Local Sportsman's Warehouse had some small pistol mag primers the other day so I picked up a couple hundred. I have always just used standard primers in my 357Mag with medium loads of Blue Dot or 2400 7 158gr JHP's. Do I need to back off my load a grain or two when using the mag primers or is there even enough difference to mess with?

Unless you're using a maximum load, the same charge will be fine as far as safety. However, I'd go a step further and vary the charge a little up and down and shoot some groups from a 25 yard benchrest. It may make no difference at all, but I got slightly tighter groups with a magnum primer as opposed to a standard primer in a 6" Python using #2400 and a 160 grain cast bullet, the H&G #51. Been a long time ago and don't remember the primer brands, but it's worth trying.

If you are at max load,you need to back off.. Guess it all depends on where you are at load wise. There is difference

The mags are called for with H110 and Lil Gun, but some years back tests showed erratic results with 2400. The results were published in HANDLOADER IIRC. All .357 data in the Alliant handbook is with standard primers. Check out the data on Loaddata.com (you don’t need to sign up to see primers,OAL etc). I just took a peek and their data uses standards except for the two mentioned plus another, A-11 FS.

Do what you have to, but be aware and careful.

If you happen to have access to old HANDLOADER magazines, there's an article on this in #251.

The Speer manual of a few years ago changed their primer recommendation with Alliant 2400. They found more consistent results using standard pistol primers vs. magnum.

Opposite boat here, have piles of standard primers and the loads I want to try all call for magnums.

If you have 2400, it thrives on standard primers, as Pappy states. I'm shooting True Blue with standard primers and it's happy. Unique also does fine with standard primers, though I've not used the newest iteration. The only handgun loads I've seen a need for magnum primers are with H110/W296.

My method when switching from standard to magnum, rifle or pistol, is to drop back and work back up. Given current availability, if all I could get is magnum primers, thats what I'd do.

Although I use 38 Sp loads in a .357 mag case, I found Universal Clays and mag primers are mor accurate. Using W231, I found std SP to be more accurate. Both using 4.6 grains

I've used them both for years. I started loading .357/.38 back in '87. And used 2400 a lot for Magnum loads and used CCI 550's. In .38 I just used standard. But, after a little while I used standard in both, and have loaded a few thousand that way. I have noticed with Magnum and standard primers, that the velocities are not consistent but, accuracy is good.