The Sierra GK bullets in this weight range are more than likely designed for American White-tail deer (about the size of our Sika), from what you are saying the bullet is doing exactly what it was designed to do. Pigs even small ones are a much tougher animal.
To get a tougher bullet you will need to either go to a premium bullet (Pricey) or go up in weight.
As mentioned above I reckon the 145 Speer Hotcore (not the BTSP) would be the first one I would try you will only loose 25-50 FPS depending on barrel length and have a bullet that holds together well but still opens up well.
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