Magnum Pistol Primers

Can anyone give me an idea about how much of a pressure increase to expect when using magnum large pistol primers instead of regular large pistol primers.
I had some rounds of 44 mag. that I had loaded with 280 grain Berrys plated flat nose bullets, 20 grains of IMR4227, Starline brass, and CCI 350 magnum large pistol primers. I was shooting this in a model 92 trapper. These loads seemed a little stout but not too bad. When I checked the brass, the primers were flattened.
The max load for this cartridge combination is 22 grains while using LP primers.
My question is could the magnum primers have caused enough of an increase in pressure to flatten the primers? I was still 2 grains shy of max when this occurred and if I switch to LP primers should this allow me to move on up without excessive pressure?
I intend to test this out by moving in small increments and small batches until I find something I like without to much pressure. I am not going for max, but I will be using this for hogs. I also don't have access to a chronograph at this time.
Any help would be appreciated.

It varies by brand. In fact, there are some LP primers that are hotter than other makers' LPM's. But to answer your question, yes, switching primers like that could have red lined the pressure. (So can shooting on a very hot day loads that were tested in cooler weather. It's a good thing to keep in mind when you have any fairly hot load.)

That is why you always reduce your charge and work back up whenever you change ANY component. You were lucky this time and got a free lesson. ;)

The other thing you need to keep in mind is that any published MAX load is a relative value. All it tells you is that a certain pressure level was reached at that charge with their components in their specific gun. It tell you NOTHING about what will happen in your gun with different components. Each gun has its own MAX level, and it may be showing pressure signs (or not) at charge levels well below what your data source lists as MAX (and rarely, it might also safely handle charge levels in excess of the published MAX charge). When you said "it was 2gr short of MAX", it was a rough guess, not a certainty. Clearly from the evidence, it was right at MAX for your rifle.

It is not unusual for me to find fired factory handgun and rifle brass in the bucket at the range with flattened, cratered, and even pierced primers. Even with a brand new firearm you can often find factory loads that are too hot for it and show clear overpressure signs. Most shooters have no idea what to look for or what it means when they appear. Handloaders are taught to watch for them.

Two years ago the guy shooting beside me was firing factory rounds in a production .30WSM. I noticed the primers on his fired brass were all pierced and I called his attention to it. I was told to mind my own business. Two shots later he got a faceful of propellent gas. One of the other shooters read him the riot act for ignoring a clear warning. I saw the rifle a week later for sale at the pawn shop, haven't seen him since. Darwin.

Thanks for the help Versifier, I really appreciate your time. Also 20 grains of IMR4227 is the starting load. I had dropped back down and this is why I wondered if mag primers could have caused this much of a pressure increase. Thanks again ISME

When you have such a narrow charge range for a given powder as 2 or 3 grains (not uncommon with handgun cartridges), then you really have to keep your eyes open. You will find some powders like that are just not acceptable for your gun(s), though your neighbor's guns might love them. Powder manufacturers would like you to believe their products can be used safely in just about anything and it is not always the case. The only way for a handloader to tell for sure that any load is too hot is to mic new case heads before and after firing. It's a PITA, but it's a lot cheaper than a ballistics lab. A chrono can give you some valuable warning data, but they are not infallible either. When there are visable signs I always pay attention to them and I will never use a load that goes beyond slight flattening of the primers on a hot day. That way I know the load is safe for year round use.

It is much more likely that a rifle will show pressure signs on the primers than a handgun, but as the experts will tell you, even that is not always a reliable method. The sobering thing is that many times rifles and revolvers show no visable pressure signs except sticky extraction. Some semi auto pistols may show no signs at all even with proof level loads. When they KABOOM, they do it with no warning whatsoever. There can be an increase in felt recoil or in the loudness of the shot, but these are subjective and not a reliable way to tell. That is why, apart from best functioning, it is always better whenever possible to stay in the middle to low end of the charge table with any semi auto, and not a bad idea with any other firearm.