There are several ways to use ironpulling that we don't see.
For example, you could use ironpulling combined with a sling to get a lead bullet up to ridiculous speeds, and use your allomancy to help time the release precisely. Needs more skill than just shooting coins, but slings IRL get ranges of almost half a kilometer. You could probably shoot with more force than your average coinshot
The idea is you can't wear full plate of the required thickness - it's too heavy. But if you just wear a single chest plate that's four inches of solid steel, that's entirely possible to carry.