Brazos bullets ... 9mm ... truncated cone 125 grain ... sized ... .3555 ... Herco and BE 86
I plan on starting at 5.5 Herco and 5.1 BE 86 ... This will be my first experience with coated bullets, so if you have some advice for me, I'm all ears.
Herco is a fluffy powder and to not compress powder charge, I used longer than typical OAL of 1.155"/1.160" for my load development with 124 gr plated RN (which I usually use 1.135") and 124 gr lead "stepped" RN (which I usually use 1.125").
I referenced 2004 Alliant load data for my Herco load development and published OAL used for 125 gr Lead bullet was longer 1.150" (Likely RN) -
http://www.thehighroad.org/attachment.php?attachmentid=182147&d=1364769070
Look at the comparison picture of various nose profile bullets and notice significantly longer bullet base (bearing surface) of SWC/CN compared to two RN bullets on the right.
The longer bullet base/bearing surface means bullet base will be seated deeper in the case neck which will increase chamber pressures.
Compared to 1.135" OAL I use with FMJ/RN bullets, I needed to seat the bullet deeper with bullets with longer bullet base/bearing surface.
The 124 gr truncated cone coated bullet you will be using will have similar profile as RMR 124 gr FP bullet and as you can see in below comparison picture, working OAL will likely be shorter than 1.130" used for RN and bullet base seated deeper in the case neck.
Since you are using truncated cone bullet, if you are going to end up with deeper bullet seating depth/shorter OAL than 1.150" listed on 2004 Alliant load data, you may need to reduce start/max powder charges. I suggest you first determine the max/working OAL and then calculate the max case fill which may result in reducing the powder charge further. You can calculate max case fill by subtracting bullet length from working OAL, then subtracting max case fill from average resized case length and marking the inside of case neck with tip of calipers and filling the case with powder to mark.
If you provide the working OAL and bullet length, I can calculate the Herco max case fill for you. Based on my experience, with the working OAL you will likely use, I would suggest you consider using 5.0 gr of Herco as start charge and test 5.2, 5.4 and 5.6 gr (Pending max case fill calculation based on working OAL) as you can always go higher.
FYI, here are my Herco range report using longer OAL -
https://www.thehighroad.org/index.php?threads/9mm-and-herco-for-jacketed-lead-plated-bullets.745656/
124 gr Berry's plated HBRN:
5.2 gr @ 1.155" - very light recoil (barely cycled the slide)
5.8 gr @ 1.155" - light recoil
6.1 gr @ 1.155" - mild recoil
124 gr Z-Cast lead RN (stepped):
5.2 gr @ 1.160" - light recoil
5.8 gr @ 1.160" - mild recoil
As to BE-86, as many THR members already found out with FMJ/RN bullets, you want to END UP with 5.1 gr, and NOT START OUT at 5.1 gr.
Many report reliable slide cycling and accuracy trend building starting around 4.6 gr of BE-86 and 124 gr FMJ/RN at typical 1.130"-1.135" OAL. Once again, since you are using slightly larger sized coated truncated cone bullet that will likely be seated deeper in the case neck, I would suggest start charge of 4.4 gr and test 4.6, 4.8 and 5.0 gr.