After performing a bunch of mods to my KelTec Sub2K, how do I know it's safe to fire? Don't want it exploding in my face.

[Dr Prepper]

When I developed 500gr sub hunting loads for my 458 socom, I did indeed clamp it in lead sled and pull trigger with a string from a safe distance. I did this with a couple rounds with each prospective powder. Even though I am a very experienced reloader I thought it prudent as there was little or no reliable data for heavy subs at the time, so I developed my own. They all were fine but it never hurts to be cautious.

There's definitely a time and place for "old school" basics. If I'm not mistaken, they also use the string method in test chambers don't they?
Can't recall what the specific name for those test guns/barrels are.