What was your final IT? I've had natural casings that were just plain tuff. I also have cooked sausages and case hardened the casing. For that reason, I bring the temp up gradually.
Homegrown 120 Gallon Reverse Flow, Homey, the Homegrown Kettle, 3 UDS's, 3 Stack Pro Q Excel 20, 18.5" Elderly(but not retired) WSM, 150lb Capacity Homegrown Electric, Weber gasserrrr,Weber Performer, Stumps XL Baby, Bicycle Fast Polder Thermo, Indisputably Fastest Purple Thermapen, SAI Satay Grill