Info about magnetic sites that fit on a ventilated rib?

I asked a question similar to this a few weeks back. I bought a pump shotgun that is a Maverick made by Mossberg. I bought the gun off of my uncle, which is pratically new, and also got 3 screw in chokes for $120. When I bought it, he told me that it shot a hair to the left; I tried it out at about 25 yards with the full choke in, and they pattern was 4-5 inchs to left; It did hold a very good pattern though. The first bead on the ventilated rib is br, like it's supposed to be, but the black "tip" on the end of the barrel, is not the proper bead for this gun, as I went to Wal Mart and looked at the exact same gun I bought. The bead on the end of the barrel is supposed to be a plastic, oblong, white bead, not a black tip. It's obvious that the original bead has been broken, and this "home made" bead was put in it's place. Well, the black tip is offset to the left a hair; This is very obvious when u look down the barrel. In my first question about this problem I made a few weeks back, 2 people gave me links to a magnetic site, made by TruGlo or HIVIC. What I want to know is, if anyone out there uses these sites, and give me any info about how good they work. Thx in advance.