Woah woah woah, clip your string Chatty Cathy. The thread is what guns do you want to see? I am not attacking your choices, I am just stating mine. It is very possible that we have different aesthetics when it comes to gun porn. When you look at the pics that Thewarsend posts you see good weapons with rails on them, the rails are a way to modify weapons that sacrifice 'mobility' for modabiity. The design aesthetic that I want to see is the ability to make these modifications internal to the weapon, like the LaserMax laser sight for pistols that takes the place of the recoil spring, you can still holster your favorite semi auto pistol in a stock holster and have a laser sight.
On the subject of piercing armor. A "needle gun" is exactly what you want, unless you are planning to use a shaped charge round. It is all about delivering maximum kinetic energy to the smallest crosssectional area. Remember you get more KE for your buck from velocity than you do from mass. A "needle gun" uses this principle just as the APFSDS (armor piercing fin stabilized discarding sabot) tank round does.
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The 'internal' mods would need to be 'custom rebuilds' or premade. But, I think that should be ok to do.
As for the 'Needle Gun', Flechette guns are in 2020 and they can be lethal against armoured targets. Of course, shotguns have the most verstile ammunition, including APFSDS, Flechettes, Slugs and my favourite, "THERMITE!"