One thing to remember, muzzleloader bores are measured from the lands, center fire bores are measured from the grooves. So, if you have a .501" land measurement such as quickdtoo, that is a very common .50 caliber muzzleloader bore. Most .50 calibers measure from .499" to .504", with T/C and CVA's mostly in the .500" to .501" range, Knights in the .502" to .503" range, and Whites in the .504". The fast twist of the .500 S&W Handi at 1 turn in 18.75" might be a little fast for most bullets, other than long heavy conicals or saboted bullets like the 435 gr SSB from Buffalo Bullets. Rifling should be about 0.0045" or so deep x 2 = 0.009" + 0.501" = 0.510" center fire bore diameter. The Huntsman muzzleloader nominal bore is listed as .501" at the lands, and .510" at the grooves. Disregard the .45 cal Huntsman info in the below pic from my 2003 Owners manual, they never brought it past production, but do look at the nominal bore measurements for both calibers. ALL of my H&R/NEF .50 caliber bores actually measure .502", just like my Knights.
http://i15.photobucket.com/albums/a377/rburns_/Recommended%20Loads/018.jpgSo, the .500 could make a good muzzleloader, but I would agree that the .45-70 with the 1 turn in 20" is better suited, and a proven twist. My .451 caliber White uses a 1 turn in 20" twist, and shoots lights out. I have a Kinght Super D.I.S.C. barrel that is a 1 turn in 20" as well. I purchased a 24" Stainless Steel .45-70 barrel with the intention of making a nice .45 caliber muzzleloader barrel for my Huntsman, but have had too many irons in the fire to get this off the back burner for this year.
Here is some information about my .45-70 barrel with the 1-20" twist I copied from another post I did a while back. Can't rely on memory anymore, CRS ya know.
The bore diameter is the same as muzzleloaders, at .4515" cal. The groove diameter is .4575".
Just remember, centerfire rifle caliber measurements are measured at the groove, such as .458". Muzzleloader calibers are measured at the lands.
I have slugged the bore, so I have exact measurements of this barrel.
The .451"-.453" conicals will do just fine, and the saboted bullets fit the bore very well too. I also have a White M-98 Elite (1:20) that has a bore diameter of .4515", a Knight Disc Elite (1:30) that is .4505", and an old Knight D.I.S.C. (1:20) that has a .453" bore.
If you are just looking to have a .50 caliber H&R/NEF Muzzleloader, there are several used ones for sale out there that you don't have to go through the work of making one. The Huntsman barrels will fit the late model SB1 and SB2 frames, and the Sidekick is a stand alone muzzleloader. I have one .50 caliber NEF Huntsman, a .50 caliber NEF Sidekick, and a .50 caliber H&R (branded) Sidekick, all in Stainless Steel. The Huntsman is my favorite muzzleloader of 13, and that includes my Whites, Knights, and T/C.
So, with all of that said, the .500 S&W Handi barrel COULD make a good muzzleloader, if the 1-18.75" twist would work for the type of bullets you want to shoot. IMO, the best rifling depth for muzzleloaders is between 0.003" and 0.0045" deep for saboted bullets and conicals, with all of the above in that ball park.
Keep us informed, if you go the .500 route.