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In the near-future story i'm developing the main character resorts to every trick in the book to improve the performance of her guns, including using subsonic cartridges for the sake of quietness, but semi-auto/full-auto guns that use the pressure of the gas expansion to load the next round are more prone to jamming because of the lower pressure of subsonic rounds.

But considering that the bullets used are made from depleted uranium that is considerably denser (precisely to compensate for the lower velocity) would there still be cycling reliability issues that would need to be fixed with a muzzle booster?

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    $\begingroup$ Tungsten is a much safer, more obtainable alternative, and is extremely dense as well. No radiation, toxic waste and contaminants. The advantages of depleted uranium are only significant at extremely high velocities when the depleted uranium burns (releasing toxic, radioactive dust, BTW). bbc.com/news/magazine-28263683 $\endgroup$
    – DWKraus
    Jul 2, 2022 at 20:58
  • $\begingroup$ @DWKraus Would the self-sharpening, penetrating properties of DU still work at subsonic velocities? $\endgroup$ Jul 2, 2022 at 21:39
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    $\begingroup$ I would guess no, although you don't need a self-sharpening projectile with a hard bullet at subsonic speeds. All I can find on DU relates to hypervelocity, often sabot flechette-like projectiles. AP rounds are actually less effective anti-personelle weapons because they tend to go straight through rather than expending kinetic energy generating tissue-destroying shockwaves or shattering into tissue-shredding fragments. Only use for heavily armored people/light vehicles, and then big, fast rounds are still vastly preferred. $\endgroup$
    – DWKraus
    Jul 2, 2022 at 22:03
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    $\begingroup$ Sabot and flechette rounds don't work well with silencers, BTW (they shed/spread inside the suppressor). $\endgroup$
    – DWKraus
    Jul 2, 2022 at 22:04
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    $\begingroup$ @PauloRaposo That seems backwards to me. A compact submachine gun with big, heavy rounds would have nightmarish recoil. I'd much sooner build the SMG for anti-personnel roles where its high firing rate can compensate for the weaker subsonic rounds, and keep the heavy armor-piercing ammo for a full-sized shotgun or rifle, where it will be easier to control. $\endgroup$
    – Cadence
    Jul 2, 2022 at 23:38

2 Answers 2

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Just use the ShAK-12.

ShAK-12 heavy assault rifle

It chambers in 12.7x55mm cartridges and is specifically designed to fire subsonic armor-piercing loads in full-auto.

The STs130PT bullet has a muzzle velocity of 295 m/s and a whopping 2.5 kJ.

High penetration loads can make short work out of 16mm RHA at 200m, which make it an excellent choice against anything short of Expanse-style powered battle suits (and even they might have a problem at short range). There are a few fancy loads with two bullets in one cartridge for extra downrange rate of fire.

Besides, it looks super cool with the mag behind the grip.

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  • $\begingroup$ It should be noted the Shak-12 uses lightweight aluminum cored rounds, nothing like the heavy rounds the OP is looking for. $\endgroup$
    – John
    Jul 5, 2022 at 3:04
  • $\begingroup$ @John: It can use lightweight aluminium core bullets. But for the purpose of this answer I believe the emphasis is on the 33 grams (509 grains!) heavy subsonic lead bullets. $\endgroup$
    – AlexP
    Jul 5, 2022 at 5:58
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Your choices are counter productive in multiple ways.

Choosing a heavier round with subsonic is counter productive since you need a larger charge and thus more noise to move the heavier round. you can have high power AP round or very silent rounds if you try to do both it just ends doing neither well. You end up with very loud low power rounds.

You can make auto cycling guns that can shoot subsonic rounds, it works well in semiautomatic rifles, but you can't combine it with high power rounds. Worse automatic fire very quickly deteriorates a suppressor so the gun gets louder as you use it. You are better off with two guns that do each job well, one for silent one for power, than one gun that does neither.

If you do decide to put heavy subsonic rounds in a submachine gun it will need a muzzle booster, if you are using a suppressor, but not if you don't have one. But using subsonic rounds without a suppressor defeats the purpose of the subsonic rounds, but as we have established you are kind of doing that anyway.

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  • $\begingroup$ But there is a supressor, every gun has an integrated supressor. I just didn't mentioned because i thought it was irrelevant to the question... $\endgroup$ Jul 3, 2022 at 4:13
  • $\begingroup$ Another key problem is there's nothing magical about DE or tungsten bullets, Their armor penetrating capabilities are a function of the velocity they are fired at as well as their density. Yes an AP round will penetrate far deeper into (or through) a non-armored target than a conventional bullet might but it also won't expand anywhere near as effectively as a modern hollow point round will leading, on average to less serious wound effects. So while I suppose there might be situations where a slow moving AP round could be useful as opposed to conventional bullets it will be a very rare event. $\endgroup$
    – Mon
    Jul 5, 2022 at 2:51
  • $\begingroup$ @PauloRaposo Don't forget the noise of the action cycling. The only way you get a truly sound-suppressed gun is if it is single-shot. $\endgroup$
    – Monty Wild
    Jul 5, 2022 at 16:29
  • $\begingroup$ @MontyWild I think there are secondary recoil management systems that might adress this issue. $\endgroup$ Jul 7, 2022 at 0:14

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