Hi all, I'm about to start a doom metal project using my Epiphone LP Custom. I want to downtune as low as possible and have no idea what gauge strings I should put on. I've dropped the entire tuning 2 half steps with my standard 10 to 46 Regular Slinky's and it sounds good and I'm just getting a little buzz from my low E.

Can I realistically drop 4 half steps to C-F-A#-D#-G-C without going buzz crazy and what gauge would be best for this?

"Music is the only religion that delivers the goods" - Frank Zappa
I tune to C Standard (like Matt Pike) and use 10 gauge strings. would not recommend for everyone, because I like the loose tension (I don't have problems with buzzing). I would say throw 11's on there, should be set.
I would say 12/60, if you like something looser maybe 11/56...
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I play 12-54s in drop b but thats on a 26" scale. you may want to just try out a bunch of different gauges
Regular gauge 7 string set. Throw away the .010 string and you'll be set. This is if you don't mind having a wound G (D# in this case) string.
"Air created the greenness. And once you've got something, that leads to otherness." - Karl Pilkington.

Look at page 6 and find the low E gauge and tuning you use. That is your current tension. Then look at the tuning you want and find the same tension. That will be the gauge you will want to use.

For EX:

A 46 tuned to E = 17.5 lbs
A 56 tuned to C = 16.6 lbs
A 59 tuned to C = 18.5 lbs

So to keep the same feel you would use either 56 or 59 low E string.
D’Addario .013 or .013 baritone sets can be ordered for less than the cost of 7 string sets. Really good guitar shops sometimes stock them, but the big chains don’t.
Thanks for the great info everybody...this should get me started!
"Music is the only religion that delivers the goods" - Frank Zappa