I'm going to ASSUME that accuracy is where you find it with primers (by experimenting and trying different ones and intensities).

But when does one decide to go to a magnum primer?  Unburned powder left in the bore?   

Thanks, Frank, that makes sense.

So apart from usging a chronograph (ok I'll dig mine out this spring) what other measures of consistancy should one use?

Would 4 close shots and one flyer be an indication?

Quote:

Here are good starting places to make the determination:
*   Propellant Profiles Fifth Edition
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* Powder Manufacturer's Loading Data website for specific powder

The principle for specific primers for specific powders is the degree of brisance that a given primer generates.  For black powder one wants to use the lowest brisance primer, even LP primers for calibers that call for LR primers 

CW Ball powders generally like a hotter primer due the amount of deterrent. What you want look for / achieve is the least amount of verticle dispersion while in the process of load development. The Alliant 300MP seem to prefer the magnum primers over all others. On the other hand AA-9, H-108 and WW296 don't seem to be as picky. In regards to unburnt powder left in the barrel it generally means you need to go to a hotter primer / longer burn with a slightly hotter brisance. To this date I have not seen a barrel with unburnt powder, shoot to its best ability. The extreme spreads might look good but the bullet has to make its way through all of that left behind garbage to make its way out of the barrel. If the amount of garbage remains the same from shot to shot then it might be ok but on the other hand you just end up with more fouling in the barrel and I have never found it to be good thing. One 10 shot group generally doesn't tell much as there are to many varibiles going on so you need to shoot several before you can start to draw any conclusions.

J.Louis

Primers are not created equally!
One brands magnum might be weaker than another brands normal.

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As a general rule of thumb, a load calls for a magnum primer when more than 65-70grains of slow powder is used.

And BP ignites quite easily so mever use magnum primers here.

Exactly what is "brisance"?

Hi John Boy,
My how times have changed !
20 some years ago, when I was active in BPCR shoots at Shippensburg, it seemed a lot of shooters were extolling the virtues of magnum primers for BP; and recommended Federals in particular.
And I would have to pull out my vintage 1990's copy of Garbe and Venturino's book to be 100% sure, but I think they also recommended magnum primers ?