Per Chubbs; measure chamber throats (pin gauge or drill bits that fit precisely- lacking pin gauge).
Your issue sounds like the classic case of under or oversize throats, resulting in bullets not obturating/"slugging up" to seal bore (this permits "blow by", or gas cutting of bullet base which will leave lead deposits in barrel). Additionally accuracy will be sub-standard or suck.
It is NOT uncommon for S&W m29's to have undersize (.426 to .428") chamber throats. This was an engineering "fix" to shoot jacketed bullets more consistently. (S&W is known to have throat diameters all over the map in both .44 mag and .45 Colt.)
This is problematic for cast bullets.
An undersize throat will swage bullet to chamber throat size (.426-.428"), the gas pressure on bullet base must then "slug up" base of bullet to obturate, or seal, the .429/.430"" bore.
If it doesn't obturate, you get gas "blow by"/cutting, which results in moderate to severe leading.
Oversize throats (which were common in the m25's in the day) result in the same issue, but because the cast bullet diameter is too small to begin with.
I have found that recent manufacture m629's and m625's to consistently have tight, or undersize chamber throats.
After discussing this at length with S&W reps- to no satisfaction, I simply purchased (Manson) throat reamers and opened throats to appropriate size for the cast bullets I intended to shoot; i.e. .431" in .44 magnum.
Your chamber throat should be same diameter as bullet, which should be one thousandth over bore diameter.
I typically run a 300 gr. WFNGC in .431" or 255 gr Keith Type GCSWC .431" at 1,000 to 1,100 FPS from my 629 (or 625-.45 Colt) MG's.
These Gas Check bullets generally "scrape" any minor, residual leading from the the bore, and both will shoot through a 300# hog in addition to being exceptionally accurate.
I'll add that a gas check is not necessary if throats are sized properly, and bullet diameter, Brinnell/"hardness" are compatible.
(Commercially cast bullets are very frequently cast TOO HARD to allow obturation at the pressures they are being shot. Hard cast bullets look "prettier".)
I use Montana Bullet Works and a couple others when ordering cast bullets for hunting. Check this link and you'll see there is option for diameter(s). I have no affiliation with them.
https://www.montanabulletworks.com/p...thentic-keith/
Brian Pearce (Handloader Magazine) is not the typical, dumb ass shill, gun rag writer.
He actually knows what he's talking about, as he is a shooter/handloader first, a writer second.
If you're really interested in learning how to make your S&W shoot cast bullets accurately, without leading, you would be well advised to read what you can find that he has printed on cast bullets/chamber throat sizing.
I had a "period" where I was a bit obsessed over this, as I was a dedicated handgun hunter- with a preference for 4" S&W's/"Mountain Guns".
I learned a great deal on cast bullet loading/shooting, so I guess it was worth it.
It is easier of course, to just shoot Jacketed bullets.
A true "Gun Guy" (or gal) should have familiarity and a modicum of proficiency with most all firearms platforms.
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