denny66 Using an oven can contribute to this because it doesn’t have as much temperature control to start low and slowly increase temps.
Jonathon did a good post on this a while back, for snack sticks, but the same principles will apply for summer sausage.
Check out #4 on the Top 5 Mistakes Made When Making Homemade Snack Sticks
What you are experiencing is what we call “case hardening”. The best way to prevent it is to start with low temps and slowly increase. Our typical guide is 125F for 1 hour - 140F for 1 hour - 155F for 2 hours - 175F until internal meat temp of 160F. That is difficult to achieve with an oven. An oven will still cook a fine and edible product, but not as perfectly as it could be.
Maybe one of the others out here have some extra tips and tricks to cooking summer sausage in an oven…?
I’ve never personally done it in an oven, but a couple things that could help might be propping the oven door to keep the temp a little lower than 180 during the initial cooking stages. Or, adding a pan of water somewhere inside the oven to help keep the humidity higher and prevent the summer sausage from drying out the outside as quickly before the inside is done.