarm and Recover all of them.
For this skill count on being able to swap to a Trapper belt that gives +15% to your skill, and holding an item that gives +7 to your skill, called a Jacknife. You can find the option to turn on those bonuses in the left column of the planner "Extras" section.
Now what about Hacking and Lockpicking? With (100) in each, you will pass around 90% to 95% of all checks in the game. The highest Hacking check is 130, and the highest Lockpicking check is (135). You can go zero in either of these or both, but I've never done that, and I would not recommend it for a first run. No quest will ever be blocked for not having one of these two skills.
Now back to Attributes for a few words. Some guns have Strength requirements. For Assault Rifles it is 7. For Shotguns it is 5. For Sniper it is 5. No need for more on your build, and if using the Dex type guns, 3 Str is ok. Although traps and mines can be heavy and you may find 3 Str annoying. But certainly no reason to go above the listed Gun requirements.
Will is a dump stat so set it to 3 for your build.
Int is very useful because it scales up all your Crafting skills. I recommend picking something in the range of 5 to 8, depending on what you decide for your other Attributes.
Con is tough for me to recommend. Because I have over 1400 hours in the game I always dump it to 3. But a new player may find that difficult. So, decide just how good you think you will be :) Setting it to 5 will give a bit of a margin for error, and going higher than that should be done only to meet the requirement for a Feat that you think is a must for you.
For Agility, go with what your Feat selections say you need.
Well I hope this helps you plan out your own build and is not what you would consider a spoiler.