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I guess I will be the sole dissenter here.

As long as you're aware that exposed lead bullets will increase the need for cleaning, I'd shoot them and not worry about it too much. All that I shoot in my USC45 conversion is .45ACP Semi-wadcutter, Lead and it's a steady diet. On average, it sees about 99% lead bullets and the remaining 1% are FMJ ball through the suppressor.

The .40 round is about twice the chamber pressure of a .45ACP, so that additional velocity and pressure is going to cause a lot of lead to be stripped off and will build up in the polygonal bore. As long as you clean it every couple of hundred rounds - complete with bore scrubbing - you should be just fine.

I would stress that you should *NEVER* fire FMJ .40 through that barrel after you've fired a bunch of lead - this will lead to even higher pressures than the .40 FMJ develops on it's own and could lead to catastrophic failure if you have a lot of barrel leading and try to shove a hard FMJ round through all of it.

As to accuracy - you can expect to lose a little accuracy pushing a bullet that fast and with that much pressure without a hard interface to the polygonal rifling. That's just part of the equation when you shoot lead.
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