No Isaiah 54:17 is not a promise that individual Christians can claim and somehow think that the bullet will not be effective against them. Let me remind you that God permitted Stephen to die of stoning in Acts chapter 7. So the final stone that knocked the life out of him did prosper. In fact, nearly all the apostles died for the faith--some weapon meant to harm them prospered. Read the devotion on this web page:
Now notice how there is some truth packed throughout the page, but the author did not consider to whom was the promise given. In Isaiah 54 the promise is given to corporate Israel. Notice how the web page author claims nothing will harm the believer. How is she using ‘harm’? Is she using 'harm' in the sense that no matter what happens to your physical body you are secure in Christ? Or is she using it like the lion will not be able to rip the arm off of the Christian in the Roman coliseums Nero? The believer is secure in Christ, but Christ did not promise that a believer's body will not be destroyed or harmed. Christ’s body was crucified, and he died. Furthermore, he told his apostles to expect the same for themselves. Read Matthew 24 slowly and carefully and remember Jesus was talking to followers.
We know weapons formed against Christians are being effective in Iraq. Even today as I am writing this note Christians are being put to death for their faith in Christ. Are you familiar with the ministry Voice of the Martyrs? (https://www.persecution.com/ ) Spend some time on that website and tell me weapons are not prospering in the hands of those who would seek to kill and destroy Christians.
If you want a promise to claim that applies to you, if you believe, read John 10. John 10 is packed full of promises from Christians can claim, and John 10 provides clarity as to who genuine sheep are. For they hear his voice and follow the true shepherd and bishop of our souls.
The promise you have as a Christians is the promise of life found in 2 Timothy 1:1!