Since you are going to use PP bullets let me give you my best advice to save time, lead and powder to work up an optimum load. "size the bullets to fit your bore" That means you need to slug your barrel and get the tightest bore and groove diameter. You need to have a push thru sizer die like Lee makes to size the bullets after patching. A PP sizer die needs to be .001 smaller dia than a GG die because they don't size down quite as much in a given die. Your sized bullets should be from .0005 to .001 under bore diameter. One final thing---cast or swaged, make your bullets out of pure lead.

That underrib and rod 'may' cause some strange barrel vibrations for a target rifle. Years ago many of the target guns were made without them. Most of the guys here in matches remove the rod. If you must have the rib install it with screws onto the bottom barrel flat and you can take it off too.

I have 'The Muzzleloading Caplock Rifle',........any one in particular you are emulating?

Welcome demac......I too have built a Rigby style sporting rifle with 1:18 twist, ebony nosecap,etc.  Mine has no provision for a ramrod but have not found it a handicap when hunting from a blind. 2 bucks have met their demise so far. I seem to be the voice in the wilderness around here about using grease groove bullets. My Gibbs and Rigby absolutely love them. Don't be afraid to try them if sized .001 under bore diameter and lubed with alox or SPG.. Also don't be afraid to use 30:1 or 24:1 alloy. A Postell or creedmoor style bullet would be a good choice to start with for target work. For hunting I use the tried and true lyman 500 grain Govt.  bullet sized down to .450 and lubed with alox. To say that they zip through a deer is an understatement.  Pictures of your project [and Z-man's] and updates on your load development would be interesting to us.   cheers    Paul

how do i post pics

Crank....sorry for the tardy response. The as cast diameter of the Lyman govt. is .458+. I size them down to .450 in a basic RCBS single stage press. The alloy is 30:1. They come out looking surprisingly good without undue effort on the press arm. As I have mentioned before, they are lubed with alox which I have found to be an excellent M/L lube, [cleaned between shots of course.] Even with the grease grooves reduced, the bullet still has enough lube to leave a faint "lube star" on the muzzle. For those of you who don't know what a "lube star" is, I will explain. When a lubed cast bullet leaves the muzzle of a rifle, there is a faint outline of lube left deposited on the muzzle. It actually isn't star shaped, but does show, more or less, the outline of the lands and grooves as the expanding gasses blow sideways out the muzzle at the instant the bullet exits. If the "star" is perfectly round and uniform, it indicates that there is a good crown on the muzzle and the bullet carries enough lube. It can easily be checked by simply looking at the muzzle after firing. It wipes off easily with your finger. However, if the star is out of round it indicates a damaged or poorly crowned muzzle, or a bullet that came out tipped slightly allowing gas to escape on one side of the bullet the instant it exits the muzzle. If there is no lube star, the barrel might be too long for the amount of grease carried by the bullet,  the style of bullet you are using doesn't carry enough lube, or you're just using the wrong lube for your application.  Leading near the muzzle could result in these instances. Hope you didn't find this too long winded.  cheers   Paul

Pat....The .458 bullets are already lubed with alox in a .459 sizing die and stored away. When I need a few to size down to .450 I just pull a few from inventory and go to it. I just thought of something. Maybe having an incompressible semi-solid already in the grooves keeps them from smearing during sizing? At any rate, being pre-lubed makes them easier to resize.  I don't use any type of wad in the Rigby or the Gibbs. I got better groups without them.    cheers    Paul