At i240 MCH dummy dps should be more or less the same as BRD, but if you stop at 3 mins before doing that Wildfire it’ll probably be under that. The price of being bursty…though I can’t remember the last time I did a burnout dummy attempt 😛 I’m guessing that’s what you mean by last minute?
It’s hard to say how to improve with only this info, but I will say that GB toggling can harm your dps if you try and do it too frequently, and ultimately it is still a VERY small gain even if done right, so there’s more to this picture (it’s mentioned often but it’s still a very min-maxy thing)! While you can toggle when all oGCDs are down and Lead doesn’t need refreshing, I would still avoid doing it if any oGCD/Lead will be needed in 5/6 seconds. Generally you should put GB back on after one GCD+Auto attack, so you’ll want to allow time to weave it back on very shortly after. That said, I’d leave GB on until identifying improvements and then focus on practising GB toggle if you’d still like to.
Things that come to mind, you probably know a lot of these, but we have limited info.
*Are all your buffs (HE/B4B/RS) active before using Lead Shot?
*Are you using the 5 ammo opener? If so this can make your 1 min CDs line up oddly later on
*Are you using Reassemble along with Clean Shot with Ammunition?
*Are you using Rapid Fire on at least 2 Split Shots during Wildfire, to eliminate cast bars?
*Are you saving B4B, HE and Rapid Fire for every Wildfire (everything else is free-use, generally)
Hmm, can’t think of much else off the top of my head.
For guides, I like Outof Names’s guide for being pretty short and succinct
Also as a shameless plug I recently made this jam-packed cheat sheet of sorts. Tried to include as much essential info as I could, but it is ultimately designed to be a single-image “guide” and doesn’t go too much into reasonings and such.