I loaded up 50 rounds of 500 magnum today to try out my new toy at the range. The recoil was not nearly as stout as I was expecting, given all the hype. My 15YO daughter even shot it a few times :-)

Loads:
(25) 46gr of 300MP under 350gr Berrys, Winchester Large Rifle Magnum primer (3 grains under Alliant max load)
(25) 25gr Power Pistol under same bullet and using same primer (two grains under Alliant max load)

Easily 20% of the rounds didn't fire on first single action attempt but would always fire when hit a second time. I attempted a full cylinder in double action, not one fired. Back around a second time in single action, all 5 fired.

All the Power Pistol rounds, after firing, had fairly clean cases and no flattened primers. No apparent powder burns on the outside of the cases.

3-4 of the 300MP rounds had VERY flattened primers, the rest looked fine. The ones that flattened were also a challenge to extract. They had powder burns back on the case which caused the sticking so I don't think it was overly excessive pressure. As soon as I tapped the extractor the case 'broke loose' and extracted just fine. I found the 300MP powder in my 44 magnums to be very dirty also, but this looked like the powder was still burning as it came back into the cylinder. Every one of the 300MP cases have some discoloration/burn marks on the outside.

1. I carefully measured the powder, checking the throw weight every 3 rounds. I also visually inspect the charges of all cases before I seat bullets. Of course I likely wouldn't see a tenth of a grain difference, but I surely would have seen something that caused a louder bark and flattened primers on the 3-4 rounds.

2. I always check the primer seating depth.

3. I've been reloading since the mid-80s and I'll bet I haven't had five total failures to fire. Today I had at least ten.

Any thoughts on the failures to fire? Maybe Winchester cups are just that hard? The Alliant data used Federal 215 LRM, are they softer? Has anyone had an issue with FTF and Winchester primers? It's especially odd that NO double action shots lit off at all. Not that I'm going to be meeting a bear any time soon but it sure would stink to have 5 clicks and no bangs in that situation!

FTF like that are 99% primers not seated fully, push harder. You arent setting the primer off seating it, but it does need to bottom in the pocket. DA firing sort of short strokes the hammer spring. I find brass win primers no harder than a cci,maybe a bit softer.

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NRA Cert. Inst. IDPA CSO

Is this the first time you fired it?

Did you check the mainspring's strain screw for tightness.

Discoloration/burn marks on the outside of the case aren't necessarily a sign of trouble. Just the case sealing up to the chamber wall a little slow.

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Waiting for the break of day

> Did you check the mainspring's strain screw for tightness

I searched around the site a bit and found several references to the strain screw. I checked it, it was tight. Going to try different primers and see what that does. Frustrating - I expect that a firearm will prefer one type of ammo to another, but it is also expected to go bang regardless.

Have you fired factory ammo, do they all go bang? Then it is your reloading process, nothing wrong with the gun. Many reloaders baby the priming process for fear they will set them off. Isn't going to happen. I have crushed primers flat sideways, as long as use a steady pressure to seat, not going off. The primer must seat to the bottom of the pocket. Even a 0.002" gap can cause a misfire, on 1st strike. If they are firing 2nd strike, that is likely the issue, not the primer brand.

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NRA Cert. Inst. IDPA CSO

I've always set primers 'just below' flush by touch using my press. When I mic them, they show to be in the .003-.005 range below the base of the case. I bought a RCBS Universal Primer Tool yesterday and seated 25 using it. I mic'd the depth of 10, they run .010-.012 below the base of the case. I've never seen primers sit this deeply in a case. Starline brass with the 'R' designation. Can't get to the outdoor range until the weekend, anxious to see if that solves the problem. I'm betting it will..... How odd.

That is certainly enough diff to cause your problem.

__________________
NRA Cert. Inst. IDPA CSO

I've used my RCBS JR3 for the last four decades to seat all my primers. Can't imagine why your press would all of a sudden not be doing a good job for you?

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Waiting for the break of day

It surely isn't the press. I use a Lyman Ram Prime for almost all my primer seating. I set them flush or just below flush. The downside to this system is that you can't really 'feel' the primer bottom out, it's very easy to crush a primer.

My problem in this case is that if you handed me a primed case with the primer set .010-.012 below flush I would think the primer was being crushed into the pocket. I've never seen primers set this low in a case. I think I might buy some factory ammo (ugh, I despise doing this), measure the primer depth, fire it, set primers and see where they end up depth-wise.

Excerpted from the Lyman setup instructions:

Adjust your die either upwards or downwards with the following
suggested method. Insert a case with a fired primer fully into the
shellholder. Move the ram to the top of its stroke and adjust the die
until the primer cup just begins to engage the bottom of the case. Now,
slowly rotate the die until the primer punch engages flat to the base of
the case. Tighten the lock ring set screw.
Inset a primer into the primer cup. ...Slowly bring the ram up and
insert the primer into the case. Make sure the primer is seated .002" –
.006" beyond the base of the case head.