Hi Gray Goat,
The "secret" to successfully cooking-smoking sausages is to do it GRADUALLY. Most people don't have the patience to do it correctly and they wind up with sausage, the texture of which resembles sawdust. The secret is to raise the temperature only a couple of degrees every twenty to thirty minutes.
Try this: After grinding the meat and placing it into casings, be sure to pinprick them to remove any air bubbles. Place the sausages into a smokehouse preheated to 130°F. (54°C.) with the damper fully open. Pre-heating is an important step. When the casings are dry, close the damper until it remains only 1/4 open, apply hickory smoke, and increase the temperature only a few degrees every twenty minutes or so, until the smokehouse temperature reaches 160°F. (71°C.). Cut the smoke but continue cooking the sausages at this temperature until the internal meat temperature gradually reaches 155°F. (68°C.). Immediately rinse the sausages with cold tap water and hang them up to dry for forty - five minutes before refrigerating them.
Note that at 138 degrees Fahrenheit (59 C.), you are destroying any possible trichinella spiralis. As you surpass 152°; F, a vast number of bad bacteria and monstrous bugs are destroyed. Monitoring the temperature with a good thermometer is critical.
Shucks pard, these "bad bugs" include the vicious eastern Utah smiling sabre-toothed "salmon-nellie" - a fish-shaped microorganism so mean and offensive, that other pathogenic bacteria just naturally shy away from it! And that's not all! They are so villainous and dissolute, they travel around in "gangs"!
Anyway, have you ever wondered why we shower hot sausages with cold water to quickly bring the temperature down? The "carry over effect" in cooking is a little known phenomenon. When hot food is removed from an oven, the temperature will actually climb a few degrees (no kidding this time). You must stop the temperature from climbing to a point where the fat will "break" and become liquid. The cold water also helps prevent wrinkling of the casings.
Best Wishes,
Chuckwagon