I agree with DPhillips, the chambering depends on the type of singleshot in hand! I have owned falling blocks, rolling bolcks, breaktops, Martini drop blocks, side swing rook rifles! All had their place in the field.

The Farquahrson is most usefull chambered for one of the .450 class of NE rounds, though many have been made in
larger, and smaller chamberings. Very few of these are converted to other chamberings, as they are vertually all collector items.

The Ruger No1s only come from the factory with the largest chambering of 458 LOTT, but these have been barreled, and chambered for all the NE rounds up to, and includeing the 577NE 3", and as low as .22lr. The No1 is the strongest of the production singleshots today, and are the most popular for conversion.

The old Remington rolling block has been cloned by many makers, but is seldom seen in anything bigger that the 45-70 Gov. A few have been chambered for 458 RCBS, and was the reason the 458 RCBS was made with a 2 3/4" case, rather than the full 3" 45 basic, or 45-120. The long case wouldn't chamber over the rolling block.

The Sharps, and the clones have habitually been chambered for 45-70, and the verious Sharps cartridges, and are very popular for hunting North America, and target shooting.

The falling blocks like the Winchester high, and low wall, and the clones of this, and the Ballard pattern are rarely used for conversion, but are popular with some folks.

Lately there has been a flood of reasonably priced breaktop singleshots, in some pritty good deer cartridges, and are becomeing very popular with folks buying rifles for their kids to learn to hunt with!

I believe you will find the most activity around the Ruger No1s than any of the others.

I have Ruger No1s chambered for: 243, 7mm mag, 45-70, 458 Win Mag, 458RCBS,with some of those in more than one rifle, and have owned them chambered for 450NE 3 1/4", 450/400 3", 500/450NE.
The 458 Win Mag, along with a Pedersoli double rifle are slated to be re-chambered to 450#2NE, soon.

One shooters make the first shot count, as a rule, and brings back the almost lost art, in many places, of stalking to get within the right distance to make that first shot count! Single shot rifle make sharp shooters!