Source: https://github.com/liurui39660/PayDay2.Mod/tree/DisableGracePeriod

There's no 0.35s invulnerability after taking a hit.
It's compatible with Disable Armor Gating Effect.
Put the decompressed folder to mod/, like most other mods.

Q: What's the grace period?
A: When you receive bullet damage from an enemy, a timer, which in Overkill or below is 0.45s and in Mayhem or above is 0.35s, will be activated. During this period, any other bullet damage less or equal to the previous plus 0.01 will be ignored. If you receive another shot that has greater damage, this damage will not be ignored; your AP or HP will be deducted, and the grace period timer will be reset.

Q: What does this mod do?
A: As the title says, it completely removes the grace period timer. So whenever you are shot, you will receive the damage.

Q: Will bots be affected?
A: No, they are filtered out in 1.1.0 version.

Comments, suggestions and ideas for the thumbnail are welcomed.

Demo
I hardcoded all damage to 0.01, so I won't die immediately.
But of course, in the game, you will receive the original damage, not 0.01.

Without this mod:

With this mod: