FWIW, I have several New Englander's and Black Mountain Magnum's, 50's and a couple of 54's. We use them for deer hunting and otherwise shoot them a fair bit. The #11 caps is all we use, both regular and magnum, and ignition is not a problem as long as the cap is seated all the way down on the nipple. Frankly I have never noticed a difference between the standard and magnum shooting at the range. When hunting I try to use the magnums just because.
Our standard load is 100gr (measured) Pyro RS. When hunting in Oregon where we can not use the projectiles that must not be mentioned here, we use cast pure lead Maxi-balls, 370gr in the 50 and 435gr in the 54. As you know, these are pretty light guns. Recoil on the 50 is bad enough, when I shoot the 54 I make sure my fillings are tight before touching off. Never shot a 600gr projectile, and I think I would want the chiropractor standing by if I ever do.
As mentioned above, the cap must be firmly down on the nipple. When I first started shooting these (and my muzzle load shotguns as well) I would get an occasional hammer fall without ignition. Re-cock and it would always go off with the second strike. I started seating the cap by gently lowering the hammer and then gently but firmly pushing on the back of the spur. You can sometimes feel it push the cap a bit further down. I can't recall having the problem since adopting this practice.
Good luck on the piggy's, share some pictures with us.