Those sound like the specs for the barrel used on a boy's rifle kit that was around a few years back. I believe Fred Miller offered it, though there may have been others. If my memory serves me correctly, it was a Bucks county pattern, using the Becky lock.
I can't swear on the details.
Speaking from experience, the .45 cal would be a good choice for a boy's rifle. I made a boy's gun years ago, with a swamped .32 cal barrel, 3/4 inch at the breach. Though the 3/4 scale gun fit the boys well, the barrel weight was still a bit heavy for them.
The lighter .45 cal with a light charge would be a better choice IMO.
Jeff