I have been loading 38 spl and +p's. I was listening to some individuals that were discussing burn rates of powder for 357 magnum and what primer is best suited. They were informing me (if I got it right) that the IMR 4756 is a slower burning? powder so it would require a magnum primer to burn through and get the maximum affect but at a costly round. But Clays for instance can use a regular small pistol primer because it has a different burn rate. The were adamant that burn rate of powder verses the proper primer (magnum or regular pistol) was a very important factor to understand to be affective at reloading. And just understanding the properties of the powder and its burning properties was important. Seems none of the reloading manuals explain this in depth. Where can I look to understand the specific's of burn rate. Or can someone just help me out?
Be Well
Ted
I hope I asked the question right.

You have the gist of it correct but you need to realize that there are a LOT variables in play and in reloading you cannot assume that anything is an absolute.

For example that 4756 you reference isn't a powder that is typically recommended as needing a Magnum Primer and it's not an exceptionally slow burning powder. Basically it falls into a somewhat "grey" area where in some cases a magnum primer will work best and in others a standard primer is hte best choice. BTW, in the cases where a magnum primer is a better choice that is typically because it produces more consistent velocities. So, you may only be able to determine that by chronograph testing.

If you're starting out with a new load or powder the safest course of action is to follow the recommendations of the reputable loading manuals. If they don't recommend using a magnum primer with your particular load then don't use one unless you see a lot of variation in the velocities for a given load. Also, if you decide to change from a standard primer to a magnum primer you'll need to start over at the minimum and work up.

Tcoburn
When I read your post - it reminded me of an old post here on the forum - similar posts are linked at the bottom of the thread page - yep -the one I read is referenced

on your thread - there is an excellent thread started by Rule 3 with a lengthy discussion regarding burn rates and powder selection - but not so much about magnum primers

here's the link of the other thread
Powder burn rates and the 45 ACP

its a very informative thread, even though it starts out disussing 45 acp

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I'd like to agree with you BUT

Thanks, I am going back through my reloading manuals and remembering what I forgot. I my neck of the woods a lot of reloading supplies are limited so I use what I have got. I never experiment I always use the information on IMR website prior to starting any new loading. Right now the primers are my concern I have limited magnum primers but am well stocked with CCI small pistol. So I was wondering what the parameters are for substituting small pistol primers in a 357 cal. load. I can say'" I prefer to keep my body parts intact and will not experiment," just hoping to be able to use what I have until reloading supplies start becoming more available.
Thanks again
Be Well
Ted

Ive used magnum primers in the past when standards were unavailable.Ive found that a different brand of primer can sometimes tighten up groups for a given load.Every variable can have an effect.This is more obvious in rifle loads,but it can change how a pistol loading works too.

Some slower magnum powders do require magnum primers for consistent ignition because they are hard to light off and the bullet is out of the barrel before all the powder gets started to burn. You reloading books tell you to use magnum primers or not. However, one of my books (I suppose for simplicity) says to use magnum primers on all .357 loads. Knowing about the powders I use (Unique, Bullseye and 2400) I can make a decision not to use magnum primers.

If I were to use powder like H110 or 296 I would need a magnum primer because they are known to be inconsistent with small primers.

You could use magnum primers exclusively if you wanted to, even where not indicated as long as you reduce the powder charge to make up for the extra pressure the magnum primer adds.

In general, long cases loaded with slowest powders need magnum primers. In general, you can substitute magnum primers in a load and adjust the powder.

Unfortunately, general rules are not worth a flip when you address a specific load. For example, right now I am using Win SP magnum primers in a PF 133 load of 9mm with Titegroup under a 124gr FMJ. When I switched to available magnum primers simply because they WERE available, I naturally got out the chronograph to see how much I needed to adjust, and the answer was: NONE.

Interior ballistics are extremely complicated: that's why we use reloading manuals and chronographs, and rely on empirical results, not theoretical general rules.
By the way, powder burn rates differ depending on the containment, which is why you will see powder burn rates with differing order listed on different charts from different sources. The results you get depend partly on how you test it.

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Science plus Art

A little note on 4756 powder.

I used it in my new 686 6" with 110 to 158 Jhp bullets in
357 brass and it was an eye opener and I was very glad that I had a chrony to see the velocity and ES of the loads.
Different crimp pressures, F100,cci500,cci550,wspm primers,all made for a wild ride.

A starting load of 9.5grs with a 110gr and f100 primer had a ES of 367 !! Yet a maximum loading with a 125gr at 1448fps with a cci500, had an ES of only 39.

It is a good powder when you figure it out but it can really toss some wild numbers at you on both ends of the loading process if you are starting from square one, as I did.

Some powders specifically call for the use of a Magnum Primer. others do not. Some manuals use magnum primers for every tested load under any Magnum caliber even though it is not needed for certain powders.

For example Hodgdon data for the 357 Mag and HP38 calls for a Mag primer but it is not necessary with that powder, They did it for constancy in all their tests

Alliant has loads for the 357 Mag and Bullseye and Unique powder but used a regular powder.

The Hodgdon data for 38 Special +P and IMR 4756 uses a regular primer.

Same powder in a 357 Mag uses a Mag primer (as I mentioned above) so go figure??


So it is confusing,

H110.Win 296 call for a Mag primer.

2400 does not.

Yes, generally slower powders produce more velocity in Magnum loads.

Only time I use SPM is H110

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Still Running Against the Wind

Part of the magnum primer issue is that the slower ball powders (H110, WW296 HS6, etc are harder to ignite, thus performing better with magnum primers.
Flake powders like 2400 are easier to ignite, even in magnum loads.
When I started loading about 1975, many manuals routinely specified magnum primers for any magnum cartridge.

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-jwk-
US Army '72-'95

Yes & no. Yes powder burn rate matters, but the only powder that needs a mag rpimer is H110/W296, maybe Lilgun. Everything else in every handgun caliber can be done with a std primer.
The term magnum has nothing to do with the primer. All the magnums can be loaded with std primers as long as you are not loading H110/W296. BTW, you will always get highest vel w/ slower powders, if you put enough in.

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NRA Cert. Inst. IDPA CSO

As others have mentioned it's not so much a question of the burning rate of a given powder as it is the individual burning characteristics of each powder. Some powders use coatings to control their burn rate or to reduce flash and these can make a powder that is usually easy to ignite more difficult.

You must understand that not all primers are alike and it will vary from one manufacturer to another, and from one lot of a specific brand to another. You might find a non-magnum primer from one company that actually burns hotter than another's magnum offerings.

As others have said, primers can make a huge difference in accuracy, not just because of how well they light the powder but in how consistent they are themselves.

The best book I have on the subject, and pretty much anything else to do with loading ammunition is "Principles and Practices of Loading Ammunition" by Earl Naramore. It hasn't been in print for a long time but you should be able to find a used copy online.

Tcoburn, something to remember about powder burn rates and all the nifty charts you'll see in the manuals and on line: these charts are the relative burn rates to the other powders. It is a major mistake to base loading/selection of powder/primer on a burn rate chart unless you're a professional ballistician with all the pressure/velocity/experience they are blessed with. Follow what's in the good books and your all set.

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Quando omni flunkus moritati.

Thanks to all got a lot of good info, But seems to be the tip of the iceberg.
Be Well
Ted

There are computer programs available on internal ballistics and the subject is itself is very deep. I'm more interested in external ballistics than internal or terminal ballistics. I only shoot targets so terminal ballistics are of little use to me. Getting an estimate of trajectory however, is of use and interest to me, especially with rifles.

There are free apps for your phone that give you basic dorp & wind numbers.

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NRA Cert. Inst. IDPA CSO