Colonel Martin L Fackler & Ballistic Gel:

This was quite a guy - He served as a surgeon with the US Navy for fifteen years (Vietnam & Japan) and then with the US Army for 16 years - before founding and working for some ten years at his Wound Ballistics Laboratory of the Letterman Army Institute of Research. 

Sadly Martin Fackler died four years ago (May 23, 2015) aged 82 after a very worthwhile life - but I think his writings about the effectiveness of bullets will long be consulted - as without a shadow of doubt - so will the 10% BALLISTIC GELATIN that he developed to simulate swine muscle tissue .. which also closely correlates to human tissue.

This following link will take you to a page containing further links to some of Facklers writings:

https://military.wikia.org/wiki/Martin_Fackler

Colonel Fackler concluded that wound depth or 'penetration' was much more important than previously thought, and recommended ammunition that could send a bullet at least twelve inches into his ballistic gelatin.

Bullets basically kill by either destroying or disrupting the CENTRAL NERVOUS SYSTEM  CNS located in the brain & spine .. or by exsanguination - blood loss that results in 'hypovolemia' - diminished body fluid (.. lovely words to remember both 😊 eh).

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Natural gelatin is collagen .. which is fibrous proteins from the connective tissues of animal bodies - extracted by slow cooking of pig skin etc .. before being cleaned & dried, & perhaps granulated for packaging.

The original gels made from natural gelatin are typically clear yellow-brown in color, and are generally not re-usable, being biodegradable - but the more expensive synthetic substitutes are engineered to simulate the ballistic properties of natural gelatin, while being colorless and clear.

 Some synthetic gels are also re-usable, since they can be melted and reformed without affecting the ballistic properties of the gels.

This next link is of a factory visit to see how gelatin is produced .. followed by some playing with differing percentage blocks for penetration:

https://www.gelita.com/en/applications/specialities/ballistic

Note: Bullets are sometimes found stopped from exiting on the far side by the animal skin - animal skin is very tough-strong and said to be roughly equal to four inches of flesh. Is this because of the concentration there of the fibrous protein collagen?

Synthetic ballistic gelatin is expensive stuff made from "unobtanium" but may consist of polymer-thickeners, stabilizer, adhesives .. so there could be some interest & fun to be had playing at cooking-up your own synthetic gels from mixes of PVA glues combined in water with Borax. The Borax cross links the PVA ... to likely make a sticky gooey mess.

https://www.scientificamerican.com/article/bring-science-home-playing-with-polymers/

Have fun, Life is good,

Marty K.