Nowadays, particularly with component shortages, primer interchangeability often comes up. I did some limited, informal testing along these lines this morning.

Using a common mild target load of 3.6 grains Bullseye powder and a 200 grain H&G #68-style cast SWC bullet from a SAECO mould, I compared the CCI-300 large pistol primer with the CCI-350 magnum large pistol primer.
The 300s were purchased in the last two to three years while the 350s were from the 70s.

I fired groups from a benchrest setup at 25 yards using a Colt 1911 A1 from the mid-1950s. I didn't measure group sizes, only eyeballed them; they were the same. The magnum primers had an average muzzle velocity 11 fps greater than the standard primers, so virtually no difference.

Again, this was very limited testing with one mild load; results might vary considerably in other instances.

Your results are typical I would say. I have done a lot of comparison tests with my chronographed handloads with different primers with loads from light to heavy. The dreaded word MAGNUM sends some handloaders into panic mode. Magnum primer effect on light to moderate loads is insignificant.

This subject has been beat to death over the past several years due to primer shortages. What you are experiencing is common.

Magnum primer application doesn't automatically translate to higher velocity, pressure or S/D!

Magnum primers by design provide a LONGER spark or brisance. It is a solution to better ignition of slower burning powders like W296 H110 and 2400.

If close to maximum pressure logic says to reduce the charge and work up looking for signs of "over pressure"!

Use Magnum primers. Use common sense. Follow safety protocols when deviating from standard loads.

Everything you ever wanted to know about primers:

Primer Info & Chart + Milspec Primers for Semi-Autos & Other Primer Applications

Smiles,

In the various tests I have seen or read about in the last year and a half I have noticed that the results will vary.

I have noticed that those who tested with a faster burning powder tend to say that nothing changed, but something slower seems to produce an increase in velocity, and you're results fit this trend.

Would be curious on what you would see if you used the same bullet and pistol with a load of 8grn of HS-6 and retested.

For me, no matter what the results, there won't be enough to draw any conclusions, but it would be more valid test data than comparing the one offs that different people have reported.

There is probably more variation in performance among different primer brands than there is between Magnum and Standard primers of the same brand, rifle or handgun. My only experience goes back many years when I ran some extensive tests of CCI SP vs. CCI SR primers in .38 Special. No significant differences were found in either MV or grouping. Since then, I have used mainly SR primers in most handgun loads.

I'm on the same page. Been using Federal SR primers for my revolvers/pistols with no significant change in any form, however, I found that some striker fired pistols will not set off the thicker cup, not all just a couple. Revolvers made no problems, go off every time. I have plenty of SR primers but my SP primers are dismal.

I apparently didn't make it clear in the original post that my testing was brief and limited to only two primers.

I did some fairly extensive testing some years back using an assortment of (eight) American-made small pistol primers, standard and magnum. Using the .357 cartridge in a 6" Python, I loaded #2400 powder and the H&G #51 160 grain cast SWC bullet. I fired five, five-shot groups with each primer.

Velocities varied up to about 75 fps but accuracy was measurably different, with the most accurate primer providing an average group size about 30% smaller than the least accurate primer.

What you found may be true, but firing five 5-shot groups is inadequate to achieve a reliable level of statistical confidence in results if you are speaking of group size (extreme spread). The absolute bare minimum to REALLY begin seeing differences in relative grouping performance requires an average ES of five 10-shot groups as a basis for comparison, and firing more than five such groups would be even better. Anything less is just wasting ammunition and time.

I have developed a rigorous computerized statistical model of group size some years ago, and the results are indisputable.

My testing protocol may be flawed, but interesting, enjoyable, and adequate for my purposes.

I normally use cci primers in my reloading.

However I have noticed in rifle and revolver that there IS................
a difference in cci and winchester primers in my tests.

A little lower fps with standard primers in 38/357 loadings but........
the standard primer in 30-06 with IMR4350 and a 180 gr bullet
cut my groups in half.

They are all good..............
you just have to get the correct "Mix" in your testings.