I'm trying to break in a new M&P-9 with enough jacketed bullets so that it will shoot quality hard cast (18 BH) lead bullets and come out with a clean, non-leaded bore. I've run about 300 jacketed rounds through it, stopping to swab the barrel out as often as every 3 rounds to avoid any shooting by-product buildup. At this point it leads a good bit when shot with a magazine of the lead bullets.

Any tricks I am missing? I'm putting lots of miles on my Lewis Lead Remover kit.

What diameter lead bullet are you using? I use a .357" in my Pro Series.

You are wasting your time just shoot the lead bullets and clean when you are done.
Put your lewis lead remover away and buy some chore boy copper scouring pads.Cut off a piece and wrap around a used bore brush run through the bbl a few times dry,Clean as a whistle.Fast easy and it works.You will never use the lewis lead remover again.

Lead bullets need to be properly sized, at least .356-.357. Some powders apparently burn too hot and can increase leading. Using a flat base vs a bevel base can frequently reduce leading, but they are a bit slower to load. My FS rarely sees anything but lead. My Shield OTOH, won't chamber my reloads. This same ammo runs fine in several other pistols. Must have a tight chamber.

I am doing the same with my M&P 9. What I just tried after couple hundred rounds is to polish the barrel (used J-B compound). It didn't take much to get a mirror finish in groves as well as lands. When I shoot now, there is little cleaning required. A LOT less than other guns in the past.
From what I know though, bullet size, as epj said above makes big difference by getting the proper seal as the bullet travels down the barrel.
I have a new 9 and a Shield that I will be going through this process with.

Regis

You need to remove all copper by using a copper solvent and/or J-B Compound. The bullets must be correctly sized. Powders on the fast side seem to lead less.

And keep the Lewis.

I have had and shot guns since the 80's. I have shot lots of lead through them. I also now have polymer guns. I have shot lead through them too. All of my guns have seen lead and jacketed/plated bullets shot through them. My M&P 15-22 AR, likes cruddy ammo (Remington Golden Bullets, it loves that ammo, even though S&W says it won't). I just use Hoppe's to clean the guns. My barrels are always clean to a mirror finish just using Hoppe's. Cleaned the 15-22 yesterday, picked up the upper and looked down the barrel to check cleanliness, and the barrel was spotless. No build up. I have tried different cleaners, but I always come back to Hoppe's. I must be doing something right, as the guns always go boom when the trigger is pulled.