I am loading some 140gr 7mm WSM using IMR4350, do I need to use Magnum primers?

I've never had to use Mag primers with that powder.

That's in .300 WinMag and .30-06.

Whatever works best, if you have the choice. Really only one way to find out……

Either should work fine. For a while I was using magnum on all LRPs thinking that the hotter ignition would lead to a more consistent burn. Did a good bit of experimenting with 6.8spc. Saw no correlation to accuracy but with the same loads about a 50fps difference. I'd still run mag primers in a mag case like that especially with IMR powder but it probably will make no difference.

Different brands of primers can make a big difference.I usually load with Winchester primers.I was only seeing about 15fps difference between WLR and WLRM primers.The Federal GM215M Gold Medal Large Rifle Primers was about the same velocity as WLR.I have some Sellier & Bellot Magnum Primers and my velocity dropped down about 50fps.I quit buying the regular primers and just buy WLRM primers now for all my loads in 308 Win and up.The 4350 powders are fairly easy to ignite.Some of the double based powders like Reloader 22 seemed to be a little harder to ignite.I had some hang fires and failure to ignite once with Reloader 22 and CCI 200 Large Rifle Primers in a 300 Win Mag.Could have been just some bad primers,but the only brand of primers I've had fail to fire was CCI.

I agree with baldhunter. In over 40 years of reloading, I’ve had about 10 hang fires and they have all been with CCI primers. And they were not at the same time or the same package.

I wish I could give credit to the original author but I can't remember were I even got this, it's been a few years back. I found it interesting but I have no idea how accurate this test is. There is a lot of variables it doesn't address.

I also ran a limited test to try to rank primers by brisance or "hotness" kind of like a burn rate chart. I established two very consistent loads, one each in .223 and .308 then shot ten-shot strings with different primers. I judged "hotness" by velocity results only.

It was difficult enough finding them all that I only did rifle primers. From "Hot" at the top to "cool" at the bottom, here's what I found.

Large Rifle Primers

Fed 215
WLRM
CCI 250
Rem 9 ½ M
WLR
F210
CCI BR2
CCI 200
Rem 9 ½

Small Rifle Primers

Fed 205
Rem 7 ½
WSRM
Fed 200
WSR
CCI BR4
CCI 400
Rem 6 1/2

Note: some results were very close together, such as the Fed 215 and WLRM. In another cartridge or another load, they might reverse.

A long time ago someone here on the 'fire said magnum primers were only needed under 'usual circumstances' with loads exceeding 79 grains of powder.

So I tested that with my 375 H&H and my favorite 270 grain bullet load of 75 gr. R15. Sure enough, I got slightly better, and measurably smaller groups using LR primers than with Magnum primers.

Now if I was in bitter, bitter cold I might swap out the LR primer for something hotter, but now I only use magnum primers in my 400 Jeffery, which uses 82+ gr. of powder.