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Yep. We've been cooking two or three 20-24 lb turkeys every thanksgiving and xmas for several generations, yes, a 20 lb turkey fits in the oven and the smoker.
I did a 24 lb last year and it came out fine, the temp was between 300* and 350* for seven hours and it didn't dry out and no I didn't brine it, I just put a piece of fat back on the rack above (over the breast) and let it drip onto it. I use a propane smoker with a drip pan with hot water. This year will be doing a 20 lb the same way I've done them in the past with no problems. Just my .02
Smoking large birds at low temps is dangerous because many times it will not get out of the danger zone in time. Thats not saying if you do it you will make everybody sick you may get away with it once or a hundred times but that one time you don't isn't going to be good.
Smoking large birds at high temps is a bit different they usually will pass through the danger zone just fine at temps of 325-350 just like in an oven. You won't get as much smoke in the bird before it gets done but you will keep your family/friends much safer. Personally I find the amount of smoke at the higher temps to be good and evidentially the people who eat it do too by the requests I get to do them