Hello! I am so happy to be able to go over this with you. Many thanks to @platinumdream, @minkhollow42, @somethingpersonarelated and the r/spanish subreddit for verifying this one for me.

This line stands out because the Japanese and English (and also the Spanish) translations are all radically different—different enough that what we have here is a stellar example of "the Xerox effect".

Let's take a look.

the scene in japanese

Let's take a quick look at the grammar of this line, which I think is dual-meaning in Japanese just as it was in English (and if any of this seems like bullshit please get in touch, as it never ends well when I try to produce sentences of my own):

Akechi does not finish this sentence—he's left off the verb. So, as often happens with him, his meaning is ambiguous. Let's take the obvious one first:

Short and sweet: "I carry one of these, concealed, and some day you will too". Note that there's nothing here about practicing, as there is in the English.

But there's another possible meaning here:

WELP. Like I say, I'm not entirely sure, but I do think the strong likelihood is that this dual meaning exists in Japanese.

I should say I don't think there's necessarily a suggestion here that he carries an illegal gun (though since Naoto has one in P4, it's very possible)—I think it's a metaphorical pocket, his inventory in the Metaverse.

Though I don't think this is really what's going on, there are also a startling number of idioms with futokoro that suggest embracing....

but what is the futokoro?

Idiomatically speaking, 懐 futokoro often translates pretty cleanly as "pocket". But something else is going on: your futokoro is explicitly your breast pocket.

Originally, it was the flap in the front of your kimono where you tucked things away. And so it also appears in a lot of idioms relating to the bosom, or the heart. 懐に飛び込む futokoro ni tobikomu is to throw yourself into someone's arms. 懐に入る futokoro ni hairu, for instance, means to worm your way into someone's affections, or to win someone's confidence—sound familiar at all?

But there's something else going on, of course, with Akechi concealing a weapon in his breast pocket:

Yep. Here's the payoff for this line in rank 5. Akechi tucks that silencer away in his breast pocket—just like he told Joker he would. And then he laughs. So did he know at rank 5 that this was going to happen, or did he bring this up for some other reason? Well, you decide.

By the way, here's Joker's question about gunplay:

The Japanese just means "Are you used to shooting guns?"—I'm not sure it has any of the more, er, fanfic connotations of the English "gunplay". @specterthief agrees there doesn't seem to be any innuendo to the Japanese line.