Tim; Do I understand you correctly?
You want to shorten the end of the barrel next to the nib end of the pen?
The short answer is, Yes, you can do that. Take the Nib, inkfill, transmission, and finial and assemble WITHOUT the barrel. Push the finial as far down as it will go. The space between the nib and finial is the minimum length of the barrel. Likely you can remove at least 1/8" and probably much more. There are several variants of the Sierra and not all are exactly identical in this respect.