Kind of new

[babygotstr12]

I've been spearfishing for a while and purchased a Rob Allen 120 about a year ago, but only used it a couple of times and really liked it. But i noticed that in and out of the water, there is alot of play in the shaft since there isn't anything holding it down to the rail. Like when pointing the gun downward in water, the shaft would pop up to the top of the closed muzzle. Leaving a lot of play! I began to think i was stringing it wrong and such, but just found out that it's normal kind of. My friend said he thinks i have a 7mm since there is so much play, should i go ahead and get a 7.5mm or just have it converted to an open muzzle?

all help is appreciated, like i said i'm new so don't "rail" on me too hard. lol

Thanks!