The new 5.7 Gold Dot appears to be a flat profile bullet. Looks quite a bit like a 22 Mag bullet....
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If Ruger normalizes/popularizes 5.7x28mm, and if the industry responds with new offerings, I wonder if "conventional wisdom" will change on the cartridge.
I'm old enough to remember a time when armchair commandos dismissed the .223/5.56x45mm as a mere varmint round. All credible professionals crowned the 12 gauge as king of CQB, "real battle rifles" were to be chambered in 7.62x51mm/.308, and the M16/M4 was for Y2K militias, not personal protection. (You can still find FUD like this at TheFiringLine or TheHighRoad.) Today, the AR platform has found widespread acceptance (and adoption) by everyone from SOF operators to soccer moms, and the .223/5.56x45mm is a consensus recommendation for home defense. This is not your father's Vietnam-era ball M193; the market has responded and bullet technology has gotten better.
tl;dr: big-and-heavy (e.g. .308, .45ACP) is fading; light-and-fast (e.g. 5.56, 9mm) is trending. Will 5.7x28mm ride the wave, or will physics get in the way?
1. You wouldn't be getting rifle grade performance. The "2200 FPS threshold" is a myth. Temporary cavity isn't solely dependent on velocity, there's no magic number where it suddenly becomes relevant.
Temporary cavity formation is likely largely dependent on drag resistance, which in turn is dependent on 3 primary factors, so far as we can choose/control from a terminal perspective: Instantaneous velocity, frontal area, and projectile shape. Inadequate frontal area and projectile form can lead to minimal wounding even at high velocities. See Fackler's testing of 5.45x39 FMJ in an anesthetized pig, where the bullet tumbled and yet caused no real additional damage in lung, intestine, or thigh tissue beyond what was directly crushed. On the flip side, shotgun slugs traveling at only ~1600 FPS can cause substantial radial wounding.
Effective fragmentation can massively increase the damage caused by the temporary cavity, as it reduces the ability of the damaged tissue to stretch.
In the case of the loading you linked, it looks like it breaks into 2 pieces, which doesn't really constitute effective fragmentation; jacket and core separation in a conventional hollowpoint would have the same effect. Otherwise, the frontal area is very low and the shape of the bullet would not likely have an inherently high drag coefficient. The muzzle velocity is high, but it's possible the bullet may decelerate quickly given the low mass. Keep in mind that the velocity component of drag at any given point along the wound track is based on how fast the projectile is traveling at the present point, not how fast it was traveling earlier.
2. It would probably reduce overpenetration, yes. Light and fast projectiles tend to destabilize and lose momentum pretty quickly when encountering barriers. A 27 gr bullet that breaks into two pieces might go through fewer walls than even some .223 varmint loadings, or at least, probably not much worse.
3. Based on my limited knowledge of cartridge variety, no. It seems like you've got most of the bases covered in that regard.
Though, technically, most service calibers can theoretically obtain velocities of over 2200 FPS given a light enough projectile.
Shot placement followed by adequate penetration will continue to be the true factors governing incapacitation.
Rifle rounds extend the distance at which these two can be applied while the handgun is best suited for applying these up close and/or when carrying a rifle around is not practical.
Shotguns seem largely overlooked in the equation due to weight, recoil and capacity. And while a slug certainly has the penetration aspect it seems buckshot may not. I remember reading something a few years ago regarding Jim Cirillo and his use of various firearms. From what I remember he said something like, "People hit with 00 buck would run away to be found dead later." Bleeding? Yes. Penetration to reach central nervous system or main arteries? No.
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Im not holding my breath, a company tried to used lightweight TSX bullets in the 5.7 a while back, the results were underwhelming. I dont think the 5.7 has enough momentum to get both significant expansion and adequate penetration, its more like pick one with the 5.7.
You're probably quite right.
Bullets built for rifle, as basically all the readily available. 224" bullets are, aren't likely to work well at 1,700 fps.
With that said, there is a 37gr bullet from Maker Bullets in GA that might work. It would be longer than a conventional cup and core so it might take up too much case capacity. But if that bullet can be driven to something like 2,000 fps it might actually expand although it is doubtful it'll reach 12-18" in gel.
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Great summary. Without mass and bullet design also being factored in, velocity would seem but one metric that can't stand alone. Intellectual exercise: if you could get a needle to travel at 10k FPS, what would its terminal performance look like? It would poke needle sized holes with a tiny perm wound track and little else due to lts low mass and very small cross sectional area etc. I suspect.
Conversely, DocGR, a student of Fackler commented that .44 mag approached rifle - like terminal ballistics dependent on barrel length, I recall due to stretch cavity effects, but I don't recall exact details.