All good suggestions and info posted here should not be ignored. A way to get into the corners is use a card or something similar with your abrasive wrapped around the edge, using just enough pressure and not sanding too much in one direction.

I guess there are different levels of shiny, nos shiny, worn shiny and oiled shiny. Also different finishes on different handguards made of different plastic. I cannot teach someone the finesse needed to make a facsimile of nos shiny handguards, but the process is likely the same, with more wet sanding and more patience. You can kinda decide for yourself how you want it to be.

Worn shiny is the easiest to accomplish and takes the least amount of time and money. I'm trying out some 1000 grit abralon pads, twice the radius so i get two for one. 5 for $15 shipped. I'm using the shit I cut off making the two pads out of one folded on a card to get to the trouble spots.

I'm working on the second one now, haven't finished the first yet. The last picture is deceiving, that was right after I was done for the day so it was wet.

I'm in it now. I'll be ordering some 2000 grit abralon because abralon seems to last a tad longer on a drill. It doesn't get into the corners as good as the autozone 800 grit though so I'm trying the process of:

1000 grit abralon
800 grit autozone pad for corners
Use cut pieces and also hit corners again
Back with 1000 grit abralon to even it out
Go finer

This is where I'm at.