Wisest.fool,
The first thing we need to establish is what you are loading. You listed a 168-grain ELD-X. Hornady does not appear to make one. They make a 168-grain ELD, and they make a 178-grain ELD-X, the lightest ELD-X shown in their line. So, if your bullet box says ELD-X, the bullet must weigh at least 178 grains. Please check and see which bullet it is.
Let's find out your actual case capacity. Take a fired case that has not been decapped or resized yet. Weigh it and measure its length. Fill it with water level with the case mouth (no meniscus, either positive or negative and no extra drops on the outside of the case) and weigh it again. Subtract the empty and dry weight from the wet weight. The result is what is called Case Water Overflow Capacity. That and the case length are what are needed for QuickLOAD. If you repeat that measurement on 30 cases and list them all, we can get a statistically very sound idea of what the variation in the capacity of those cases looks like to see how much chance of a case having extreme variation there is.
For everyone discussing them, the CCI #34 primers are identical to CCI 250 magnum primers except the anvil is shaped to be a little shorter and with wider-angled legs. That anvil design reduces their sensitivity to military levels to better tolerate the inertial impact of a floating firing pin on a primer cup. The cup is no harder than the CCI 250. I got that information straight from the horse's mouth by calling and speaking to CCI about it. The lady on the phone looked up the cup and the priming mix and priming mix quantity and confirmed the CCI #34 and 250 are identical in those regards and only the anvil is different. The only military sensitivity primer currently on the U.S. market that reduces commercial sensitivity by using a thicker cup is the Federal GM205MAR small rifle primer for the AR. Federal gave me that cup thickness information by email. Otherwise, that primer is the same as the GM205M standard match small rifle primer. They don't make a military sensitivity large rifle primer.
Too much primer? IMR 4895 does not need a magnum primer to ignite well, and, indeed, shooters often report the best accuracy with primers that are on the mild side. Palma Match brass from Lapua even has a small rifle primer pocket with a smaller-than-standard flash hole (0.069" instead of 0.079") to cut down on primer influence on final pressure and is intended for stick powders like N140 and 4895. The Remington 700 does not have a floating firing pin, nor do most people work a rifle bolt so fast that a floating firing pin would pick up the inertia needed to fire a cartridge even if it had one. For these reasons, there is no point in using military-sensitivity magnum primers in this load for that rifle and the hotter primer is possibly raising pressure unnecessarily. Add that to what is possibly an overweight bullet running a couple of grains of powder over its maximum, and you have elements to contribute to gaining some pressure.
That said, a case should withstand proof load pressures at least once. The top of the .308 Win proof load range is 92,000 psi. This expanded case head has gone well over that. If it were simply a case head flaw, you would not expect all that primer pocket expansion. It clearly tried to hold up to press that was grossly excessive. Hodgdon's data for a 168-grain Sierra MatchKing in a Winchester case (a couple of grains more room inside than LC) shows IMR 4895 loaded to 45.5 grains maximum and compressed about 2%. After tweaking QuickLOAD to match their pressures, it takes almost 51 grains of powder compressed about 15% (very difficult to actually load) to get to the top of the proof range. If I switch to assuming a maximally fast lot of 4895 I can reach the top of the proof range with about 48.3 grains and 9% compression, which is possible, though just barely. The above makes me doubt a simple overload is responsible, though if the bullets were 178-grain ELD-X types, the 168-grain proof load would get up to about 110,000 psi. But that takes 114% compression, and that's very hard to achieve.
Wrong powder? We seldom interrupt the flow of the loading process once we are setup and running. So unless you had to interrupt to use a different container of powder, that seems unlikely, but odd things happen. Occasionally someone will “return” a powder to the wrong container without realizing it. Letting someone else load one of your rounds could do it. Letting someone else’s load get mixed with yours could do it.
In any event, with the uncertainty, you are right to want to pull the rest of the bullets. IMHO, the best bullet puller for doing large numbers is the Hornady Cam-Lock Bullet Puller. It works with different collets for different bullet diameters and you would have to get their .308 collet separately to go with it. It takes a little setting up to get it adjusted, but once you do, it is way faster than the T-handle type collet pullers, much less the inertial pullers. I have pulled down thousands of surplus rounds with mine.
Did something get into the case? I once had a piece of Lake City 30-06 brass that felt odd in my hand. It turned out to weigh 35 grains more than other cases. My inspection found a lump of bullet core lead in the case down near the head and impressed against the brass by firing, so I had to get it out with a pick. Anything that can use up case capacity will raise pressure. An insect crawling in, if it hasn’t dried out, will take up space and raise pressure.
Was there a bore obstruction? If a bullet is fired into the bore and gets stuck, you experience essentially no recoil, which you ought to notice. The case doesn’t seal against the chamber well, so a little smoke comes out around the bolt. You still get some sound, but it feels wimpy. No bullet hole appears in the target. There are other kinds of obstructions left by cleaning tools. A broken Bore Snake; an unscrewed or broken cleaning rod jag and patch, etc.
Undercharge of powder? In Dr. Lloyd Brownell’s 1965 study of absolute pressure for DuPont, he finds charges of IMR powders (3031, specifically) at around 30 grains of charge weight in the 30-06 (about 55% case fill) begins to demonstrate very erratic pressure. This is at its worst at 25 grains (46% case fill). It appears to be due to charges that light being able to spread out in the case enough for more of it to ignite at once than occurs in a normal powder column burning from the breech end forward. Later, writing in Handloader Magazine in response to another writer opining that reduced charges of that sort were fine because he’d never had a problem with one, Dr. Brownell pointed out the issue is statistical in nature, meaning very high pressure excursions had a low occurrence probability, but it wasn’t zero as his lab had observed up to double the rated cartridge pressure occurring in this circumstance. It might only happen once in 20,000 rounds, but it can happen, and bad luck can make it happen the first time you try one.
Had powder gone bad? Many powders, when they start to break down, will consume their deterrents early on in the process, leaving the remaining powder burning much faster than it should, which raises pressure. The Navy documented roughly 50% pressure increases caused by this. I have seen powder breakdown in individual cartridges in a lot while others remained shootable (in the short term). This is randomly initiated when a group is near to going bad, and one of the cartridges has to be first. However, unless you have a bad lot of powder or have exposed the individual cartridges to excessive temperatures for some time period, it is another improbable cause of the problem. Just not impossible. Since some lots of some powders wind up recalled for premature breakdown, carefully inspect the powder in the cartridges you pull down. Make sure none is getting oily looking or clumpy. Make sure none has an acrid acid scent to it. Pour it on a sheet of white paper and slide it off, looking for rust-colored dust to be left behind.
I, too, would be concerned about cracks in the receiver lockup area. The idea of sending it to Remington is the best one. They will tell you if the receiver is salvageable or not. In the event there was a blockage in the receiver, you want them to look for a bulge in the bore, too.