have had dismal results in Glocks, which also have polygonal riling. I have had 4 G-17s over the years and if I was lucky I could get 50 rounds down the pipe and then blast it out with a few jacketed rounds, this is a hillbilly way of doing it, but have been doing it with all sorts of handguns for 30 years.With some Glocks I would stop after a mag or two because of leading, and hit it with a Lewis lead remover, depends on the gun, and only once had any luck with a long string.My 10mm, and 40 cal Glocks do not like lead. Now if there is heavy leading, one risks bulging a barrel (worked at a gun shop years ago, we did service work, and would usually see about one gun a year with a ringed barrel due to lead, more due to oil believe it or not).
I shoot mainly wheel weights, and have found very hard cast pistol bullets more likely to lead a barrel unless magnum velocities are used. The commercially cast pistol bullets sold around here are too hard IMO, and the guy does it because people think their bullets need to be as hard as possible, just not true for stuff like 45 acp and the like. His pistol bullets are nearly as hard as linotype bullets I cast for use in 30 cal rifles.
With my G-21 I did an experiment, it is commonly thought that to get the best accuracy, a barrel should be void of copper if shooting lead and vice versa . I used a scary strong copper solvent, required flushing with denatured alcohol. Once I could see no copper, I burnished the barrel with Hoppe's moly paste, about 4X the recommended # of passes. Shoot 6-700 rounds of 180 gr bullets cast with wheel weights with no leading at all. To me, this is too much bother, and that gun may get an aftermarket barrel.
If one has a polygonal rifled barrel that shoots lead, count your blessings, I have not shot cast out of my HKs (and I am a long time bullet caster) because I value my HKs more than my Glocks, and I am not likely to shoot lead through the Glocks either, after decades of doing it and only one true success, after extraordinary measures, or if I do, I will keep a close eye on it, especially around the throat. Now when I go shooting a gun can expect at least 3-400 rounds at minimum, usually a lot more, unless were talking big magnums. My 44 mags get thousands of cast loads with out ever seeing jacketed, and they are fine with midrange loads. I can hit cans at 100 yards with them(245gr Thompson type lyman over 8.5 gr unique, ans any old lube I can get)
My advice is if you try it with a stock barrel, keep a close eye on leading, and have a lead remover ready, either abrasive patches or bore paste, like J&B, or a lewis lead remover makes quick work of it. I have seen barrels ringed, and would hate to see an HK suffer. I have been using plated bullets in my HKs with good accuracy and no abnormal fouling. Good luck!