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The Annoyed Man wrote: ↑Fri May 31, 2019 2:51 pm Very good explanation of pressure and volume, but I have to ask what’s the advantage of 7.62x40 WT over 7.62×35mm (.300 Blackout)? Wikipedia says that:

Wilson had been hunting feral hogs with both the .30 Remington AR and .300 AAC Blackout. However, since most feral hog hunting is performed at night, he did not like looking for proprietary brass cases after he had fired them and designed the 7.62 X 40mm around the inexpensive and readily available 5.56 NATO cartridge.

Well... the 7.62x40mm is also a proprietary case, and .300 Blackout cases can also be made from inexpensive 5.56 brass. Even the bullet weight range is similar up to a point....except the Blackout range includes heavy subsonics.

I’m not knocking the WT cartridge, but I’m unclear as to its advantages over .300 Blackout. Please enlighten me.

If you're limited to buying factory ammo, the 7.62x40 is awful. Wilson relaunched a slightly modified (and worse) version called the .300 HAMR that means even the maker will probably stop selling ammo for it. It is also useless if you want subsonics.

The advantage is 2-300 FPS with 150 gr bullets, matching a .30-30 at 100 yards and beating it thereafter. The .300 Whisper was built to replace the MP5-SD using a standard 5.56 platform. The 7.62x40 was built to replace the .30-30 and 7.62x39. While the lack of factory ammo is a huge issue for most folks, the round is a reader's dream. It is NOT a proprietary case (the reason that Bill Wilson hired Kurt Buchert was partially that he got sick of losing 6.8 SPC cases while killing hogs at night on his ranch), but a cut and resized 5.56 NATO case. Most of my brass was made from range pickups that I cut, formed, annealed, and fireformed. As long as I can get 5.56 NATO cases and primers, 150 grain .308 bullets, and either CFE BLK or revolver powder, I don't need to care what factory ammo prices do.

Here is how I think this issue. I'm not putting it forth as the only way, just as a thought experiment that may be useful.

A cartridge and the barrel are a single pressure vessel, much like the piston in an internal combustion engine (replacing the air/fuel mix with nitrated hydrocarbons and eliminating the ability to reciprocate). In general, the higher the initial pressure and the lower the final pressure, the more efficient the piston will be at converting chemical energy into motion (see Carnot's theorem of heat engines, where pressure is replaced by heat in a more rigorous and technically correct explanation....but it's much easier to think in terms of pressure for our limited purposes).

The greater the ratio of expansion volume (bore area*length) to chemical energy (powder charge weight, ignoring small differences in smokeless powder energy content from, say, more nitroglycerine), the more of that energy will be converted into motion of the projectile. This is why black powder arms have huge barrels: adding expansion volume was all those designers and Smith's could do because the chemistry (black powder and early nitrocellulose) and materials science (barrel steel and later case brass) put hard limits on their maximum initial pressures. All they could do was wring every foot-pound out of a low power system. Smokeless powder changed that, allowing far higher initial pressures to produce very high velocities from small expansion volumes. This led to everything becoming a "carbine" (historically, a rifle with a 24" barrel was a carbine, c.f. the Mauser 98k).

This technological revolution doesnt change the laws of physics; it merely improved our ability to harness them. Today, we have the ability to generate useful velocities with extremely short barrels, and the tradeoff is muzzle blast. The high initial pressure that can be so useful can produce deafening and blinding blast rather than useful velocity if not given a sufficient expansion volume.

To that end, I suggest a website called ballistics by the inch, at bbti.com, where they shot a variety of calibers out of varying length firearms. There is also a software package called Quickload which can estimate the effects of barrel length, powder burn rate, charge weight, and bullet mass on velocity and muzzle exit pressure. I'd be happy to run a few numbers for you if you like since the program isn't cheap and has limited utility for most folks.

David Ricardo once said in reference to WW2 aircraft engines (supercharging was just taking hold then) that we must get away from measuring engine power in displacement and instead think instead of how much air the engine can be made to efficiently consume. When building pistol or rifle, we must think in terms of how much powder a barrel length of our chosen length can be made to efficiently consume. That's why folks are suggesting the .300 Blackout/Whisper over the 5.56 NATO for your chosen barrel length.

My preference is a .357 Sig with a 5.7" barrel or a 7.62x40 WT in a bullpup with a 16.5" barrel. The IWI bullpup shotgun might be the best choice for a pure home defense gun.

A rifle normally releases gas at the muzzle at 3-5000 psi. A .50 with a carbine length barrel (29" vs the 45"found on an M2) will do 9000 psi. An SBR can't match the amount of gas from a .50, but it will exceed the pressure. Ian McCollum of Forgotten Weapons has a YouTube clip that demomstrates this issue:

https://youtu.be/oU_sOb7Fkdo