...it all fit into the front tube of a mac sionics supressor.
The reason that this tube was specified is quite simple; it was a specific length & both ends were internally threaded. Obviously, threaded endcaps would provide a better gas seal than friction fit with set screws. It would depend on the skill of the builder to choose which method was perhaps more expedient.
There was a virtually uncontrolled proliferation of silencer parts kits back in the '80s & ATF was NOT amused. Several manufacturers skirted the legal ramifications by not supplying complete kits. Internal baffle stacks might be provided by one company & tubes from another. Sometimes these outfits were the same people who provided both sets under different names. Tubes for SIONICS type two stage cans usually included the correct front tube with an overlength rear tube (to be sized later).
This created a *mix & match* scenario that BATF put a stop to by classifying EACH SILENCER PART as a SUPPRESSOR in & of itself.