Good point Tim the student. When on looks on the box of tampons, does it tell one how to treat a GSW with it? Is there a medical school or combat lifesaver course that teaches proper tampon use for GSWs? Does any agency or military branch issue tampons as part of their office first aid kit?
There is no "knowing how to use a tampon to treat GSWs" because that's not what they're for. Anyone using a tampon for that purpose is by definition, going outside the scope of their training and equipment since no training course or issued equipment kit issues tampons for GSWs or teaches how to use them. They are by their very nature, an improvised item.
Actual medical items such as gauze, hemostatic agents, pressure dressings, etc are all recognized by both the medical field and the "shooter" field (LE, military, etc) as legitimate items to be used with training for treating gun shot wounds. Military, law enforcement, and even civilian training school teach their use and issue them to those who have been trained/certified. They don't issue tampons. And more importantly, they don't train you on tampons.
If I'm dying of a GSW and your choice is between plugging it with a tampon that you just happen to have, and doing nothing, please use the tampon. But we're not talking about seat of the pants improvising here. We're talking about stuff that you would equip yourself with ahead of time. Why would you settle for anything less than the best?
A comparison: can you use a pointy stick to ward off an attacker? Yep. But if you're expecting to be attacked do you purposely just bring a pointy stick? Of course not. You use it if that's all you have on hand. First of all, you avoid the situation where you'd get attacked. If that's impossible, we've all heard the "bring a rifle and friends with rifles" quip.
Well medical trauma is just the same. First of all, avoid getting hurt. Obviously. But if you're making a backup plan, you bring the best. You don't deliberately cripple your attempts by preparing yourself with substandard stuff. A tampon, or a t shirt, or towels, etc are all the pointy stick version of medical equipment. Stuff you use if that's all you have. But if you're already making a plan on possibly needing it, you bring the best. If you're expecting an attack, you bring the rifle and friends with rifles, and if you're planning on the possibility of a GSW, you bring real medical supplies and real training to go with it.
When you thankfully have the opportunity to prepare for a GSW ahead of time, there is absolutely no reason not to find the absolute best materials and training you can. Why would anyone set themselves up for anything less than the best shot at surviving? I'm not saying tampons are the worst thing ever. But if you've got the opportunity to prepare ahead of time, why waste it on getting less-than-the best?