As far as usefulness goes, this GTA Online glitch isn't on the top of the list.
GTA Online, like most online multiplayer games, has its fair share of glitches. These bugs or errors are not induced by the players using third-party hacks, mods or scripts.
Instead these are the tiny loopholes that exist by virtue of the game mechanics themselves or certain unpatched bugs. They do not require the game to be modified in any way to be exploited, thus their use is not a bannable offense.
However these usually do get patched pretty quickly.

In the past most glitches found by the ever vigilant players of GTA Online pertained either to vehicle duping for a quick buck, bypassing cooldowns on various jobs or the saving of outfits exclusive to certain game-modes. Save for the outfit glitches, these are pretty useful in practical terms.
Today we bring to you a glitch which is as useless as awesome.
With a relatively simple glitch, you can apply a new coat of finish over various weapons on top of a luxury engraving, resulting in an overly fabulous gun. The flare of the shiny new gun is enough to blind an opponent from the other side of a map.
Depdending on the desired result, you should use the gold tint for an orange-on-black effect, pink tint for deep purple and platinum for shiny-shiny black.
All you need do is hop in to your closest Ammunation store, select the desired weapon (the glitch has been confirmed for the Combat MG), apply the luxury engraving, the colored tint and then the engraving again. Once this is done, you'll need to jump over into a new session and select the Combat MG or whichever weapon you glitched. Voila, you have the raddest gun in Los Santos.
The luxury weapon engravings were added to the game in the second part of the Ill-Gotten Gains DLC update last year.