Military cases are heavier, (more brass with slightly less volume capacity).
That's only true in 7.62. In 30-06, the military and commercial capacity differences are negligible for most applications. In 5.56, LC actually has slightly more internal capacity than its commercial counterparts. You will find a good comparison of weight and case water overflow capacity by scrolling about
1/3 of the way down on this page.
Notice the correspondence between weight and capacity doesn't always track. The Guatemalan IMG brass weighs 95.4 grains and has 30.1 grains of capacity, while the WCC99 is slightly heavier at 95.5 grains but has a slightly greater capacity at 30.5 grains. I've found with 7.62/.308W brass in the past that a difference in capacity is only predicted within about 20-25% by weight difference. You just have to measure water capacity if you want to be sure.
The advantage the military brass has in most semi-autos is the heads are double-struck and significantly harder than most commercial brass. This withstands hard extraction better and with less rim bending than softer brass does. An exception is Starline 5.56 brass which has harder heads, too.
The poor weight-to-volume correspondence occurs for two reasons. One is
different brands use different alloys with differences in density. But the bigger contributor is the exterior tolerances for the head. Even if you keep the height of the head the same so it doesn't impact internal capacity, the differences in diameter, rim thickness, rim relief angle, extractor groove depth, and extractor relief angle can add up to significant weight differences. I've calculated it for 7.62/308W size head before and it came out to over seven grains of difference being possible without changing internal capacity. Real cases never get all the way the tolerance extremes, but seeing three or four grains difference from the head tolerances is well within the expected range of variation.
The differences in 5.56 and 223 pressure have a lengthy and convoluted history. The cartridge was originally developed at 52,000 CUP, and when Remington commercialized it as the 223 Remington in 1962, that pressure number is what they used, together with the original chamber dimensions. Over the next couple of years, in order to maximize the number of rounds fired between failures, the military altered the chamber dimensions, making the chamber bigger, and when it finally made the cartridge an official military cartridge as M193, that chamber expansion had dropped the pressure to 50,000 psi by copper crusher (what SAAMI and the Aberdeen lab techs call CUP, but the military tech manual editors insist on calling psi, just expecting everyone to know what instrument measured the pressure, leading to a great deal of confusion).
So, here's the really confusing part. When the U.S. pushed NATO to adopt the same cartridge, NATO found it didn't pass their minimum standards for 350 m penetration and accuracy. So they got FN to develop the SS109 with 62-grain projectile and operating at a pressure of 55,000 psi by copper crusher (CUP). When SAAMI took the reference ammunition for the 223 Remington that was developed in copper crushers and fired them in the conformal piezoelectric transducer, the peak number they got was 55,000 psi (by transducer). Those two different unit numbers, both with the same magnitude of 55,000 have caused a lot of misunderstanding of the differences in the test standards and what is expected of a cartridge. Also, as powders have gotten better, the number that allows reaching the military requirements for velocity and gas port pressure with lower peak pressures has increased. As a result, what you really get by way of pressure in 223 and 5.56 ammunition is often not really different.
This test shows the same ammunition fired in two AR's and one sporting rifle, and the two ARs gave the highest and lowest pressure reading with the sporting rifle was in the middle.
There is no question the SS109 and its U.S. counterpart, the M855 should be running about 10% higher in pressure than the 223, but they don't always. In a SAAMI conformal pressure transducer, they should read about 58,000 psi. In the
NATO EPVAT system using Kistler pressure transducers sampling propellant gas through a channel (aka, a channel transducer) these same cartridges read a maximum of 4300 bar or 62,366 psi. The CIP adopted that number and uses the same channel transducer except where the NATO transducers sample gas from the bore just in front of the case mouth, CIP uses a point 25 mm forward of the head and samples pressure through a hold drilled into the case. This channel location tends to read a couple thousand psi higher than the NATO case mouth location, so European commercial 223 Remington can be expected to read about 60,400 psi in a NATO test barrel, and about 56,200 psi in a SAAMI conformal transducer. I have no way to confirm those numbers; they are just based upon averaging published values. It would be fun to have a complete set of the three equipment setups to see the differences in action, but I can confirm I have one declassified military document that shows conformal pressure transducer readings of 58,100 psi is used for one particular 5.56 round.
Today, the new "green" military round with lead-free everything is the M855A1. It is being loaded to even higher pressures than the SS109 and original M855 and is reported to cause increased throat wear. Personally, knowing that SAAMI pressure variation allows individual outlier rounds to reach up to 18% above the SAAMI Maximum Average Pressure (MAP; the number we usually see used as a pressure limit) and that and any of the above-mentioned pressures are is still below proof pressures, I don't have a problem shooting M855 in a 223 chamber. I just recognize that I am going to see the barrel wear out faster with a steady diet of that.