RE: 21 inch GMB carbine barrel
Peep sights are without doubt an accurate way to go. Have you considered a Tru Glo Ghost Sight? I was looking at them. It is a peep but also has fiber optic rods on each side of the peep for when the darkness creeps in.
This little barrel was more of a... I did not need it but wanted to play with it. I hope it shoots conicals well. With the 1-28 twist is should do fine with some Goex 3f.
Congratulations on the doe. It is funny how sometimes you can make all the noise in the world and they deer do not spook and other times your chair makes a creek and they are over the next ridge.