3F Pyrodex is pretty reliable in a sidelock. T7 not quite as much. BO of course is the most reliable.

The ROA will shoot any sub except BH 209.

BO = BP?

BH 209 won't work well at all with caps will it?

Would it be bad practice to use a 5 grn starter of 3F Pyrodex/sub under 3F 777 (or any variation of powders)?

What makes subs, especially 777, less reliable? Is there a way to remedy this? And what exactly is less reliable (ignition, hang fire)?

Subs have a higher temp to ignite them. Not a problem with inline guns shooting 209 primers, but can be a problem with sidelocks shooting percussion caps.
3F of subs will work better in sidelocks as far as reliability is concerns, because of the smaller granules. It also buns a bit cleaner and more complete than 2F.

If you're going to prime a load for easier ignition. You should use real black powder. It has the lowest temp to ignite it. I've never done it, so don't believe it's needed. Keeping everything clean and clear should be enough for caplocks. I like Remington #11 caps for rifles, and Rem #10 for revolver/pistols. Everybody has their own opinion on this, but it's what has given me the best luck. I used to shoot CAS, and couldn't afford misfires.

If you were shooting a flintlock. You'd have to use BP for reliability. That shows you how close to failure you are with subs in a caplock. It will work, but you have to make sure everything is right.

I notice that Traditions gives a max load for their Deerhunter as 110 2F/105 3F. Not that they are the same rifle, though they seem quite similar, but I find it odd that Lyman's gives 110 also for 2F, but 90 for 3F. Are they just being cautious?

Yes, they're being cautious. You might not like the recoil and accuracy if you beyond that anyway.

What kind of a load is similar to a 12 ga dove load? That and a 270 Win are the higher end that I'm familiar with.

Since accuracy is as important as horsepower for hunting I'm certain I'll not be near max loads.

With a max load like our talking about, and the conicals you just mentioned in your other thread.

About like a H&H .375 mag. Maybe more.

Not really needed. I'll be shooting a 250 gr bullet with 80 gr of BH 209 for elk this year.

Last year I killed a cow elk with a .54 round ball and 80 gr of Swiss black powder.

I like to get close though. To me that's part of hunting, but especially ML hunting. (for me)

I'm curious if you have an idea as to how much power you are getting from that load you have used for elk.

I've had elk once, and given a chance to hunt them I'd have to give it a go if I could figure out how to pack it back out! Best tasting critter I've had. Right up there with axis.