I'm planning on reloading some 243 ammo and the only thing I have large rifle magnum primers is this a problem???

No but start with below max loads. Which you should do anyway. But with the powders you are likely using, it won't make any difference.

The 243 is notoriously squirlie, pressure wise, so I would definitely take Tom’s advise and start good and low!

While I've never seen anything squirrelly about loading for, or weird pressures with, a 243 (over 53 years of loading for multiple rifles), I will agree to reduce your favorite load by a grain or so.

The "big boys" say the mag primer is like unto a grain more powder so use your head and load accordingly

I just formed a ton of 22-250AI brass and had a pile of mag primers that I don't normally use so they were called upon during fire forming. Worked like a champ in a sacrificial barrel before the new one was installed.

Zeke

Handloading 243W for soms years I have experimented with various components including comparing standard and magnum primers. Agree with the comments that magnum primers can produce slightly higher velocity compared to an identical charge ignited with standard primers. Never gave me any pressure issues though. The only time pressures got too high was when trying lighter bullets and faster powders but these were with standard primers. Just do as suggested here, start low and work up until you find the sweet spot.

As cool as temps are not, you will never know the difference. Summer time, different story if you are top of the chart. I would guess drop down a grain from top of the chart but work up to it, just like the book says for any top of the chart loads not matter what the conditions.

You learn a lot about pressure the hard way living in a hot climate like Houston Texas. I've loaded more than one top of the chart loads in the hot Texas Summer and all indications show high pressure. But may not be high in other parts of the country. I am one that strongly belives in the pressure effect of high temp. 85 degrees is not hot, 105 is.

Thanks Billy

No difference.

Many years ago, a friend brought his BRNO 243 rifle, asking for help.

It was blowing primers off SAKO factory ammo.

First I thought it might be due to the ammo.

The Sako ammo primers fall off all by themselves, after we had to hammer the bolt open!!

I tried Norma ammo.

Same results.

I loaded some ammo with 10% less powder than suggested starting loads.

Very stiff bolt.

We were stumped.

I slugged the barrel, and if memory serves me right, it had a 0.239 as the largest dimension.

I have had some really freaky rifles from Europe, but never as bad as this one.

I took the barrel off and replaced it.

I have loaded max with std Federal 210s and was good with that, so came up short one time and instead of going to town for some 210s I used 215s and the same old load, got sticky sticky bolt lift and a locked shut gun..Got home and ran dowl in it and punched it out and light head seperation and ugly primer..Time to pull bullets, powder and cut the grs. x 2 and all was well again..Tried the same on my 7x57 and 308 and decided 210s for non magnums and 215 for magnums with max loads was a pretty good way to go...I like that better than cutting a gr. or two of powder for the same results..

Because of the primer shortage of primers, I will be using mag 215 Fed primers with my standard loads hence fourth with a 1 or 2 gr. cut in my loads,depending on the caliber and the rifle, and its working fine so far. I will continue this until this shortage mess eases off..