There are no rolls:
It grants protection +12 (more if you mess around with it).
So how do you do enough damage with a pistol using the MgT rules to kill someone protected by such a device?
Buried on page 27 (well not buried, it's a column-wide text box, but still, obscured in a supplement to a supplement) of the Mercenary Specialist Forces book is a section titled Snipers and Assassins:
If the criteria for a one-shot takedown are met –
the target is unaware and not moving much, and
the attacker is free to act without distractions
such as being under fire – an aimed shot can
be made at DM-2. If this shot hits, damage
is increased by 1D for every point of Effect.
Now this would only work if Strephon were distracted and looking the other way (and Iolanthe and Ciencia - maybe doing some Vilani-ish ceremonial thingy - or more likely only Strephon was using energy armour, 'cause maybe there's an interference effect if multiple shields get too close) and of course the Aslan ambassador would never insult the Emperor by wearing anything other than extremely elaborate but tissue-paper thick ceremonial armour.
Would also help if Dulinor had a smart gun, lots of skill and maybe a smart bullet or five to boot.
Or, this could all just be a way to say the assassination was botched - welcome to the Lorenverse.
(Which wouldn't hurt my feelings much - the Third Imperium could persist nearly another century, if able to withstand the 'secondary effects' of the oncoming Wave, mainly hordes of Vargr and Zhodani heading rimward...
(Plus, for some inexplicable reason, I always liked Trin, and got a bit pissed about that little incident in 1139 (I thought it was '35, but the wiki says '39 and I don't have access to all of my
horde piles selection of books right now) @%*!# the Virus!)