Standards

The Third Imperium, 11,000 plus worlds.

Considering that the US and Europe can't even agree on what kind of electricity to pump out the wall and Wal Mart will have about fifteen hunting rifle cartridges in the case (optimally) getting something to standardize and STAY standardized across that much space and population is a major undertaking.

For the Imperial armed forces, it's simpler.  "Here are the plans, go for it."  They have a pattern room sort of deal going and anyone who wants to make issue weapons for the Imperium must build to the exacting standard.

It will mean that changes to the armament of the troops will change very slowly.

Some things will become standardized and ossified out of practical reasons.  You're going to be able to find Type A Free Trader parts just about anywhere because it's the most common ship.  It's a risk to introduce a similar class of ship because there's no several hundred years of infrastructure already in place for it.  (I used this as the limiter on the players concerning the Far Trader)

The gear you can find everywhere is going to end up being the most popular and cause it to be more available in a self-reinforcing loop.

Individual worlds might go their own way, some forms of planetary governments kind of encourage it too.  There might not be standard 9mm Automatic Pistol ammo on such worlds! but the high port will have standard ammo, and will likely also carry the local stuff too (for collectors).