The Mandolin is a pure normal damage gun from the recently released Spike event. Since it is Normal you might expect it to perform evenly well in every scenario but that is far from the case. I'll go over the strengths it has and the weaknesses.

Strengths:

As a Normal weapon it does Neutral damage to all monsters but do understand that it's unusual bullet behavior does lend itself to certain matchups more.

One other benefit it has is the boomerang bullets which deal more damage on the return, this can also be leveraged to hit blocks a normal gun can't, and sometimes from angles an Alchemer can't. This is because the Bullets always try to return to the player after they are shot and not to the point from which they are shot. This lets you hit things from behind by shooting and moving horizontal from your bullet.

Front Shielded enemies like Trojans, Mecha knights, and Mortafires can all be beaten abusing the feature above to shoot bullets past their side before causing them to careen back into their rears.

Dodgy Enemies like Thwackers, Menders, Devilite, Stalkers, all struggle with the gun's wide bullets and return shots, often not being able to dodge everything or ending up hit in the back by the returns.

Gorgos are also much easier to deal with because of how their floaty mechanic is dealt with by the wide returning bullets.

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Weaknesses:

As a Normal weapon it will have places where it is outperformed in DPS by more specialized guns such as the Tundrus or Needle shot lines.

Tier 3 Wolvers end up dodging behind the player in reaction to gunfire which makes the return shots much less useful so pair it with a Flourish or Cutter sword.

The wide bullets end up causing them to hit walls and other obstacles easily and can make sniping through a spot that's just one or two blocks wide more difficult... get into more open areas to see it work better.

In a similar vein the wide bullets make trying to snipe healers difficult, you'll have to kite around groups of enemies or use another gun to take out the horde faster or remove the healer.

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Is the gun worth getting? I'd argue yes... it's a very strong generalist gun with a focus on kiting enemies, it's a great backup sidearm and the Stun status it deals can help you keep that spacing even more easily. Just be aware of it's downsides and it should perform well.