I'll break down from beginning till recent chapters.
Standard reincarnation type story. You either get a loser or an awesome guy just dying and recincarnating, simple. The "twist" is that this MC watched a fellow bullied classmate suffer and die. Sure, okay.
Next he reincarnated and it was pretty slow. I don't mind it but each chapter was basically 0-1 years. Two random chapters were basically repeats of the female girl's perspective. It wasn't entirely pointless but it added very little. There's also cool yet poorly executed parts about guns. It's literally like wikipedia. The author needed to spice it up, somehow.
Point being, MC spent 10 years making two handguns and one rifle. Slow but cool, not that bad. Then he became an adventurer. This is the part where the author should've learned from The Walking Dead. Those who mess with Rick (MC) gets killed, lol. I totally loved how he got tricked. But he should've gotten away somehow. And then some stuff happened about being a blood slave or something. I mean, it's all over the place to make a harem.
I know people will say, if you don't like harems, don't read them. Well, it didn't start off as a harem. A nice sweet girl to look forward to is nice. Not a harem, but I suspected, just hoped it wasn't as focused on the harem part. Like more on the gun and stuff.
Characters are also...not amazingly well developed but I don't mind. Light/web japanese novels usually don't have characters super developed. It's usually really short and vague, like Shieldbro.
It has a pretty good concept but it just strayed away and became a harem or something like that. A better plot would be while transporting, somehow he broke away and killed all three of the slave dealers. And he continued on his gun harem, not harem. Like not trusting people and doing solo work. Then make more guns, farm his own metal slime, I don't know. Stuff that's not just a perv.