Is the caliber I'm using enough stopper?
Are ballistic gel tests really enough to better understand this ?
Nope. Ballistics gel is nothing better than an idealized and hypothetical best case scenario using a tissue simulant that is actually nothing like the tissue it simulates except in basic overall average non-bone density. It tells you most about how well the bullet is apt to expand in a perfect sort of situation. Depending on the particular gel, it may or may not accurately represent the correct amount of penetration that may be attained in an ideal situation. With enough tests, it should tell you how stable the bullet is passing through the medium, whether it flies straight or is apt to veer off course, but again, this is in an ideal situation that does not actually mimic living tissue.
Ballistics gel is meant to be a consistent and uniform medium, a control standard by which evaluations can be made that throws out all of the nasty real world variables like whether or not your target is moving, different tissue densities, amorphous versus fibrous tissues, bone (thick bone, thin bone, deflection from bone, damage to bullets striking bone at various angles, etc. Yeah, I know some folks will buy a side of ribs an place it just so in front of the gel, or place a random bone or two inside the gel and see what happens for what is usually a singular example, but these are limited tests only. Sometimes, they use a simulated bone material inside a simulated tissue gel to up the reality of the simulation, LOL. These tests with bones that brought me the most laughter is when I see these guys using obviously sun bleached or boiled bone they have found somewhere out on the ground, which isn't a good choice because sun bleached or boiled bone is actually structurally different from living bone. This altered bone is actually much more brittle, less elastic, and less dense than living bone.
Also, gel tests tell you nothing about actual "stopping power" (if this concept is acceptable as a real thing) as the power of stopping reside not in the bullet or the gel, but in the living creature being shot that is to be stopped. While the gel gives you some idea of how well the bullet may expand or penetrate in an ideal situation, it does not tell you whether or not that expansion or penetration will necessarily produce the desired stop in real life. The general consensus is that expanding bullets tend to do more damage than non expanding bullets and more damage tends to equate with and increased ability to produce a stop more quickly, but these are nothing but generalities and not any sort of hard and fast laws of nature.
Gel tests will not tell you if the hydraulic shock produced is enough to produce a stop with a given shot.
Gel tests will not tell you if there is hydrostatic shock and if that shock will produce a stop in a given shot. Hydrostatic shock, in and of itself, it not even consistently produced in a manner to produce a stop in people/animals that have been shot. We know it is not likely to be produced with most pistol shots and that with rifle shots, that it is at best, is inconsistent, even with calibers such as .308
Gel tests will not tell you if your given shot will create enough expansion for a bullet to clip the aorta as is passes just missing the heart or not.
Gel tests will not tell you how fast the lungs will collapse or fill with blood.
Gel tests don't take into account whether or not the person you are shooting is adrenalized or stoned beyond the point of even realizing s/he has been shot.
Gel tests do not tell you how long it will take for a person to bleed out.
Again, all of this stuff happens inside a living animal inside of a real situation for which gel tests are NOT a simulant.
Bottom line, if you are a subscriber to the notion of "stopping power," there is not gel test that will actually serve as a reliable predictor for stopping power. Despite the dramatic slow motion videos of temporary wound cavities produced in fairly small blocks of gelatin that look absolutely horrifically devastating, the gel block expanding very briefly by as much as a factor of 2, you pretty much never see that same result in the shooting of real animal tissue unless you are shooting tiny animals.