Now, to address toughness. You can think of meat fibers like rubber bands. You want them to relax so as not to be tough, but have the good, satisfying chew between your teeth of good meat, and that requires rest and proper cooking. I won't get into freezing, which can have a tenderizing effect due to the damage the ice crystals do to the meat fibers, but along with hanging, that is really another issue. I think most small game gets eaten more quickly than large game anyway.
So, step one is to let the meat rest at a cool temp. I like to salt and pepper my game and lay it on old oven racks, over plates in the fridge, so that as the salt pulls out moisture, it can drip away from the meat. I usually leave it in the fridge over night. Then, I leave it on the counter at room temp for several hours - up to 12 - before cooking. That sounds absolutely crazy and opposed to every modern health recommendation, I know, but I think it is essential. This process alone will tenderize at least 75% of meat to my preference. This "quick hanging" allows the muscle fibers to fully relax and to break down somewhat just due to temp. But, a lot more is going on. The salt pulls some liquid and natural sugars out of the meat, to the surface, while creating an environment that inhospitable to most bacterias that can make you sick. This creates the ideal environment for bacterias like lacto-bacterias - the same bacterias that turn cabbage into sour kraut - wild yeasts, etc. So, there is a preserving effect, the bacterias pre-digest the meat to some extent and the flavor is enhanced. I like the meat to get that good cheesy, rich smell before cooking. We could call this the George Herter Method, as Bull Cook fans know..
Just before cooking, I poke or squeeze the thickest part of the meat, like the thigh. It should feel very tender, almost as soft as your earlobe or the muscle between your thumb and first finger with your hand completely relaxed. If so, I cook it fast and hot in a pan, oven or a hot grill, to medium rare. Carry over heat means that the meat continues to cook on the plate, so take it off the heat before it is fully cooked to your liking. Put some butter on it. Cover it with foil and let it rest for 15-20 min so that the juices can redistribute. Not letting meat rest is a big mistake that often results in dry, tough meat and a plate full of all the good juices going to waste.
If the meat is not tender enough after that long pre cooking rest, then we have to take measures to tenderize it. To do so, we must understand what cooking actually does to meat. Cooking meat does 4 essential things:
1) It firms the proteins. The meat fibers contract. This is a chemical reaction that can be caused by heat or acids, like when you put citrus juice on raw seafood and it seems to cook.
2) It degrades the proteins. It breaks the muscle fibers down, making them easier to chew and more digestible.
3) It enhances flavor. This is a simple concept, like browning meat. But, it is scientifically very complicated, so I won't go into that.
4) It adds flavor by allowing the meat to be flavored by additions like salt, pepper, herbs, spices, onions and other vegetables.
Obviously, the first two of those aspects are in conflict. How can cooking meat make it firm and more tender? That is the art of cooking. There are several options for tenderizing meat, and each has its place.
1) Chemical. A meat tenderizer like pineapple or papaya will chemically break down the muscle fibers. This should be used very sparingly, so you don't end up with mush.
2) Mechanical. You can grind the meat, of course. Another great option for the larger cuts is to bone it out and pound the hell out of it with a mallet. Make a thin cutlet, bread and fry it like chicken fried steak or schnitzel. Or, you can mix up some cheese, herbs and bread crumbs, roll that up in the cutlets, tied them closed with string, brown them and cook them in sauce... which is awesome... and the scraps can go in with the sauce - think of a big Italian Sunday dinner!
3) Parboil. This is the classical method that most old cookbooks and good restaurants use. The reason it works is that it is a two step process. To parboil is to boil briefly, take the meat out and let it cool, before cooking it fully in another manner. The parboil firms the proteins without cooking the meat fully through. Therefore, after the meat rests and the juices re distribute, when it is cooked for the second time, the already firmed muscle fibers will not further contract to the extent they would with a one step cooking process.
4) Braise. This is my favorite method. Braising is like parboiling in reverse. Brown your meat thoroughly in a hot pan or under a broiler. Take it off the heat to rest. Then, either in the same pan or a deeper roasting or casserole dish, add the meat and enough flavorful liquid to come about half way up the side of the meat (not enough to cover it). Then cook it low and slow like you would a pot roast, until the meat falls from the bone. Because you cooked it on the bone, all that good gelatin and collagen and some marrow will melt into the liquid and it will have the richness of bone broth.... and so much flavor! This is what people go crazy over in rustic European restaurants where they make great braised dishes out of shanks, oxtail, trotters and other super cheap, tough cuts of meat that people think are worthless. A good braising liquid is red wine, some stock, onions, garlic, thyme, etc. Some folks like using tomato juice or just chicken broth. Whatever you like is fine.
5) Pressure cooker. A pressure cooker is another one of those things that is an easy concept with a long scientific explanation that we can skip. Basically, it cooks the meat in a moist environment, under pressure that mechanically tenderizes the meat. It can be a good option because it saves time and the results can be almost as good as braising if you do one thing - let it cool down and release pressure completely before taking the lid off. The pressure squeezes moisture out of the meat. As it cools and the pressure lowers, the meat will reabsorb the liquid and all the flavor. Taking the lid off early can be dangerous and result in dry, flavorless but wet meat...which often confuses people.
6) Barbecue. This is my favorite option for larger small game that can be cooked whole - like a big, fat coon, groundhog or beaver. Barbecuing is simply cooking low and slow over the dry heat of wood coals. A major benefit is that you don't have to trim off all the fat, sinew, etc, because it will melt away as it cooks low and slow (fast, hot cooking causes sinew/silver skin/etc to toughen). All I do is salt my meat and let it rest as described earlier, then cook it 10 hours directly over live oak and hickory coals (NC style). Most of the flavor comes from the fat dripping onto the coals and the smoke rising from that. If your barbecue tradition uses dry rubs, wet mops, offset smokers, that will work too.
7) Stewing... http://reclaimingsouthernfood.blogspot.com/2016/12/stew-basics.html
All of that may seem complicated, but the basics are: 1) Handle it right in the field. 2) Rest it before cooking. 3) Cook hot and fast for tender meat or low and slow for tough meat. 4) Rest after cooking. 5) Don't over think it. Don't do something just because someone else said to do it. Start simple. Cook to your own taste in your own style. These are all "perfect case" suggestions, where you have everything controlled. In camp, cooking on hot coals or low and slow in a camp/dutch oven yields results just as good.