Although there are several different rifles/calibers I would be very happy using under the conditions mentioned, for big game up to the size of large moose, in heavy timber/thick brush/swampy areas, inside 100-125 yards... the gun I would often reach for would be this:
Stainless steel Marlin "big loop" 45-70 carbine with standard factory-installed ghost ring open sights and/or low magnification Leupold scopeMy choice of load would more than likely be this:
Buffalo Bore 350 grain at approximately 2,150 fps (2,070 fps from the stubby 18 1/2" carbine barrel)This load is a bit much for deer but is an excellent moose and big bear load. It is also quite proficient in dispatching those pesky wild bovine such as Asiatic water buffalo.
All this having been said, I'd be just as comfortable with an old bolt action 30-06, souped stoutly with 200 grain Nosler Partitions or 240 grain Woodleigh Weldcore bullets. If big bears are not in the area, hell, just your typical 180 grain "
name-your-brand" bullet will suffice... or a handy little .308 Winchester with almost any bullet over 150/165 grains.