It is common a mold manufacturer to design molds for a weight of say for 125grs when poured with Lyman #2 or Linotype alloy. This can be especially true if the mold was made by Lyman.
Then the home caster comes along and pours using clip on wheel weigh alloy, and the bullets come out weighing a bit more. It sounds like this is what has happened with your bullets.
If you have one of the Lyman Cast Bullet manuals, or even when looking at their standard manual in the cast bullets, you will notice they DO list the alloy up at the top by the bullet weight. Most are listed as 10-1, #2 or Lino.
Just go with the cast bullet 125gr listed loads, and start low and work up. You will be fine. Lead is pretty forgiving to a point. Usually you will get some pretty serious leading in the first few rounds if things aren't quite right. That is if your load is too warm for the bullet alloy. Just be sure to check the barrel after the first few rounds with each change in powder weight.
Just examples here,
45 Colt with 100+ rounds through it and a pretty smart load using HS-6,
Load work up with my 41mag, bumped up the charge weight and this after only 4 shots,
Oh and yes it only gets worse the further down the barrel you look.....
Classic example of overpowering the soft alloy with the load pressure. Had I kept going pressures would have been climbing steadily resulting in some not so friendly loads real quick.
Years ago I shot some custom "hard" cast bullets I got from a fellow. He sent his load data and such as well. I had never shot anything cast before and didn't have a clue on sizing or slugging the barrel and such. He sized and lubed them and sent them on to me. The first shot was about what I expected. The second similar, the third through 6th were increasingly more and more recoil and noise with the 6th being really something. When I looked at the muzzle there were actually strings of lead hanging out of it and the bore was solidly leaded up to the point I could not see any rifling what so ever. It took me a good week to get it all out scrubbing and actually scraping out the lead from the grooves. It took me close to 20years or so before I shot another cast bullet through any of my revolvers again. Now I know to look, and stop before things get that way.
