A theory on JHP expansion methods - "squishers" versus "spreaders"?

[Jim March]

Folks,

We know that it's possible for rounds completely lacking a hollowpoint to expand - jacketed softpoints in, say, 357 moving at REALLY high speeds against hard targets will mushroom, although it'll take so long in some cases that they're best against quadrupedal furcritters rather than bipedal crookcritters.

On the other hand, we have rounds out there that are just barely going fast enough to expand, have huge JHP cavities and they "petal outwards"...the pics we've seen so far of the Speer 135grain 38+P Gold Dots are classics in this regard.

I think we're seeing different "mechanisms" for expansion. Some rounds allow the lead at the leading edge of the JHP to "squish inwards" and then flow outwards into a mushroom (like a jacketed softpoint can if it goes fast enough) whereas others have the lead at the forward lip moving "outwards first" - "spreaders" instead of "squishers".

If this is correct, it explains why certain very-small-hollowpoint-cavity "primitive" hollowpoints still work pretty damn good as long as they go fast enough. The classic Cor-Bon 115+P 9mm at over 1,300fps in a duty-size gun is an example of this sort of "squisher", as is the Remington 125grain 357 full-house combat load.

IF I'm right and these primitive hollowpoints are acting like "slightly enhanced jacketed softpoints", they should be somewhat resistant to clogging, in that the lead is going to "squish inwards" and begin mushrooming at least some no matter what they hit.

Starting with the Hydrashock, we've been going a different direction, relying primarily on "spreading" effects. But if done wrong, this can clog. Speer took a good step forward by reducing jacket thickness, Hornady's XTP and the Ranger/Talon series use a thick but pre-scored jacket.

But...I'm not sure we've seen the end of the "squishers". Cor-Bon's Pow'R'Ball is combining "squish" and "spread" construction. Bufallo Bore's new 158 38+P plain lead at 1,000fps from a 2" tube will probably have the same modest hollowpoint as the Winchester and Remington equivelents (Keith-profile) so will probably do at least as much "squishing" as "spreading".

Which might be a good thing...because I suspect that no matter how good a "pure spreader" design you have, it'll always be possible to clothes-clog them in at least extreme circumstances.

If this is correct and "squishers" are the "cold weather kings", Cor-Bon has maybe made a mistake going too light on the Pow'R'Balls, esp. the 38/9mm/357 100grainers.

Test data on four-layer-denim gelatin is going to tell the tale.

 

[sm]

"squishers" versus "spreaders"?

Interesting Theory Jim, I can see where you are coming from.

Being in Revolvers , first thing comes to mind is a snubby .38spl for CCW.
The old 158 gr LSWC - HP of standard vel., Seemed do well , hit POA/POI and recoil was managable in most guns...especially the all steel guns.
Basically the old "big and slow" idea.

The sharpeness would "bite" and less chance of bounce off bone than say a LRN. Even clogged, the bite would aid in penetration.


Just some thoughts that first hit me when I read your post Jim.

 

[RWK]

Jim,

I agree with your fundamental thesis and had some similar thoughts – not nearly as refined or articulated – when I tried to understand the “expansion mechanisms†of PowRBall and EFMJ (when they were introduced a couple years ago), versus traditional JHP (HydraShok, Golden Sabers, etc.) and also the old, reliable 158 grain LSWHPs and LSWCs in .357 magnums/.38 Specials (i. e., the "FBI load", which performed well for so long).

My thought is some rounds expand by having the bullet’s leading edges CAVE INTO the cavity under dynamic pressure; this forces a “mushrooming†of the projectile. On the other hand, “modern†JHPs (of the HydraShok and Golden Saber era/design) expand by having greater momentum centered at the bullet’s core, thereby forcing the serrated leading-edge segments to FLAP OUTWARD. I apologize to you – and to others – for this unscientific explanation and for the terminology. I believe I understand the mechanics, but trying to express it succinctly and cogently is not easy.

At any rate, there are “cave into†and the “flap outward†approaches. It would be interesting and instructive to paint interior and exterior surfaces of various expanding loads with distinct colors (however, this “paint†could not alter the projectiles’ performance). You could then ascertain if a bullet caved inward or flapped outward by firing them into ballistic gelatin (or even through plastic water/milk jugs).

My theory may be completely fouled-up, and I will certainly be interested in reading your comments -- as well as others – concerning its accuracy.

Best regards.

 

[Jim March]

I don't think the various small-hollowpoint designs are caving in...I think they're collapsing more or less straight in for a bit, then outwards.

If they were caving in the first instants, we'd have seen evidence of that in gelatin photos of "almost opened but failed partway through" cases, which are common as fleas (see also ammolab entire collection, they're scattered throughout). Not one shot shows such a collapse.

Some good examples:

http://www.ammolab.com/38spcl_-1.htm - look at the 38+P 158grain LSWC-HP Remington (#2) - it's clearly barely started to expand, not collapse.

There's one example here: http://www.ammolab.com/9mm_section1.htm - #24, but it's a 147grain 9mm silvertip load that didn't break 1,000fps. Still, it DID seem to collapse inwards in bare (not clothed) gel.

All the rest are linked from: http://www.ammolab.com/Test Results.htm

I've seen a lot of these, from a lot of sources, and "hollowpoint collapse" isn't a common failure state at all. If some rounds were doing this, we'd see that more often.

I think there's two reasons:

1) On impact with clothing, the clothes "stretch open" before giving way, imparting a slight "outwards momentum" to the HP cavity leading edge.

2) Once moving in fluid, the fluid moving past the bullet is at lower pressure than the stuff crammed into the cavity. The lead will flow towards the area of lower pressure, all things being equal.

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However, they're NOT equal. Look at the Remington 125 full-house 357 as a classic example: it's a fairly distinct "cone profile", small hollowpoint cavity, more or less straight interior walls. I suspect that the inwards force on the cone shape is somewhat "balancing out" the outwards force from within the cavity, so that the straight inwards pressure from the leading hollowpoint edge is the main force in play. So that leading edge flows straight in and with a slight outwards direction, until it gets to about the core lead zone and moves outwards at a rapid clip.