[Michael Thayne]

For the post part, Ultra-Tech conspicuously does not use calibers that are popular in the real world. Instead, it mostly uses calibers that make nice round numbers in the metric system: 5mm, 7mm, 10mm, 15mm. This got me wondering: in a universe where these calibers were standard, what stats would lower-tech firearms using those calibers have? You can imagine a variety of reasons why people would create such weapons, from colonists who are mostly TL9 but don't have a full-blown TL9 industrial base, to post-apocalyptic survivors making 10mm black-powder weapons because the metric system is the only system of measurement they know.

Looking carefully at various weapons tables, for TL7-8 weapons firing cased ammo I think the main difference is that they'll hold about 1/3 as many shots as their TL9 equivalents. Damage is AFAICT likely to be unchanged—for example both High-Tech and Ultra-Tech rifles generally seem to inflict dice of damage equal to about 90% of their caliber in millimeters. Other stats seem mostly to be similar—though Ultra-Tech guns also seem to have slightly lower ST requirements on average for reasons that aren't entirely clear to me (ergonomics?). Maybe lower-tech equivalents to Ultra-Tech guns would have as much as +2 to ST requirements, though that seems to produce too-high ST requirements in some cases. Maybe only +1.

Much less certain about post-apocalyptic guns using round millimeter calibers but I'm open to suggestions.