This is almost certainly the result of incompletely seated primers, as already mentioned, and that can be in combination with grease in the bolt. Strikes may look light compared to fired rounds because the pressure of normal firing will push the primer back over the firing pin before it withdraws, deepening the mark. So the primer in a successfully fired cartridge case looks like it was struck more deeply than the one in the cartridge that failed to ignite does.
Primers are designed to be seated until the feed of the anvil touch the bottom of the primer pocket, and then to be compressed further in by -0.002" to -0.006" to "set the bridge" (the thickness of priming mix between the tip of the anvil and the bottom of the primer cup). It is also called "reconsolidation" of the primer (the first consolidation being assembly at the factory, and the second, or "re" consolidation being this squeeze you put on it in the primer pocket). U.S. Naval Ordnance at Indian Head did testing in the late 1970s to show the best and most consistent ignition occurs when reconsolidation is -0.003". However, Federal, specifically (and uniquely, AFAIK) says to use -0.002" with their small rifle and pistol primers and -0.003" with their large primers.
Getting reconsolidation exact requires special tools, like the
K&M Primer Gage Tool or an adjustable primer seating tool combined with measuring primer pocket depths, primer heights, and depth of the primer in the pocket afterward. It's a good bit of bother. Also, you frequently find it is just plain hard to seat primers this deeply. Simpler methods include seating -0.004" below flush with the case head and assuming that covers it. The integral priming tool on the Forster Bonanza Co-ax press does this. You can also set the adjustable priming tools to do this. A still simpler method is to go by feel, just seating primers pretty hard. You can learn the fell by measuring a few and then just pushing hard.
Headspace Not The Issue
In a typical high-power rifle load, after the firing pin pushes the cartridges forward to its stopping point against the headspace determinate, growing pressure sticks the case body to the chamber walls. Shortly after that, it proceeds to force the head back against the breech face by stretching the brass at the pressure ring. When that happens, a protruding primer is reseated so it is flush with the face of the case head when you extract the case. Only when pressure is too low does the head not stretch to the breech face to reseat the primer. So a protruding primer is a low-pressure sign, and not an excess headspace sign.
The sign of excess headspace is a mushroomed primer spread out to start filling the width of the radius at the mouth of the primer pocket. This happens because, when the primer backed out, it backed out so far that so much of it was unsupported by the primer pocket walls that it actually was slightly inflated behind the case head by the chamber pressure. Then when the head stretched back to reseat the primer, it flattened that swollen portion, which is how it got out into the edges of the primer pocket and wider than the rest of the primer cup (that is the definition of mushrooming in a primer).
CAUTION: The mushroomed primer is also a high-pressure sign in a normally headspaced gun because even when the primer protrusion into the head clearance space is the normal small amount allowed by average headspace, it can still be inflated by excess pressure with the same result to its appearance. This is why you need to check headspace independently of primer signs. If your rifle uses a rimless bottle-neck cartridge with no belt, it headspaces on the chamber shoulder. Assuming the cartridge is at least a few thousandths shorter from head to shoulder than the headspace is for smooth chambering, the gap between the breech face and the case head when the firing pin pushes the cartridge forward is called the head clearance (sometimes mistakenly called the headspace even by "experts", an error mentioned in
the SAAMI glossary).
The easiest way to check for excess headspace is with a case comparator adapter for your calipers, such as the ones sold by Hornady. Ideally, you compare the case-head-to-case-shoulder measurement on a case that was fired in your chamber (and not yet resized) to the same measurement made on a headspace GO gauge for your chambering. A GO gauge is made to be the minimum headspace length. Because the case will spring back a thousandth or so after firing, you then expect the average result to be anywhere from -0.001" below the gauge measurement to about 0.007" over before it is excessing. Some gunsmiths and the military allow a bit more, but IMHO, over +0.008 it is time to look at setting the barrel back or replacing it.
If you don't have a headspace GO gauge for your chambering, compare your case to measurements taken from commercially loaded new ammunition. Such ammunition is usually -0.002" to -0.003" shorter than a GO gauge, so you would expect your normal measurement to be +0.002" to +0.010" bigger than the commercial load cases to be in the normal range.
If you don't have a comparator, you can improvise one with a spacer that has a hole that stops somewhere in the middle of the shoulder and, with it slipped over the mouth of the fired case and again over the GO gauge or the commercially loaded new ammunition, you can make the same comparisons. It is just more fiddly to have to keep the spacer, case, and caliper all lined up correctly.
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