The Rifts Conversion Book on page 10 had a section called "Dodging and Parrying Energy Weapons" and its 7th paragraph (after the old "O.C.C. Dodge and Parry Note") mentions:
The Crazies, Juicer, Borg, Simvan Warrior, Cyber-Knight, Dragon and those operating power armor or bots (excluding the Glitter Boy) can attempt to parry a bullet or a single energy blast if they have a suitable item/shield with which to parry
Interesting pluralization dichotomy between the first two... it doesn't go into much detail about what items besides shields are suitable for that task, but in the combat example on page 11, in Five: Antagonist Y shoots back with his Coalition laser pistol (his third attack or melee action) Antagonist X parries using a vibro-blade, which sets the example that it is acceptable to use vibro-blades to parry energy blasts. I'm guessing bullets too, since lasers are faster and possibly narrower than a bullet.
I think it would be interesting to zero in on the ramifications of a Glitter Boy's inability to parry and what that means for them. Namely, when they are battling any of these opponents... can you parry a boom gun blast?
It seems to operate like a single shot, but should we get particular with the word "bullet" and exclude rail gun rounds and not consider them to be bullets?
Page 10 gives a list of things which can't be parried:
Grenades, missiles, explosives and other area affect weapons/damage cannot be parried
Rail guns are conspicuously absent from that list for such a major weapon in the RMB. Rail guns do not appear to be treated as area affect weapons. This leads me to think they would be considered 'bullet' weapons.
Most rail guns fire bursts so they wouldn't be considered 'single bullet' but the way boom gun rounds get treated, they do seem that way, even if the round does flechette into 200 pieces at some point.
Does anyone recall if we're told at what point in time it splits? Is that when it is fired, or when it hits? If the former, it would seem to be too spread to be counted as one bullet (sort of like shot from a shotgun) but if it's the latter, then if it split after a parry redirected, it wouldn't matter too much.
"Disadvantages Playing a Glitter Boy" on page 15 of the Conversion Book did reference the boom gun bursting...
if the Glitter Boy has engaged the recoil suppression system, which engages in an instant, and is thus properly secured, he can fire one accurate and devastating burst after another (+1 to strike for an aimed burst and an additional +2 boom gun bonus)
The RMB 221 illustration is labelled "100 rounds of 20,00 slugs" then "50 slugs with 3 more sets of 50 behind them" and describes the AMMUNITION as having a 7" shell casing with a fall-away sheath "holding 200 slugs".
So I suppose each shot of 1 round could be considered a burst of 200 slugs... but if you want to play that way (counts as a burst, so it can't be parried) then using the +1 to strike for aimed bursts instead of the +3 to strike for aimed shots should be mandatory.
This would also mean, per the conversion book's updated rules on Shooting Wild (page 9), that anybody without the WP Heavy needed to use the Boom Gun, would fire Wild (-6 to strike) with the Boom Gun. This wouldn't harm the Glitter Boy OCC but would harm any random person who piloted it... which doesn't seem like a bad thing.
Conversion Book Revised page 19 "Parrying Energy Blasts" appears to have narrowed the possibilities. It made you -10 to parry at less than 400 feet and -12 to parry at more than 500 feet, so the only place you could parry without penalty is the sweet spot from 401 to 499 feet. Given that the section is about "Energy Blasts", I don't believe it applies to bullets and so revised would have to defer to the original conversion book's rules on the list of people who can bullet-parry.
Page 33 of the Game Master Guide does have this:
Are there any types of physical attacks you cannot parry? Like a dragon's bite?
Definitely. A parry is generally used to deflect or physically block an attack, so if an attack can not be deflected or blocked, it cannot be parried. This means that falling boulders, energy blasts, bullets, etc., cannot be parried.
Since it uses the plural "bullets" though, it actually does not contradict the Conversion Book, because it only ever allowed a SINGLE bullet to be parried. Multiple bullets (bursts) were never parryable.
Same with single energy blasts, this does absolutely nothing to contradict that since it does not say a single energy blast cannot be parried.
For the same reason, you should be able to parry a single falling boulder, assuming you're reasonably strong enough. GM judgement is always required on what's realistic. Like RAW a 3 ft gnome with a PS of 3 could parry a power punch from an NGR Devastator, but that's a case where GM should intervene and say "no, that doesn't seem plausible".
A Devastator could easily parry a falling boulder, but a normal human couldn't due to the weight, GM judgement would be needed.
A Devastator could not parry multiple falling boulders though, because that would work like a burst and be too many to keep track of.
Apparently the only multi-strike that you can parry is melee attacks, since a Quadruple-Strike from a Warrior/Super-Warrior from Xiticix Invasion can be blocked with a single parry.