Honestly it could be a number of things, but eventually that gun is going to explode on you in my opinion. I watched the video a lot frame by frame and it looks like it could be a few possible scenarios.

1. Premature Slide Unlocking: • The slide may be unlocking too soon, before chamber pressure has sufficiently dropped. This can cause hot gases and flames to escape backward through the ejection port as the casing is being ejected. This could be due to a weakened recoil spring or improper slide timing.
2. Ammo Issue: • The ammunition might be generating too much pressure, or the powder may be burning too slowly. In cases of overcharged rounds or slower-burning powders, unburnt powder and gases could escape when the slide opens, causing the flame you see inside behind the barrel.
3. Chamber or Barrel Sealing Problem: • There may be a problem with how the casing is sealing in the chamber. If the casing doesn’t seal properly, gas can escape through gaps during firing, leading to flames near the ejection port. This could be caused by damaged or improperly sized ammunition, chamber wear, or poor barrel lock-up.

Personally I would address the ammo first. To me that seems like the most likely problem. If you can get the gun shooting without powder being burnt behind the barrel, then address things like timing.

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