Ported Barrel

A few factory and numerous aftermarket smiths provide barrel porting on handguns, rifles, and shotguns. Properly done the ports will reduce muzzle jump to varying degrees depending on caliber, barrel length, and port design/efficiency. Recoil reduction is claimed, too, but that is not as effective as the reduction in muzzle jump. Probably dangerous only if you put your hand over it while shooting or standing too close to one side.
Off the top of my head factories that have offered porting are S&W, Taurus, and Springfield Armory. Gunsmithing shops are nearly limitless with one of the best known being Magnaport. I do not recall any specific shop offering only horizontal ports. Post clear closeup photos of your and perhaps we can identify who might have done it.

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Not dangerous, but loud and dubiously effective. A compensator would be preferable in my opinion. There is some minor loss of velocity with ports, not really consequential.

For porting: Magnaport

For comps: lots of options depending on revolver.

For a discussion of pro/con (albeit in s/a pistols) and list of comp makers: https://www.shootingtimes.com/editorial/recoil-reduction-ports-vs-compensator/99220.

I have Taurus 44s 1 is ported the other not, there is a difference in noise between the two, I haven't tried shooting fast to see if I can get back on target faster, 44 mags I usually shoot slow. The ports sometimes is hard to clean and leaves soot on top of the stainless barrel. This is after 12rnds

Be sure to wear eye protection, I bought a Glock 23c at an auction and it had the ported barrel and slide. I don't really notice any difference (I bought an aftermarket 40S&W barrel and also have a 357SIG barrel for it). It's not so bad outside, but inside, it seems to blow gasses and burned powder back in my face a little, enough that glasses are absolutely necessary.

Porting is done by using an EDM machine, if the ports are trapezoid shape it was done by MagNaPort ai have 2 revolvers done by them and the porting does help reduce muzzle flip but does increase muzzle blast