I've been reloading a long time but I'm new to reloading mag cases and the 375 H&H. I will be trying IMR 4064, 4350 and RL-15 powder with 270gr bullets. My question is, is there anything to be gained in using mag primers with these powders? Some reloading books say to use them, some not. Any advice would be appreciated

I find no advantage to "magnmum" primers. On the other hand, the only disadvantage is a few cents per hundred difference in price. For your .375, use whatever you have available.

I use magnum primers on all magnun cartridges..Over the years that has worked best for me...

it was the idea behind mag primers that they would ignite large loads of slow burning powder better, thereby giving better accuracy etc. I guess I still use them here and there on big stuff, but I really don't think there's any differance

Specifically regarding the .375 H&H:

I worked up a load of 68.0 grs IMR-4064 with the Hornady 300 gr Interlock round nose. (Winchester cases.) With Winchester magnum primers (WLRM), I was getting extreme velocity spreads of around 90 fps for a 10-shot string. Switching to the Winchester standard primer (WLR) reduced the extreme velocity spread to around 40 fps. Both loads produced almost identical average muzzle velocities (about 2480 fps).

-Bob F.

I would say use what works for you. I don't think I have ever reloaded .375 H&H without using magnum primers, usually Fed. 215.

I use magnum primers in magnums.

I did a test with the 7mm WSM shooting the 140 gr SPT with RL 22 by loading some with Fed. 210's instead of 215's and the group opened up.

One could argue that the orginal load might need to be changed but that was enough for me.

Use mag. primers for slow powders.
The powders that you listed are not what most of us would consider slow. I've used the std. primer in most of my loads. Used the mags with WW760 because you are suppoesd to with ball powders but I think that might be hogwash too. My favorite load for the .223 uses WW 748 and Rem. 7 1/2 STD. primers.

I have been using standard wlr primers in my .375 and they seem to work fine. Accuracy is good with the 300 gr Hornady RN.