Isn't that intentional though?[ . . . ]
By TL15, ACRs have 5 tech levels of obsolescence and Gauss rifles have 3. One tech level of obsolescence is enough to give the IRA access to M1 Garand rifles and armour piercing ammunition in the 1970s, and most irregular forces having access to relatively modern assault rifles for the better part of half a century.
Shaka Zulu's forces had a lot of firearms, mainly obsolete muzzle loaders that were about one TL below the capabilities of the British forces at the time, and they still gave the British a run for their money. A lot of the indigenous forces in the NZ Maori wars were quite well armed as well (do a web search on Te Rauparaha if you want a rollocking good story).
All of these examples happened within an era where the world was dominated by a large imperial or quasi-imperial power, so I also don't buy reasoning based on the Imperium controlling the market.
By that analysis, ACRs could be expected to be widely available on the grey market by TL11, Gauss Rifles by TL13 and X-Ray laser weapons and 1-2 gen PGMP's by TL14, so the disparity in tech levels doesn't wash with me. I don't really see a disparity of more than one tech level
1 surviving in anything resembling open market conditions.
If you assume a practical maximum tech level in the 12-13 range for most regions, though, I can see Gauss Rifles being less prevalent on the open market, but there would certainly be a demand for them.
What doesn't resonate with me is a TL 11 free trader crew in a TL 13 universe armed with TL 6-7 weapons when significantly better kit has been available for millenia.
It sort of works in a LBB1-3 style universe where the kit is assumed to have not been significantly improved on (i.e. a Laser Rifle isn't that much more powerful than the TL5-7 kit) but from LBB4 onwards, it's obviously rendered obsolete by the high tech kit such as Gauss Rifles and PGMPs.
1 Maybe two tech levels at the outside, but I'd like to see a good story about how the region came to be such a backwater while much higher tech forces are cruising around on its borders.