Black Jacque: just about the easiest way to establish bullet seating depth is to set up your seating die with a new factory load. Set your seating die so that you would seat your 124 grainers at the same depth as the 115s. About the only problem you might have is with the bullet ogive if the one on the 124 grain slug you are using is a bit fatter than on your normal 115 grain bullets, in which case you would want to go just deep enough to get the case to feed and chamber. How many lube grooves show above the case mouth doesn't really matter. If you seat the bullet too deeply, so that all the lube grooves are covered, you may have seated the bullet too deeply and increased the pressure too much. Since you are using a truncated nose design I doubt you will have the feeding problems I mentioned.
If you could feel the die 'do something' as you fed the case/bullet into it, then it has done it. You shouldn't see any difference as those crimp dies make it look just like a factory loading.
I'm willing to bet that the only problems you may encounter is with any lubrication that gets shed during cycling, and may get dumped in either the magazine or on the loading ramp or in the chamber.
Also, if that truncated bullet you are using has a square shoulder, that would have to be seated at the case mouth or you may wind up with a longer than normal cartridge that won't chamber properly - it would chamber on the bullet shoulder, not the case mouth. If this then seats the bullet lower than max overall length you may have to reduce your powder charge to avoid increased pressures. If there is no shoulder on the bullet then seat it to the depth that is most conductive to proper chambering and you should be alright. Hope this helps. Mikey.
