I've heard National Park Rangers give the effective range of the Brown Bess (standard British musket of the American Revolution and the Napoleonic Wars) has being 40 yards. I've also heard of testing by the British army which claimed that a man-sized target could be hit with some regularity at 100 yards.
I've never fired a smoothbore musket and defer to your experience, but it is important to give a little more information regarding the target size.
Target size at 125 yds? How about a dinner plate. We use a knockdown target with a 10" plate in the kill zone of a bison sillouette. If you don't get the hit the plate doesn't go down.
Also you must remeber that smoothbore muskets were anywhere from 58 to 76 caliber roundball or anywhere from double to triple the size of a 45 caliber wadcutter. That much lead running at over 800 ftpersec would hit a man and could rip his arm or leg most of the way off of his body...
40 yds... I guarantee that you wouldn't want to face a Brown Bess at 40 yds...
There are a couple of differing points of view here: those of an individual firing on another individual, that of an individual firing on a mass or ranked target, or against horse and rider, or that of a formation of troops firing on another formation, in which the volley fire of massed muskets becomes formidable, indeed.
But the real value of a musket was double-edged: as an extension of the bayonet that allowed a single shot upon closing with a row of enemy troops, using that single shot to open a hole in the defending group that could be exploited with point and butt; or in close quarters such as scrub brush or thisket in which a horse and rider could not operate, but from where a few gathered foot soldiers could bring them down.
Can I hit a man at 40 yards with my own frontloading smoothbores? Usually. And a rank of targets in a row is no greatly difficult feat, so long as I have my elevation about right; I'm more likely to hit one than miss, and a hit anywhere on a horse from the broadside is probable and would probably result in unseating the rider one way or another; a shot front on or to the rear is more problematical.
But facing a row of trained soldiers with bayonets, while I'm in a similar sized group facing them? No thank you. I'd prefer to go and ruin the trip for their supply wagons.