I picked 44 magnum as I have many rifles and this would be different enough yet still fill the role as a backup or loaner rifle for my night time hog hunting. I have several bullet molds, one in particular that throws an as cast diameter of .433 and 2 boxes of 240 grain XTP bullets. My plan really is to use the 250 gr. keith design cast bullet at 1300-1400 fps, figuring recoil won't be bad and it will still kill hogs at less than 100 yards. My next scheme will be to get a 1 in 9 inch twisted 223 as I have a bunch of 60 grain Hornady HP's to shoot up. Just how much trouble is it to send your reciever into the factory to get a barrel fitted?
They've been putting the Williams WGOS sight on them for several years, before Marlin started making the barrels with tight bores, if yours was made before 2007, it will likely be overbore and won't shoot the jacketed bullets well, particularly lighter loads, the 1:38" twist just isn't friendly in combination with the loose bore. Those that were able to get reasonable accuracy went with .432" or bigger cast bullets, or pushed the 300gr XTP real hard with 110 or 296. You can find the build year info in the FAQs and Help sticky, although it could still have a late model accessory barrel on it which would be a good thing, slug the bore and go from there.

As far as getting a barrel fitted, read the Ilion Satisfaction Poll sticky, those that have had barrels fitted recently have been happy. Instructions for getting one fitted are on the Accessory Barrel page at H&R's website, you can save the order form that's in the FAQs and Help sticky here lots faster than trying to download the form from H&R, then print it. My preference for getting an accessory barrel is send the frame only and order a stock set at the same time, the frame can be shipped to them in a USPS Priority video box for about $10, insured with deliver confirmation. You may want a monte carlo stock for the 223 barrel if you're gonna scope it, depending on which barrel you order.
Tim
http://www.hr1871.com/Support/accessoryProgram.asp