Technical question on slides, for a change

[YVK]

I got induced into thinking about quality of steel that goes into making modern pistols. Following that, I took a micrometer and measured side wall thickness on several pistols. I presumed that most of "pressure load" happens around breech face so I measured a wall opposite the ejection port.
Four different full sized guns measured at 0.20+, with HK P30 being thickest at 0.24 (I again admired how intricate P30's design was).
SIG P365 measured notably thinner, at 0.17.

Obvious question is why is that? What allows slimline gun be made with a thinner slide, or are full sized guns way overbuilt, or what else is a factor?

[TheNewbie]

I don’t know the answers here but this sounds like an interesting topic.

Did you measure any non 9mm guns?

[JCN]

YVK did you know that by the breech face some slides are cut so thin… that they actually have holes to see brass like a loaded chamber indicator or even complete cut outs so the cartridges can be ejected out of them!

(am I misunderstanding something? The slides don’t see the pressure contained by the case and the barrel….)

[YVK]

No. I have only two non-9s, both described as mature designs in a neighboring thread. Didn't think they would help in understanding how modern designs are made.

[YVK]

(am I misunderstanding something? The slides don’t see the pressure contained by the case and the barrel….)

Not as much of a full explosion but sees enough pressure to be moved backwards and cycled in 0.05 sec. The ejection port side has, naturally, ejection port that will bleed off some pressure. I presuming (but not stating as a fact) that breech face and wall opposite the ejection port will see the most pressure load. We have certainly seen breech faces cracked.

So the question remains: why SIG makes their wall 0.17 thick while HK makes theirs 0.24, while taking an effort and cost to thin out that wall towards muzzle (P365 does that too but to a much lesser extent)?

[MattyD380]

Well, most of the explosive force would be contained within chamber/barrel walls… so it wouldn’t be putting much blast pressure directly on the slide walls. Which I guess is JCN’s point.

By the time the breechface separates from the chamber, most of the pressure would have already dropped, and blown out with the bullet. At least in a locked-breech gun.

Unless the chamber walls need the slide walls on the outside to resist the explosion. But it doesn’t seem like it would work like that? Then again, I got Cs in science.

Nevertheless, this is interesting. I feel like we need more “how guns work” threads.

[AdioSS]

As an owner of many Berettas, this thread is amusing.

[YVK]

Obviously, slide's wall thickness may have nothing to do with pressures generated during ignition. I simply found it interesting that, with the same caliber, wall thickness on full sized guns was 15%-35% beefier than on a slim gun.

[MattyD380]

Obviously, slide's wall thickness may have nothing to do with pressures generated during ignition. I simply found it interesting that, with the same caliber, wall thickness on full sized guns was 15%-35% beefier than on a slim gun.

Yeah, it’s a worthy question.

Maybe it has more to do with the mass/momentum/cycling speed of the slide, than strength/durability.

[zaitcev]

It would be very interesting to polish a little chunk of SIG and HK slides and look under a microscope.

[JCN]

Not as much of a full explosion but sees enough pressure to be moved backwards and cycled in 0.05 sec. The ejection port side has, naturally, ejection port that will bleed off some pressure. I presuming (but not stating as a fact) that breech face and wall opposite the ejection port will see the most pressure load. We have certainly seen breech faces cracked.

So the question remains: why SIG makes their wall 0.17 thick while HK makes theirs 0.24, while taking an effort and cost to thin out that wall towards muzzle (P365 does that too but to a much lesser extent)?

So bizarre.

Slide walls aren’t to contain blasts.

That’s what barrel chambers are for.

The only pressure slam the slide sees is BREECH FACE backwards.

So if you were asking about breech face thickness, sure.

Or barrel chamber thickness, sure.

But slide walls?

Riddle me this.

If it took any side loading, don’t you think you’d see a lot of slide side abrasion?





So you’re asking about this area?

88187

Your question would be valid if your barrel was made out of rubber, lol.

The pressure is contained in the barrel chamber.

88188

The slide walls don’t see it.

Breech face does get hammered but not exploded.

Case base contains that.

If the slide walls saw pressure through the walls of the barrel chamber, you’d see marks on the slide walls from friction in a shot up gun.

88189

I still feel like maybe I’m misunderstanding what you’re asking because it’s like asking why Dodge used a certain hood sheet metal thickness when the explosions are in the engine.

As another data point…

Have you ever shot a Glock with a thumb over the back plate to prevent the slide from cycling?

It actually doesn’t take much force to do so.

Even the breech face isn’t getting super slammed because the case is pressed against it at ignition by the recoil spring and locking lugs of the barrel.

Breech face is also reinforced laterally in the unsupported area (yellow).

88190

The red area doesn’t see pressure.

That’s my best understanding of how a semiauto works, anyway.

I could be wrong.

If you took a revolver and extended the question, the cylinder chamber would be like the barrel chamber and there is no slide thickness at all! Just the frame below and above. To the rear where the case is, pressure on breech face but if you held the rear of cylinder when firing (if you avoided cylinder gap to barrel in the front), your fingers would be fine. No pressure.

[Oldherkpilot]

No. I have only two non-9s, both described as mature designs in a neighboring thread. Didn't think they would help in understanding how modern designs are made.

I thought the consensus was "vintage?"😁

[JohnO]

I'm going to go out on a limb and suggest that slide thickness in the area of the breach is strictly structural. We are talking about an area analogous to the non-existent roof on a convertible and the design considerations implemented to maintain structural integrity throughout the useful life of the product.

[JCN]

I'm going to go out on a limb and suggest that slide thickness in the area of the breach is strictly structural. We are talking about an area analogous to the non-existent roof on a convertible and the design considerations implemented to maintain structural integrity throughout the useful life of the product.

Well said!

[YVK]

I still feel like maybe I’m misunderstanding what you’re asking

I thought I clarified that. I found that slides on my full sized guns are considerably thicker than on my slim gun. I measured that specific area simply because it was easier to do for a right handed person. If it isn't related to any pressure, as you explained well, then to what? The suggestions so far have been "related to cycling" or "structural integrity", sounds fair. If the latter is the case, that would be particularly interesting to me because P365 is one of my carry guns. It'd be nice to know that its structural integrity not compromised by making its slide appreciably thinner.

[DMCutter]

This is what I think from an engineering standpoint. Bear in mind I'm a civil engineer, not a mechanical, so what I deal with is generally static loading. The barrel chamber contains the outward force of the blast, so the walls are under hoop stress, which doesn't make it to the walls of the slide. The bottom of the cartridge is sitting against the breech face and subjects the face to some compressive loading during ignition and as the slide begins to move to the rear. Since there shouldn't be significant space between the bottom of the casing and the breech face, the shouldn't be significant impact loading-a push vs a punch. I surmise there's a minimum thickness requirement for durability under thousands of cycles but it's probably pretty thin, and I think it would vary with the specific composition of the steel used for the slide. The elastic modulus of brass is about half that of steel so you would see more deformation of the base of the casing than the breech face, and if there was significant compression of the bottom of the casing you wouldn't be able to reload them numerous times before they mashed down and widened out enough to have to be trimmed.

[GearFondler]

I've heard of at least one RDS milling shop that had to change its design because the pockets and windows they were milling into the slides were starting to crack in places but that was a result of the battering the slide takes as it cycles. And no, I can't remember exactly who it was but they did readily admit it.

[hiro]

@PensFan

ETA, hmm, mention isn't working. Perhaps the #shitmods (https://pistol-forum.com/usertag.php?do=list&action=hash&hash=shitmods) can ask his opinion?

[YVK]

I surmise there's a minimum thickness requirement for durability under thousands of cycles but it's probably pretty thin, and I think it would vary with the specific composition of the steel used for the slide.


Thnx.

I've zero understanding of anything engineering, and my only other thought was whether the situation is somewhat comparable to what's described in Laplace law. I.e. larger in internal diameter slide of a full sized gun has higher wall tension and therefore requires to be thicker to withstand that. Needless to say, I don't really know what I'm talking about, just being curious.

[DMCutter]

Larger bore gives proportionally greater surface are (see "Associative Pet Peeve" in general discussion) but larger bore doesn't necessarily mean higher chamber pressure-SAAMI spec for .45 ACP is 21k psi vs 35k psi for 9mm or .40, so walls would need to be thicker in any 9mm vs a full size 1911.

[Clusterfrack]

@PensFan

ETA, hmm, mention isn't working. Perhaps the #shitmods (https://pistol-forum.com/usertag.php?do=list&action=hash&hash=shitmods) can ask his opinion?

Who exactly are you looking for? You can search the members with "fan" in their username here:
https://pistol-forum.com/memberlist.php

[Jim Watson]

I think that the part of the slide that is stressed by chamber pressure is the portion between the breech face where the case is pushing back like a piston and the locking lugs holding the action closed. Right where the OP is measuring. Right where Walthers, Berettas, and even Colts are known to break.

Why is a P365 thinner there than a HK? Because HK is promoting strength and durability and Sig is promoting convenience.
Has anybody shot a P365 a LOT to know its durability or in the abuse "tests" we see on youtube?

[MattyD380]

I think that the part of the slide that is stressed by chamber pressure is the portion between the breech face where the case is pushing back like a piston and the locking lugs holding the action closed. Right where the OP is measuring. Right where Walthers, Berettas, and even Colts are known to break.

Reaching here... but...

Wouldn't the chamber pressure acting on the breech face, in essence, transfer to the barrel-camming lugs (and ultimately the frame)... more than the slide? I imagine that the stresses on the slide are caused more by impact/shock against whatever arrests its rearward motion in the frame (as opposed to blast force). And, maybe, the impact of returning to battery.

But I agree: in 6-figure round counts, my money's on the HK.

[Jim Watson]

I don't think it works that way, the frame is not stressed until the slide and barrel start back in recoil.
You could probably strip the gun, duct tape the loaded barrel into the slide, and hit the firing pin with a hammer without breaking anything.

W.W. Greener went into that on double barrel shotguns, promoting the Greener Crossbolt, of course.
He said other makers emphasized their "holding down" bolts, with double Purdey underbolts and doll's head extensions.
But he pointed out that you could hold the barrels down with "a turn of twine" and that the main load was against the breech, flexing the action and taking the barrels "off the face." Which a properly fitted crossbolt would prevent.

[JCN]

I thought I clarified that. I found that slides on my full sized guns are considerably thicker than on my slim gun. I measured that specific area simply because it was easier to do for a right handed person. If it isn't related to any pressure, as you explained well, then to what? The suggestions so far have been "related to cycling" or "structural integrity", sounds fair. If the latter is the case, that would be particularly interesting to me because P365 is one of my carry guns. It'd be nice to know that its structural integrity not compromised by making its slide appreciably thinner.

If it’s not a load bearing part, it’s like wondering why the fender on a truck is thicker than on a 911.

Unrelated to the engine and more to the role of the vehicle.

[HCM]

I thought I clarified that. I found that slides on my full sized guns are considerably thicker than on my slim gun. I measured that specific area simply because it was easier to do for a right handed person. If it isn't related to any pressure, as you explained well, then to what? The suggestions so far have been "related to cycling" or "structural integrity", sounds fair. If the latter is the case, that would be particularly interesting to me because P365 is one of my carry guns. It'd be nice to know that its structural integrity not compromised by making its slide appreciably thinner.

I’d be less worried about structural integrity and more worried about how the weight /mass of the slide effected reliability.

[zaitcev]

Has anybody shot a P365 a LOT to know its durability or in the abuse "tests" we see on youtube?

Yes.
There were users with 50,000+ rounds.
Which is frankly amazing given how spindly it is, which is OP's point.
If 365's slide is that strong, why don't other gunmakers make slides like that? Look at retail prices of P365, clearly it's not using any space technologies.

[GJM]

I just grabbed some calipers, and I measured the 365XL at .175, the 43X at .155 and the Shield Plus at .145.

[Hot Sauce]

Why is a P365 thinner there than a HK? Because HK is promoting strength and durability and Sig is promoting convenience.
Has anybody shot a P365 a LOT to know its durability or in the abuse "tests" we see on youtube?

https://www.maxonshooters.com/blog/sig-sauer-p365-gun-review-100000-round-torture-test

[YVK]

Yes.
There were users with 50,000+ rounds.
Which is frankly amazing given how spindly it is, which is OP's point.
If 365's slide is that strong, why don't other gunmakers make slides like that? Look at retail prices of P365, clearly it's not using any space technologies.

GJM was kind enough to measure several slim guns, as shown above. Seem like everyone is making those slender slides, most likely to match the slim profile of frames. Full sized guns probably have theirs thicker to both match wider frames and for reliability purposes, and HK likely makes theirs visibly overbuilt in that area because HK.
That's my conclusion.

[jh9]

HK likely makes theirs visibly overbuilt in that area because HK.
That's my conclusion.

Your P30 is still largely based on the USP that was designed 30 years ago, isn't it? Unlike the VPs, the .40s weigh the same (https://hk-usa.com/hk-models/p30l/) as the 9. So either way I assume they were .40s first and foremost (or at least designed to accommodate the .40) which is probably part of the reason why they're thicker.

I wonder how an analogy to the F-117 and F-22 works. The former being a product of the limited modeling ability of computers available at the time.

The 365 is probably closer to what a modern 9mm "should" be in terms of shedding extraneous weight. Without commenting on shootability... just function and lifespan. Another few decades of more data and better modeling will usually produce a more streamlined design.

[balance]

My guesses to this would be reliability, durability, and maybe even safety.

Reliability because with more mass the slide should slow down and allow everything to move into place even if, say, the mag springs have fatigued, durability because there's more metal there, and safety because the last pistol I'd want to have a KB with is with a pocket pistol chambered in a cartridge that was originally designed around a full-sized pistol.

[JCN]

My guesses to this would be reliability, durability, and maybe even safety.

Reliability because with more mass the slide should slow down and allow everything to move into place even if, say, the mag springs have fatigued, durability because there's more metal there, and safety because the last pistol I'd want to have a KB with is with a pocket pistol chambered in a cartridge that was originally designed around a full-sized pistol.

Regarding KB, it seems like most modern pistols are meant to blow mag down and extractor out as sacrificial parts.

So unlikely that the slide would blow out, the pressure just isn’t contained and held there to rupture the metal of the slide even if thin.

I am more concerned about barrel and chamber thickness of some of the paper thin 380s when shooting over pressure rounds. The LCP has had a history of barrel rupture when subjected to high diet of hot rounds.


88246

[fixer]

I got induced into thinking about quality of steel that goes into making modern pistols. Following that, I took a micrometer and measured side wall thickness on several pistols. I presumed that most of "pressure load" happens around breech face so I measured a wall opposite the ejection port.
Four different full sized guns measured at 0.20+, with HK P30 being thickest at 0.24 (I again admired how intricate P30's design was).
SIG P365 measured notably thinner, at 0.17.

Obvious question is why is that? What allows slimline gun be made with a thinner slide, or are full sized guns way overbuilt, or what else is a factor?

I've done component design before for an automotive oem so here is my take...

There is probably a minimum thickness needed for high cycle fatigue. I think the entire discussion is due to a level of safety factor in design on top of double stack vs single stack dimensions.

This area you are looking at is loaded in tension mostly. However the vibration from the ignition/explosion likely contributes to some overall stress in the area. It'd be good to know if an ignition puts the slide near its mechanical natural frequency because this would drive further stress analysis...

With a full size pistol, double stack, there is likely also an assumption that the pistol will get dropped on this particular side wall area, etc. So the assumption is that the thickness should accommodate for some cracking and still maintain integrity.

With slim guns the safety factor here is much less but still workable.

So its like this generically

Double stack gun. Width will be around 1.0 inch. Slide internals need to be 0.6 inch. That leaves .2 inch per side for the sidewall area.
This area can have a calculated max stress, yield point, and fatigue point determined with modeling software and inputs.
Then a crack is simulated, and same stress analysis is done. 0.2 inch good? yep...ok...
No good? ok...what mass or dimensions are sufficient? 0.25. ok...drawing board time...need to get overall stress lower in this area so that 0.2 inches will pass our standards.

Single stack gun will have similar considerations but the safety factor in the model will be substantially smaller than a double stack gun.

[fixer]

Regarding KB, it seems like most modern pistols are meant to blow mag down and extractor out as sacrificial parts.

So unlikely that the slide would blow out, the pressure just isn’t contained and held there to rupture the metal of the slide even if thin.

I am more concerned about barrel and chamber thickness of some of the paper thin 380s when shooting over pressure rounds. The LCP has had a history of barrel rupture when subjected to high diet of hot rounds.



Ernest mentions a safety design feature of the 92 at about 20 seconds I thought was interesting and relevant here.

https://www.youtube.com/watch?v=Db8t-f54Im0&t=310s