Unfortunately, there's the rub - hardness alone is not the whole problem.
You might have high surface hardness, but less through the meat of the part. This would be good for wear, but might not hold up against shear. At the opposite end, a fully through-hardened piece of steel might become too brittle for some uses, and shatter under load. The wiki article on
heat treating is just that - but if you're bored, it can help you start to see some of the complexity.
If you want to just get a ballpark, this is
the cheapest set of hardness files I could find on Amazon in a quick search. For 70 bucks you could scratch everything you own for the lulz. Maybe
@warwickben or
@Boris has a set, and can be convinced to test your barrel*? You could go so far as to test one of your other barrels at the same time just to compare. Note that this method only tells you about the surface of your part...
*dinner and a movie first?