I just went through the pain of this exact issue, and while I'm definitely no expert, here's what worked for me:
Tap the barrel in a little farther than you need. Then, using the top edge of your vice, flip the rifle so the barrel is now facing down and one of the corners of the trunnion is now resting on the vice. Using your brass nut or roll of pennies gently tap the barrel "out" while you twist the barrel in the direction you need to go. Tap gently, but twist as hard as you can. Once you are lined up, tap the barrel home in either direction. Remember to use an old slant brake or muzzle nut to protect your crown. Also, it may seem like you need four hands at first, but I used a little tape to hold in my brass punch so I could focus on twisting.
I knew I was completely lined up when I could run a cleaning rod against the bottom of the barrel pin hole without getting hung up.
I was very happy with the barrel pin was seated. It was a good feeling.