Well, let's see...
Good pork butt is usually 20-22% fat and summersausage tastes the best when made so that the finished product is around 25 percent fat, not to exceed 30 percent. I think that 30 percent is too much. Therefore, you need to boost it up by adding fatback or hard fat from beef (not leaf fat).
With venison, trim all of the venison fat off the meat, it will turn rancid easily and that is a big yuck! Trim all of the sinew and other connective tissue, too; it just makes higher quality sausage.
Old-time summersausage makers typically cured their meat in the frig for several days, like maybe 5 to 7. That allowed time for the nitrate to turn to nitrite and thence to nitric oxide for the curing--all done by natural bacterial fermentation. The same was true for the tangy taste: the long cure allowed the Lactobacillus to produce enough lactic acid so that the finished product had a tangy flavor and the acid content protected the meat from growing other bad bugs. How much and how long were the secrets.
Nowadays, curers use cures that are more-or-less instant and there is no time for the developement of the lactic acid, so most add acid. Lactic acid can be added and phosphoric and citric are others. However different acids have different tastes (all are sour, it's the other character that is different). A good example are dill pickles made with vinegar (acetic acid) and those left to naturally ferment (lactic acid). The two pickles have markedly different flavor profiles. So, add your agent to lower the pH and preserve the sausage and add the flavor; it is O.K. BTW, some add a fermenting culture to speed up the process, but it is less certain and even less than that is a product that is simply a bacterial food to speed up the process. They can all work with the right care.
The final grind is usually specified in the recipe, e.g., twice through a 3/16" plate or something like that. The meat will have to be cut or ground into relatively large (1" or less) chunks for the curing process and for the addition of spices. After that comes the final grinding. Add the binder when the recipe says so: some say early some say late. A sausage with a binder added too early can set up like concrete and make further grinding difficult or impossible.
Simply squeeze a ring around the tube with your index finger and thumb, letting the filled sausage pull the casing off the tube as it builds pressure. Stuff the summersausage very tightly. The tube does not have to be full-sized to be workable.
Hope that helps.
