I've been told that in hammer forging, there are optimum ratios for tube diameter to bore diameter... for final results.
@Franklink's barrel is a 15 mm x 711 mm (28"), as are 2 that I purchased. The measured bore size on mine is larger than specified by Troy so I've been very careful about much polishing. The bores on mine are fairly rough and after much testing, decided to lap it. It is MUCH better but not that close in accuracy to the factory barrel on my Safari. I have not experimented with slugs yet. The velocity gain was minimal on the Safari as it is tuned to be fairly efficient (little wasted air). Now I bought mine as blanks and
@Franklink's was machined by Troy or under his supervision, for what that's worth. I have occasional groups that are quite a bit better than those above but overall , probably could consider those as the typical.
Slugs could be a different story because you can get different diameters.
A different outer diameter will likely deliver a different result... as I understand it.
Bob