Metal Slug 3 if it was actually good.

The main issue I have with 6 is just how the weapon and ammo systems are both handled. Two weapons sounds interesting on paper, but the weapon pickup system just doesn't work that well in practice. When you get ammo, it applies to the weapon you are holding, or the leftmost weapon if holding pistol. If you pick up a new weapon, you keep the weapon you are holding, or the leftmost weapon if holding the pistol; this will also always move your preserved weapon to the rightmost spot. Additionally, ammo counts are substantially reduced compared to previous games, and there is no quickswap stance when swapping to or from pistol, with the only quickswap being between two active weapons (from either position/stance) or when picking up a new weapon.

The lack of quickswap combined with the extremely low ammo counts makes weapons a lot less satisfying, and near useless outside of a few bossfights. Basically, you are outright punished for picking them up because you have to swap to your preserved weapon to prevent losing it (one second animation), pick up the new weapon, then swap back to pistol (another one second animation). So a single weapon pickup just wasted two seconds where you are completely defenseless in animation and moved your preserved weapon to the less ideal rightmost spot. Weapons like Dropshot, Iron Lizard, Rocket Launcher, and Heavy MG were already not all that impactful in most situations, but now with the massively reduced ammo counts and the long transition animation any time you swap into or out of your pistol, they are a detriment that you want to actively avoid. This is further exacerbated by the game constantly bombarding you with every variety of weapon so you can't stockpile ammo and must constantly avoid the weak ones, but between the spongy enemies, the low ammo counts, and the long transitions, you just don't bother outside of a few edge cases where you need a specific weapon for the boss.

Yep, that's why MS6 sucks the same way MS7 does and both fall flat when compared to original trilogy.
Even titular Slugs became scarcer, making the series turn into Contra but with funny artstyle.

Did you even play MS3?

Actually there is quickswap from pistol to one of your two weapons, just not vice versa. Seems like intuitively it would be the opposite. Also you automatically swap immediately to your other active weapon whenever one runs out of ammo, so you have to be very careful when routing a weapon for the boss because you can burn through that weapon's ammo in just a couple seconds if not paying attention.

It definitely requires time to adapt to the two weapon system, and I would consider the implementation a downgrade overall.