I've been thinking about trying some cast bullets in my 9mm. I'm not really familiar with how hard they should be for an auto loader. I was looking at some hardcast bullets on GB and they were described as having a Brinell hardness of 18. Is that sufficient?

Personally, I'd be more concerned with bullet fit than BHN. Slug the barrel of your gun and load bullets .002" larger in diameter than the barrel's groove diameter.

More than sufficient, if they actually measure that. It is close to linotype hardness. It is probably harder than necessary. I too would be more concerned that your bullets are big enough.

I used to use lead bullets that measured as soft as ~ Bhn 10 in a few different 9mm pistols. I did not have any real leading problems, but did have trouble with accuracy. One gun in particular was a real nuisance with its factory barrel, but a new barrel fixed the problem. What little leading there was I found I could clean pretty easily with simple techniques like letting the bore soak for just a few minutes with wetted Hoppe's Nr. 9 patches and then brushing out. Rarely had to resort to the Lewis tool.

If you are lazy and don't want to slug your barrel (like me) and you start having leading trouble, try a larger bullet. I don't use lead much any more in 9x19, but when I did, all my bullets were .357" or .358". I remember it made a dramatic difference in my Beretta 92.

The BHN is fine, particularly if you are loading full pressure loads.

As others have said bullet diameter is important, larger will give best accuracy but some 9mm chambers will not accept ammo with bullets over .357. The fast twist in 9mm barrels often negatively affects accuracy and harder bullets, of the largest possible diameter that will chamber safely, are the answer.

I double checked just to make sure and they give the Brinell hardness as 18 and the bullets are sized to .356.

.356 could be too small. Most of my 9mms prefer .358, which chamber fine, and shoot well. Slug the barrel and be done with it. Seriously, you'll be happier in the long run.

Like said above, if they are actually 18 BHN that is more than hard enough for full power 9mm ammo. With bullets size does matter so if they are .356" that are probably the correct size for current 9mm pistol barrels.

__________________
Freedom is never free!!
SWCA #3437

There are nominal dimensions, then there are actual production dimensions, and they rarely ever match. For .356 to work properly without leading, the barrel bore should be .354 to .355. If the barrel is polygonal, all bets are off.

Proper fit is key for leading.

Hardness can greatly affect accuracy just like powder can affect accuracy. As hardness increases, accuracy increases, until it reaches its limit, and harder than that causes accuracy to drop off. Just like powder, there is no magic number for hardness, as each gun will be a little different than another. If given a choice, always choose a little softer than a little harder.

Too hard and too small.

Too hard; bullet may not obturate/seal bore causing leading from gas bypass.
Too small; won't seal allowing gas to bypass and melt bullet causing leading.
Too hard or too small = barrel leading...

I regularly shoot .356 @18 hardness through my 9mm handguns, and traveling over 1000fps from my 6" with little to no leading. Bore size is important but most current production firearms are standardized to .355 bore.

All the 9mm/380/.38super, handguns I own are .355 bore.....(and yes they have all been slugged)

And this is how it's done folks....