I have only used the Fed 155 Mag LP primers for all the testing thus far. . .VV N-120 is at the top of the heap for this handgun/cartridge combination. AA1680 needed 1.5 grs. more to achieve basically the same MV, but ES/SD was high. 1680 will be a back-up powder though and H-110 wasn't working out at all. With a max load (pressure signs on primer) of N-120 at 32.5 grs. the average MV was 1772 fps ES/SD of 17/5 with groups of 3" at 100 yds. The load I'm favoring is 30 grs. with a average MV of 1652 fps, ES/SD 10/4 and 1.5" groups at 100 yds. These were 5 shot groups and the inside of the barrel is clean and clear. I'm very happy with the 30 gr. N-120 load, but there are two reason I've asked about the LR primers; 1. because of a few reloading manuals along with load data from Dan Wesson shooters who use it. 2. I have another fella I chat with online and he strongly believes that the LR give him better performance and ES/SD in the single digits! He is shooting a 12" 445SM T/C Contender.
I probably should leave well enough alone, but I'm one of those guys who likes to tinker and experiment with cartridges to get the most out of them. And its a new toy and I REALLY have enjoyed everything about it so far!
I am high in analytical thing (one of my aptitudes) So I do not blame you for wanting to experiment some. If you do want to use the LR primers , you should be able to do so. According to info I have, Mag primers burn around 23% hotter than regular primers.
The flame duration of the Mag Primer is longer than a regular primer. Intended Mostly for Ball powders and cold temps.
So I am sure that all, or most all of your powder is getting burned. 15" barrel is in your favor too.
So let's begin questions,,, Does the fella online that you chat with, have all the data for you? such as temp that day,, altitude and Barometric pressure? The reason I ask this is in colder weather that primer change just might make a difference.
Is he in Colorado or Montana and you in Louisiana (sea level VS high altitude ) see where I am going?
Is your brass the same as the recipe you got? or how about your chat friend? Same as his?
I am not familiar that much with that bullet, and when Bruce bought Montana bullet works, he changed the website and stopped showing the measurements on the OAL and crimp to nose. Why would I bring that up? I like knowing so that I know how much capacity there is and know if and when I begin to compress a load
N120 powder is temp stable !!! SO no need to worry about that.
All the new batches of VHIT powder have the copper eraser stuff in them. To quote the sales pitch from Vhitavouri it increases velocity, and since it prevents fouling, it is supposed to be more accurate. Say,,, after 30 -50 shots even. So they say.
Here is my other thoughts on this. First off you have a good barrel. Some might even call it a fast barrel. It seems to me that you are getting a little better velocity than test barrels. SO you might want to stay with what you have. Also you are shooting lead VS a Jacketed bullet. Lead usually gives a higher velocity, with less pressure. If you do switch to a jacketed bullet, you for sure might want to go to the LR primer for pressure reasons.
Primers, and changing them with a given load does make a difference. 260 Rem and the 308 Win are similar agreed? well the Fed Gold medal match primer would not work in the 260 and yet it is supreme with the 308.
And just for the record I have tried different brass too !!!
243 necked up Win brass Federal brass and of course the Remington and some Hornady.
While I am talking about brass I want to bring up another piece of information that may and may not be relevant. Hornady started using a SR primer with the 6.8 SPC because in their test findings, They found that the LR primers were causing too much pressure with the rounds.
I hope that I have given you plenty to think about and spark questions. This is how we learn.
Different primers do make a difference. Try 3-5 cartridges only and see what the chronograph tells you. As you know you can look at the primers for flatness and it is better to measure near the base of the cartridge. After firing if you measure .002 larger is too much pressure. I am not talking about at the rim. I am talking about near the end of where the case is Full Length sized. Use a sharpie and measure in the same spot.
Do empties fall out of the chamber? Did accuracy fall off?
Loose Primer pockets ? (usually doesn't happen till after 3-5 shots with a max load or over) depending upon brass brand.
But I have to say that I am in favor of staying with what Elgin Gates might have formulated. He was the creator of the super mags. I do not know if he came up with your exact recipe.
BUT,,, At the same time I do also know that Comparing VHIT N110 to Hodgdon H110.... with the N110 there is usually less pressure. Sometimes less velocity even with more powder.
You have answered your own questions in some ways, already.
Stating the max load of 32.5 showed pressure signs and accuracy dropped off. I would be inclined to try the same load or even start at the 30gr of N120 with LR primers and go up from there.
Good luck and let us know how this turns out for you.