There are many tests that show that clothing inhibits or even prevents many hollow point handgun bullets from expanding. The majority opinion is that the cavity of the hollow point becomes clogged with the fabric.
I suspect or theorize that the fabric does not enter the hollow point but instead covers and wraps around the tip of the bullet before it sears off the shirt or jacket. In effect a fabric patch that is larger in diameter than the hollow point cavity is created and this fabric patch covering the front of the bullet is what inhibits expansion.
I further theorize that hollow point bullets that tend to still expand when shot through clothing are doing so because their profile is cutting through the fabric without creating this patch or is creating a patch that is small enough to enter the hollow point instead of wrapping around it.
I suspect or theorize that the fabric does not enter the hollow point but instead covers and wraps around the tip of the bullet before it sears off the shirt or jacket. In effect a fabric patch that is larger in diameter than the hollow point cavity is created and this fabric patch covering the front of the bullet is what inhibits expansion.
I further theorize that hollow point bullets that tend to still expand when shot through clothing are doing so because their profile is cutting through the fabric without creating this patch or is creating a patch that is small enough to enter the hollow point instead of wrapping around it.