You can mimic a beadblasted finish with careful application of a hammered abrasive cloth on the metal surface.
It takes some experience and patience.
Best not to use a steel face hammer. That can leave marks in the metal surface if you get a little over agressive while tapping or tilt the hammer a bit and leave some cresent moon marks.
A brass or copper hammer or even a wooden or rawhide mallet is best.

The abrasive paper/cloth to use can be anything from garnet paper used for wood sanding to any of the metal polishing cloths or wet=or-dry papers. (Use everything dry).
Nothing but practice and experience will tell you what grit and what particular brand or cloth or paper will do the trick and match the existing beadblasted finish now on the gun.

The Garnet (or even flint) wood sanding papers wood well but break down rather quickly. Not a great disadvantage on some surfaces and finishes in that the smaller broken down particles still impart the necessary look when impacted. It could be an entirely different story on another job and look completely out of place.

The carborundum strip metal abrasive cloths work well. Tear off a piece and straighten out the tendency of it to roll back up.
Start hammering the back side of it with the cloth held just off the metal surface. Keep moving the cloth around as you hammer and keep feeding new cloth with new abrasive area under the hammer as you work.

Work from multiple directions to avoid a pattern to develop in your work. Start with the finer grit and end with the heavier to bring the look up to match the original. If you start heavy, it's hard to reduce the look back to a finer matte finish to match as you work.
During the process it helps to use a medium scotchbrite or even steel wool to lightly go over the worked area to cut down any heavier turned up burrs or spurrs from the particle hammering. These can be almost microscopic but will disturb the overall look and stand out plainly if not toned down.

Doing this type of work is kind of like fixing the little scratch in the wood finish of the stock. It starts out as a little fix-it,,then gets bigger as you try to blend it in better and before you know it you can have half the side being re-done trying to match everything in.

Go slow, go easy on the hammering and start with fine grit and work up to heavier to blend with the orig finish.

When in doubt...disassemble the gun,,mask off the area carefully. Take the frame to an auto body refinish shop and ask them to low psi glass beadblast the surface. Takes about 1min.