I've shot up to a 460gr conical with a 1:48 twist with excellent accuracy. If you cant kill with that, take up golf! lol
The old story is that sidelocks misfire a lot more. Very possible with new shooters. What powder is being used plays a huge part in that. Most importantly, the nipple is what will cause a misfire rather than the powder. Inline shooters learned this when blackhorn209 came out. Its the same story with a sidelock shooting #11 caps. A poorly designed nipple with a sub black powder, can cause the shooter to have a ton of misfires simply due to the nipple not provided the right amount of flash to the powder.
I shoot both, but favor the sidelock a lot more. Much better triggers than any inline out there, especially since they are adjustable into the ounces.
Here's a semi custom 58cal CVA Hawken @ 100 yards with patched round ball and open sights.