I was reading one of the reloading manuals for 243 and it says for H414 and 760 they recommend using magnum primers. Is this your experience or does this matter?

I use magnum primers behind ball powders like WW760 and H450 in most cartridges, as a matter of habit. Just yesterday, I was loading up some .243 cases with these two powders; I employed Federal LR Mag primers.

One of my favorite loads in the .22-250 features WW760. I've found that standard primers give better consistency over Magnum primers here. But in more capacious cases, I routinely use magnum primers, but only because all the experts and manuals recommend them.

Apparently ball powders are a little difficult to ignite, thus the recommendation of higher brisance primers.

Do the magnum primers affect pressure very much?

Yes, they increase pressure, all other factors being equal.

I use IMR-4350 as where the std primers stop and mag primers start. For powders including IMR-4350 and faster I use std primers. All powders slower than IMR-4350 I use mag primers.

Per Accurate Arms website FAQs:

17. Do Magnum Primers cause higher pressures than standard primers?

Not always, and unfortunately it’s hard to predict and quantify. It depends on the combination of powder-caliber-pressure/performance level. However, in most cases the difference is small especially in the case of rifle calibers. In the case of handgun caliber certain magnum primers can cause higher pressures, however this is more the exception than the rule. Also the number of handgun calibers that really requires a magnum primer is limited to large capacity calibers using relatively slow burning powders.

There are also sometimes big differences between makes of primers which complicates the issue even further.

The accuracy aficionados like to tweak that kind of stuff. On non-magnum calibers I use non-magnum
standard CCI primers period. I forget what caliber it is was, maybe 260 or Creedmoor, but the accuracy
nuts had the manufacturer take what normally would be a large rifle pocket and go down to a small rifle pocket
then they use a small rifle magnum primer... I'm sure it's accurate but dang!

Sometimes primer selection can be very effective, sometimes it just won't amount to a hill of beans.

Yes. My findings is a mag primer will increase speeds/pressures equal to about .3 to .6 grains of powder. It depends on the type of powder used, and the amount of powder being used how much it will effect the pressures. In a 243 Win case, a standard or mag will be fine for any powder used.