Short barrel Ruger revolvers are notorious for this; they install a too-short front sight at the factory, and this is the result. It will be even worse with heavier bullets. You can return it to Ruger and ask for a taller front sight, or you can order a taller front sight blade and install it yourself - the blades are pinned in place on the stainless guns. Those are your options.
Personally, I would call Ruger and explain the problem, and politely request a replacement sight blade. They'll probably send it at no cost.
And personally, I think you should slug the bore again. It's certainly possible that there is a constriction under the frame threads, but you need to verify that. Drive one slug all the way through the bore and mike it. Slug the bore again, but stop short of the frame threads, then push the slug back out the muzzle and mike it. Compare the two measurements. Fire lapping is one solution to that particular problem.
