Different sources say different things. Some have said Eric and some, Dylan, as in this 911 extended transcript  while other sources like this Jeffco ‘Finding of Library of Events , simply mentions  ‘a witness heard one of the gunmen say’  This seems to be because it isn’t perfectly clear as to who said what. The 911 transcripts vary too.  The one I mention above that lists Dylan as saying it doesn’t even have the first interaction they make with hiding students upon entering the library where you can clearly hear the two say on the very indistinct audio clips  Eric: “Get up!” Dylan: “Eeeeverybody get up NOW!” 
Many of the students in the library didn’t really know specifically which dude said what in the library let alone when they’d first entered into the library. The 911 audio tape that’s on youtube is tbh, a really horrible quality and I wonder how much of a clearer version Jeffco actually has/had in it’s possession. I tend to think not all that better. Also, their 911 transcripts are really incomplete and sketchy. 

As I said, many of the students were confused as to which gunmen said what. Only that they appeared in the doorway through a cloud of smoke – probably from Eric’s shotgun and the pipe bombs and crickets tossed just outside the library door. The fire alarm goes off soon after because of the smoke billowing in the air..

The reason I tend to believe that Dylan is the one who’d said it is in relation to the testimony Bree Pasquale gives in conjunction with Evan Todd who was the very first person to be shot but only injured.  Todd mentions that Eric is the first to fire into the library (from just outside the door) at Todd who is hiding behind a pillar at the front desk.  Eric doesn’t speak just notices Todd; the two make eye contact and Eric racks his round and fires two from his shotgun at Todd who ducks and the damage hits the pillar and slightly injures Todd. According to Bree, Dylan enters into the library slightly ahead of Eric and this is when both begin to yell to everyone to ‘get up now!’ at which point Dylan would then logically say in response to nobody cooperating and doing as they commanded “fine, I’ll start shooting” and would then be walking towards Kyle, who was supposedly very visible not hiding too well if at all, and aiming his shotgun at him. So , Eric shoots first and injures the first person from outside the library and Dylan shoots first within the library and kills the first library victim, Kyle Valesquez. It would make sense that Dylan said ‘fine, I’ll start shooting’ once they’d entered and no one did as they demanded and got up. Unfortunately, there is just no crystal clear way to verify and determine exactly everything each boy said in the library because the evidence released is sketchy.

While I agree with you that Dylan’s weapons of choice were a bit shitter than Eric’s (I don’t know about ‘10 x worse’ as you suggest), I don’t feel it is the only viable reason for his lagging kill count.  It seems a combination of Dylan being more interested in wrecking havoc, terrorizing and raising hell as well as him discovering rather quickly that his selected weapons didn’t perform as efficient or ‘cool’ as he had fantazied about in his head.  For example, outside, Dylan’s non factory 50 magazine for his 9 mm Tec supposedly jammed and was dropped on the grass still full of bullets but he still had other clips, two 36s and a 24. I think there was even a magazine found in his BMW that I recall. If he was eager to kill, he still had time to jump right in, reload his Tec 9 to get in some more warm-up target practice but for whatever reason, he didn’t do so. Instead, Dylan chose to mix it up by messing around with the arsenal at his disposal: he chose to wander off near the parking lot and lob pipe bombs. According to record, he tried a mere 2 rounds of his double barrel shotgun – and likely realized it was a horrible distance shot on running human targets. He spent a lot of time going up and down the stairs near the commons and was the most scene by witnesses through the windows. He was the show off presence that shot Lance in the jaw within an audience left to gasp in horror at the windows. It was reported that Dylan even walked in the commons as people were fleeing in a panic up the cafeteria staircase and yet, he refrained from shooting or throwing any of the incendiaries at his disposal.  In the hallway, he got in 31 rounds of Tec 9 mm target practice and 4 shotgun rounds but he spent a fair amount of time taking it all with the journey and not the destination: enjoying the site and sounds of destructive impact on Columbine’s interior, running down the hallway chasing screaming students and shooting at the walls and the lockers along the way. He happened to maim a couple of students including aiming, for whatever reason, super low and succeeding a blast on Stephanie Munson’s ankle. Ok, yeah, they’re moving targets and somewhat long distance but an ankle just seems silly.  And again, Dylan didn’t wander into the bathroom to hunt down the girl who ran from the payphone. Maybe he hit his limit in murdering etiquette that a gentleman hitman does not hunt down a girl in the girls restroom?  lol 😏 

Inside the library there was a lot of opportunity for close range kills especially with the shotguns.  Dylan succeeded using his shot gun straight off the bat and wounded Kyle Valesquez’s head and shoulder with deadly accuracy, killing him instantly.  He started off boldly in the library, as if he were electing to set the tone there by volunteering a  “fine, I’ll start shooting!”   Thereafter, Eric spent 21 shotgun rounds and the library yet Dylan only a total of 6.  Dylan 21 9 mm rounds and Eric 13 with his carbine.  I’m not really certain if Dylan’s Tec jammed there, I don’t think so?  At any rate, at such close range, you’d think he’d have had better chance at success.  Did he perhaps spend a fair amount of time shooting glass display cases or blasting holes in computers and tv sets to supersede Eric’s 9 mm count by 8 ?   For some reason, Lauren was the only one Dylan went hog wild on drilling Tech bullets into.  Yet, all in total in the library, at close range with sitting ducks under tables or half tables, Eric manages to surpass Dylan at the kill count. Most of Dylan’s kills are in tandem with Eric on a person, even.  In the library, Dylan was said to be the more vocal one, whooping it up, hollering and having the time of his life and Eric more quiet and methodical.  He spares Valeen after taunting her. He inflicts his verbal wrath on Evan Todd, yet let’s him live and gives him to Eric even, with a ‘you can have him..if you want.’   All in all, to me, it seems that it wasn’t just that Dylan’s weapons sucked but that he was simply less motivated to murder with deadly intent and accuracy and more interested in terrorizing and enjoying the power trip.  Here’s a total shots diagram for anyone interested.