Sometimes the gun is the cause.
Is there damage to the crown? Is the bore in good condition?
Is the ammunition correct for the gun?
Not knowing all particulars, but anything between the gun and target? A bullet hits anything in flight can make it tumble. Even a 50BMG will tumble after hitting a quarter inch twig.
For the message of the Cross is foolishness to those who are perishing, But to us who are being saved, It Is The Power Of God. 1Corinthians 1-18