TBH I don't think heavier weight will really reduce stamina (or affact in a bad way at all). But like MJ9 said. it's weight distrubution what affacts it's stamina and I think heavier weight can keep them keep spin velocity longer
for example. Virgo is heavier then Burn but it has better stamina due to having better Weight Distubution
as for magnum. Tbh I doubt there is anything chosing it over other Top-Tier disks. It's quite light and most of it's weight is toward center (which isn't always bad btw) due to most of it's weight is that metal part.
if you compare it to other disk
for light weight (burst defence see
this for more info on burst defence):- Ring, Armed are much better
For Stamina:- Ring, Knuckle, Heavy, Gravity and Spread are much better due to there weight distribution
For defence:- again Heavy, Gravity, and Spread are better (in some cases Armed if you want more burst defence)
for attack:- you would want your attacker to be heavier so it can do well against other attacker and suffer less recoil so Heavy, Gravity, Knuckle, Spread are better
for Synergy (with attack layers:- this is the place where Magnum can shine due to it being 3 sided like V2 and Valkyrie which are best attack layer but again it's 3 sides are plastic and has very less weight of there own so Triple is better choice
Well it still has some chance to shine on destablizer or something but it hasn't been tested much so we can't say till someone test it. but I think for stamina Polish and for Defence Jerk would be better as they have more useful pastic part (polish can provide LAD and Jerk can block attack from bellow)
I haven't did formal test but so far I found Jerk to do well.
Once I get bunch of other parts (and some time from studies lol) I will post some tests)