To date, the rifling of the acrylic has been done prior to casting. Came about as a requirement since the bullet would either get sucked into the brass, or displaced from the brass under vacuum and pressure curing. So the transparent rod filled the brass, and held the slug evenly for the cure. Spiraling it was to simulate the round leaving the bullet.

But I gotta say I like where your thinking is going as far as a transparent 'barrel' and the bullet traveling through it. That will require a different approach, but is really a neat evolution or twist on the idea. Thanks for the idea..

Another limitation to the idea is the overall length I am working with. Reason and proportions have me wanting to keep the finished brushes under 4" height and even at that bigger knots are almost a must to keep it all to scale. 25 mm knots minimum I think.
Thanks again mate..