Thompson Center introduced the "Cleland" Match Hawken rifle as a precision grade .40 caliber cap lock rifle intended for serious target shooter.
Of course Thompson Center traditional type muzzleloaders have not been made for several years now and I'm not certain how many Clelands were produced. However it does beg the question, why T/C picked .40 for the Cleland. They must have considered the accuracy potential of .40 caliber, I'd think.
I've picked up a few .40s over the years. I have an Ozark Muskrat .40 finished from one of their kits. I shot it very little until recently. The way powder prices have soared and we've experienced shortages I have been moving from my .45 and above bores to the smaller, .32 to .40.
My pushing 70 year old fingers make handling, when loading, .32 and .36 caliber components a lot tougher than my .40s. I'm really coming onto this .40 caliber thing. Accuracy has been outstanding, mostly 25 yard shooting. I've done limited shooting at 50 yards however I've seen the potential as I'm confident it will shoot well at that distance.
The Ozark Muskrat is not my only .40, I have a couple by the late Ted Fellowes. In my Muskrat I shoot a .395 ball with a pillow ticking patch. Mr. Fellowes used Green Mountain barrel in his guns and for those I run ball up to .400 out of an old Green Mountain mold.