Grunkybouy

Just a quick hello from Mississippi. I have built a few "15's" but this is my first 10 or .308 which is really going to be a .243. I have a feeling this is not going to be easy. To many patterns, to many parts that work here but not there. Barrel extension undersize by .003 Is it the bore on the upper or is the extension to small? And this is just the first day.