Keith Slatowski; Bianca Cemini Slatowski v. SIG SAUER, INC., On Appeal from the United States District Court for the Eastern…

PRECEDENTIAL
UNITED STATES COURT OF APPEALS
FOR THE THIRD CIRCUIT
_______________
No. 24-1639
_______________
KEITH SLATOWSKI; BIANCA CEMINI SLATOWSKI,
Appellants
v.
SIG SAUER, INC.,
_______________
On Appeal from the United States District Court
for the Eastern District of Pennsylvania
(D.C. No. 2:21-cv-00729)
District Judge: Hon. R. Barclay Surrick
_______________
Argued: April 30, 2025
Before: KRAUSE, BIBAS, and MONTGOMERY-REEVES,
Circuit Judges
(Filed: August 1, 2025)

-- 1 of 16 --

2
Robert W. Zimmerman [ARGUED]
SALTZ MONGELUZZI & BENDESKY
1650 Market Street
One Liberty Place, 52nd Floor
Philadelphia, PA 19103
Counsel for Appellants
Kristen E. Dennison [ARGUED]
LITTLETON JOYCE UGHETTA & KELLY
2460 N Courtenay Parkway, Suite 204
Merritt Island, FL 32953
Jonathan T. Woy
VSCP LAW
2001 Market Street
Two Commerce Square, 41st Floor
Philadelphia, PA 19103
Counsel for Appellee
_______________
OPINION OF THE COURT
_______________
BIBAS, Circuit Judge.
Do not underestimate jurors. Pennsylvania law requires
expert testimony for complicated questions beyond a jury’s
knowledge. But not every hard question is too hard for the jury.
Here, the District Court rightly excluded two experts’ testi-
mony about what caused a gun to fire accidentally. Yet it went
on to grant summary judgment, reasoning that without an expert,

-- 2 of 16 --

3
the jury could not determine causation. That was wrong. Given
the other admissible evidence, a jury is well equipped to figure
out what caused this gun to fire. So we will affirm the District
Court’s exclusion of the expert testimony on causation but
reverse its grant of summary judgment.
I. AFTER SLATOWSKI’S GUN FIRED INTO HIS LEG,
HE SUED THE GUN MAKER
A. An officer gets hurt, allegedly because his gun was
unsafe
This case is about a simple question: Why did an officer get
shot in the leg? The story starts at a quarterly gun training for
federal immigration agents. There, ex-marine-turned-agent
Keith Slatowski was practicing with the pistol he had been
issued, a Sig Sauer P320. He fired about four magazines,
reloaded, and put the gun in his holster. When he next went to
draw it, it fired a bullet into his hip and out his thigh. He says
he did not touch the trigger, but only the grip.
If that sounds unusual, it is because the P320’s design is
unusual, making it much easier to fire—intentionally or not. To
start, it is a single-action pistol; its trigger does only one thing.
That differs from the trigger of a double-action pistol, which
does double duty: It both cocks the firing mechanism and
releases it. By contrast, on a single-action pistol, the shooter
cocks the firing mechanism by (on the P320) pulling and
releasing a slide on the top of the gun. Then the trigger does its
single job, releasing the cocked firing mechanism. After this,
the shooter need not re-cock the gun; the mechanism re-cocks
itself using the recoil generated each time the gun is fired.

-- 3 of 16 --

4
That mechanism makes a single-action gun’s trigger quite
sensitive. Like a coiled spring in a mousetrap, the cocked firing
mechanism stores explosive energy, requiring less of a tug on
the trigger. That increases the risk of accidental firing, so many
single-action handguns use external safeties. An external
safety forces the shooter to deactivate it before firing. One type
is a thumb safety, a switch near the back of the pistol that the
shooter can flip with his thumb. Another is a grip safety, which
prevents firing until the shooter squeezes the grip tightly. Both
types of external safeties do the same thing: Each stops a gun
from firing until the shooter does something to turn the safety
off. Until he does, an external safety stops the trigger from
being pulled altogether.
Internal safeties, by contrast, do not stop accidental trigger
pulls. All they do is keep the gun from firing without a trigger
pull. That addresses the risk that jostling the gun might release
the tension inside the firing mechanism and fire the gun—not
the risk that someone might graze the trigger and so shoot a
bullet by accident.
Slatowski’s P320 had no external safety. Though the ver-
sion issued to the military has a thumb safety, the version
issued to law-enforcement officers does not. But it has two
internal ones. Keep in mind this diagram, from one of Sig
Sauer’s expert reports, mapping out the mechanisms described
below:

-- 4 of 16 --

5
The first internal safety immobilizes the striker pin, the part
of the firing mechanism that ignites the gunpowder. The striker
pin is first cocked by pulling back the slide. Once cocked, it is
ordinarily held back by another part, the sear. The safety is a
spring that keeps the sear in tension against the striker pin until
the trigger is pulled. When that happens, the sear moves,
releasing the striker pin, igniting the gunpowder, and firing the
round in the chamber. Without the spring, the sear could more
easily get dislodged without a trigger pull.
The second internal safety is a lock that works as a catch
for the striker pin. If the sear slips, releasing the striker pin, the
safety lock catches the pin before it can complete its journey.
But internal safeties are not foolproof. The safety lock can
be disengaged by pulling the trigger back as little as 1/13 of an
inch—about the thickness of a quarter. That would leave only
the sear, which can be jostled loose.
That background sets the stage for Slatowski’s theory of the
case. He sued Sig Sauer, arguing that it designed the P320

-- 5 of 16 --

6
defectively because very light trigger pressure is enough to fire
the gun. On his telling, something could have gotten into the
holster, lightly pressing the trigger and so disengaging the
safety lock. Then, when he went to draw the gun, the motion
jostled it, dislodging the sear, releasing the striker, and firing
the gun.
Slatowski argues that the P320 would not have fired if Sig
Sauer had used a different safety design: a tabbed trigger,
shown below. A tabbed trigger is a bit like an external safety.
It is a mini-trigger attached to the main trigger that must first
be pulled before the rest of the trigger will move. It forces the
shooter to put his finger squarely in the center of the trigger,
which could reduce the chance that debris in the holster would
accidentally pull the trigger. This is what a tabbed trigger looks
like:

-- 6 of 16 --

7
B. The District Court did not let him proceed to trial
because it excluded his experts on causation
To make his case, Slatowski offered the testimony of two
experts. He offered them to show both that the gun’s design
was defective and that the defective design caused the gunshot.
Those two experts are at the heart of this case.
The first expert was a gunsmith, Dr. James Tertin. After
inspecting Slatowski’s gun, Tertin opined that (1) the P320’s
design makes it easy to fire, (2) its lack of a manual safety
makes it “unique among single-action pistols and uniquely
dangerous,” and (3) a tabbed trigger would lower the risk of
accidental firing. App. 204 (semicolon omitted).
The second expert was a firearms instructor with a doctor-
ate in ergonomics, Dr. William Vigilante, Jr. He analyzed data
about accidental firing and watched videos of the P320 in action.
He opined that (1) if Sig Sauer had added an external safety,
“it would have significantly reduced the risk of an
unintentional discharge,” and (2) its failure to add one “was
most likely a cause” of the accidental firing. App. 245, 247.
The District Court excluded both experts’ causation testi-
mony. It ruled that while both could opine on whether the
P320’s design was defective, neither could opine on whether
those alternative designs would have prevented this particular
accidental firing. Because neither expert had simulated the
conditions of this shooting, the court reasoned that neither’s
testimony was reliable evidence of what had caused it.
That exclusion doomed Slatowski’s case, the District Court
thought, so it granted summary judgment for Sig Sauer. Under

-- 7 of 16 --

8
Pennsylvania law, it reasoned, the causation question was too
complicated for the jury to decide without an expert’s help.
Slatowski now appeals both rulings. We review the District
Court’s exclusion of expert testimony for abuse of discretion
and its grant of summary judgment de novo. Oddi v. Ford
Motor Co., 234 F.3d 136, 146 (3d Cir. 2000); Tundo v. Passaic
County, 923 F.3d 283, 286–87 (3d Cir. 2019). As the parties
and the District Court agree, Pennsylvania substantive law
governs this state-law tort suit.
II. THE DISTRICT COURT PROPERLY EXCLUDED EXPERT
TESTIMONY TO PROVE CAUSATION
As the plaintiff, Slatowski bears the burden of proving each
element of each tort, including causation. Berkebile v. Brantly
Helicopter Corp., 337 A.2d 893, 898 (Pa. 1975), overruled on
other grounds by Reott v. Asia Trend, Inc., 55 A.3d 1088,
1098–1100 (Pa. 2012); Spino v. John S. Tilly Ladder Co., 696
A.2d 1169, 1172 (Pa. 1997). No one disputes that the gun
injured Slatowski. The dispute, rather, is whether that injury
was caused by the gun’s defective design. Under Pennsylvania
law, Slatowski could prove that by showing that if Sig Sauer
had used a different design, he would not have sustained the
same injuries. See Oddi, 234 F.3d at 143; cf. Gaudio v. Ford
Motor Co., 976 A.2d 524, 532 (Pa. Super. Ct. 2009).
To carry that burden, Slatowski offered opinions from his
two experts, Tertin and Vigilante. They explained the P320’s
design and potential alternatives. They included technical
details about the firing mechanism as well as opinions based
on videos of accidental firings.

-- 8 of 16 --

9
The District Court allowed that testimony but faulted some
of the experts’ conclusions about what happened that day,
such as:
• If Slatowski’s gun had had a manual safety, it “most
likely would not have discharged.” App. 15, 208
(Tertin).
• If the gun had been safely designed and Slatowski’s
finger or an object had touched the trigger, “it most
likely would not have discharged.” App. 16, 208
(Tertin).
• “The defective design of the P320 was a proximate
cause of [Slatowksi’s] accident, in the event that his
finger or a foreign object touched the trigger.” Id.
(Tertin).
• “Sig Sauer’s failure to integrate an external manual
safety into the design of the P320 … was most likely
a cause of the subject unintentional discharge.” App.
18, 245 (Vigilante) (internal quotation marks omit-
ted).
• “If there was a tab trigger in there that required a
deliberate and square pull of the trigger, there’s
nothing to indicate [that pressure from the holster on
the trigger] would have occurred in this instance and
therefore, the gun wouldn’t have unintentionally dis-
charged.” App. 19, 287 (Vigilante).
The District Court rightly excluded those conclusions.
Though they speak to whether the gun’s design caused the
injury, they are not based on testing specific to the context of

-- 9 of 16 --

10
the shooting. Instead, they are based on mere analysis that the
gun could fire more easily in theory.
Consider what an expert analysis of this situation could
have looked like. Slatowski’s experts could have tested designs
against each other in a context that mimicked what happened
that day. They could have simulated a situation in which a gun
was drawn from a holster. But neither expert did. Instead, they
both resorted to another method: speculation.
That is a problem for Slatowski. Expert testimony offered
in federal court must be “the product of reliable principles and
methods.” Fed. R. Evid. 702. “The hallmark of Daubert’s reli-
ability prong is the scientific method.” Soldo v. Sandoz
Pharms. Corp., 244 F. Supp. 2d 434, 559 (W.D. Pa. 2003)
(explaining Daubert v. Merrell Dow Corp., 509 U.S. 579, 593
(1992)). In turn, “the scientific method [is] the generation of
testable hypotheses that are then subjected to the real-world
crucible of experimentation, falsification/validation, and repli-
cation.” Id. at 457; see also Daubert, 509 U.S. at 590, 593 (requir-
ing “appropriate validation” and focusing on “whether [a
theory or technique] can be (and has been) tested”). Specula-
tion does not cut it. After all, “the word ‘knowledge’ connotes
more than subjective belief or unsupported speculation.”
Daubert, 509 U.S. at 590.
Yet Slatowski never bridges the gap between theory and
reality. Instead, he just touts his theory. His experts explained
how easily P320s fire, reviewed statistics about how often
other P320s fire accidentally, and analyzed why those acci-
dents might be so common. But neither expert considered how
the conditions that could cause accidental firing might have

-- 10 of 16 --

11
manifested on the day of Slatowski’s accident. Their testimony
is reliable about whether the P320’s design could have caused
an accident, but not whether it did cause this accident. That
would have required not just theory, but factual context.
So the District Court did not abuse its discretion by exclud-
ing Tertin’s and Vigilante’s conclusions about causation. But
that exclusion is not fatal to Slatowski’s case, as we will see.
III. EVEN WITHOUT A CAUSATION EXPERT,
THIS CASE CAN GO TO A JURY
Though Slatowski’s experts may not opine on causation,
they may still testify that the gun’s design was defective. That
means that experts will explain to the jury the most compli-
cated part of the case: how the P320 and the safeties work. And
a lay juror can understand the rest—namely, what happened
that day. So the District Court should not have granted sum-
mary judgment.
A. Pennsylvania law requires experts for overly
complex questions
To decide when a jury may consider an issue without expert
testimony, Pennsylvania’s law takes a functional approach. “If
all the primary facts can be accurately described to a jury and
if the jury is as capable of comprehending” them as a specialist,
“then there is no need for the testimony of an expert.” Reardon
v. Meehan, 227 A.2d 667, 670 (Pa. 1967). But “[e]xpert testi-
mony becomes necessary when the subject matter of the inquiry
is one involving special skills and training not common to the
ordinary lay person.” Storm v. Golden, 538 A.2d 61, 64 (Pa.
1988).

-- 11 of 16 --

12
So when is a question too complicated for the jury? That
depends on whether, given all the admissible evidence, the jury
can answer it without speculating. We distill that principle
from three lines of cases.
First, experts are often needed in cases where the theory of
harm is abstract or technical and so requires expert analysis.
One example is whether water erosion or flooding came from
runoff from a neighbor’s property or from some other water
source. Landman v. Calvary Full Gospel Church, 2014 WL
10752219, at *2 (Pa. Super. Ct. Dec. 3, 2014); Schirmacher v.
Crawford, 2013 WL 11266147, at *1 (Pa. Super. Ct. Apr. 1,
2013). Another is whether a mold infestation was caused by a
leaky toilet or by a fungus. Jemmerer v. State Farm Ins. Co.,
2004 WL 87017, *3 (E.D. Pa. Jan. 19, 2004). The story of harm
in those cases depends on sophisticated analyses that try to iso-
late individual factors’ roles, requiring an expert. After all,
common knowledge and life experience are not enough to answer
questions about mold and fungi.
Second, an expert is needed in products-liability cases
where something seems to have gone very wrong with machin-
ery, but the plaintiff is not sure what. For instance, a wheel
might fall off a delivery truck or forklift. Brandon v. Ryder
Truck Rental, Inc., 34 A.3d 104, 105–06 (Pa. Super. Ct. 2011);
Bibbs v. Se. Pa. Transp. Auth., 2012 WL 8704635, at *1 (Pa.
Commw. Ct. Apr. 13, 2012). Or a truck’s engine might fail cat-
astrophically. Am. Power, LLC v. Speedco, Inc., 2017 WL
4084060, at *1 (M.D. Pa. Jan. 17, 2017). Or its bumper might
ride up onto a guardrail, causing a crash. Oddi, 234 F.3d at 141.
Or a van might suddenly catch fire. Miller v. Davies Ford, Inc.,
2019 WL 1530237, at *1 (Pa. Super. Ct. Apr. 9, 2019). In each

-- 12 of 16 --

13
case, complex machinery allegedly malfunctioned, but the
plaintiff was not sure why. So in each, the court held that the
plaintiff needed a causation expert.
But third, no expert is needed when, even though the sub-
ject matter is complex, lay testimony can tell the story of cau-
sation. Some of these are cases where courts let juries decide
causation based on eyewitness testimony paired with expert
testimony that explained the complex background information.
For example, after an expert explained how the pressure in a
beer bottle could make it explode spontaneously, a jury could
rely on eyewitness testimony to decide whether that had in fact
caused the explosion. Bialek v. Pittsburgh Brewing Co., 242
A.2d 231, 233, 235 (Pa. 1968). And when an “expert mechanic”
testified that a car’s underside was rusted, a jury could decide
based on the driver’s testimony whether the rust created a brake
failure that caused the crash. Topelski v. Univ. S. Side Autos,
Inc., 180 A.2d 414, 418–19 (Pa. 1962). In other cases, a jury
can decide causation with no expert at all because, even though
the subject matter is complex, a combination of lay testimony
and pictures gives enough background. Padillas v. Stork-
Gamco, Inc., 186 F.3d 412, 415–16 (3d Cir. 1999).
Those cases decided under Pennsylvania law teach us to
look functionally. Putting ourselves in a juror’s shoes, we ask
whether he would need more explanation.
B. No expert is needed here
Here, a jury will be armed with enough expert knowledge
to decide causation. To explain why, we summarize how
products-liability suits differ from ordinary negligence suits.
Ordinary negligence suits require proof that the defendant

-- 13 of 16 --

14
behaved negligently and that the negligent behavior caused the
plaintiff’s injury. In products liability, the defect takes the
place of negligent behavior. See Sullivan v. Werner Co., 306
A.3d 846, 849–50 (Pa. 2023). Whether a design is defective
depends on whether it is unreasonably dangerous. Id. at 849. A
plaintiff can carry that burden by showing either (1) that an
ordinary consumer would not know of the unacceptable dan-
ger, or (2) that the risk and seriousness of harm outweigh the
cost of a safer design. Id.; Tincher v. Omega Flex, Inc., 104
A.3d 328, 388–89, 417 (Pa. 2014). Either way, the plaintiff will
have to explain to the jury both how the product works and how
it could cause an injury. Whether it did cause an injury might
then become a much simpler question. See Davis v. Sig Sauer,
126 F.4th 1213, 1231 (6th Cir. 2025).
And it is a simpler question here. The P320’s design is tech-
nical and probably needs explaining. How the sear, the firing
pin, and the trigger interact is not common knowledge, nor are
the mechanics of different safeties. And Slatowski’s theory of
the case turns on how they interact: that something compressed
the trigger, releasing the safety lock, and then when he reached
for the gun’s grip, the sear was dislodged, causing the gun to
fire. But Slatowski has experts who may explain how all these
internal mechanisms work together.
The remaining causation question is not beyond the average
juror. It turns on whether Slatowski is telling the truth and
remembers it accurately, whether something could have gotten
into the holster, and whether a tabbed trigger might have
stopped some debris or the holster itself from depressing the
trigger. During the trial, jurors would have the benefit of the
experts’ descriptions of the different safeties. And jurors could

-- 14 of 16 --

15
view up close a P320 as well as a gun with a tabbed trigger and
Slatowski’s holster and ponder whether his story really holds
up. That requires no degree in ergonomics or scientific study.
Those factors distinguish this case from the products-liabil-
ity cases that required experts on causation. Those cases lacked
any expert at all but required one to explain what went wrong.
Here, by contrast, we have two experts to explain the design
defect. From there, like the plaintiffs in Bialek and Topelski,
Slatowski must rely on his lay eyewitness testimony. 242 A.2d
at 233, 235; 180 A.2d at 418–19. It may not prove persuasive.
But that is up to the jury, not the judge.
True, Slatowski’s theory of causation is fuzzy. He does not
know whether the trigger was depressed by some debris or the
holster itself. And his experts neither considered the gun’s hol-
ster nor tested how it would have interacted with the gun. Yet
all that goes to the strength of his case, not whether a jury could
decide for itself how his theory adds up.
* * * * *
We are not the first federal court of appeals to decide
whether a tort suit about the P320 can proceed without expert
testimony on causation. We join the Sixth Circuit in holding
that these two experts may testify against Sig Sauer that the
P320 is defectively designed. Davis, 126 F.4th at 1232. And
we depart from the approach that the Tenth Circuit took in an
unpublished opinion. See Herman v. Sig Sauer, 2025 WL
1672350, at *6 (10th Cir. June 13, 2025). But those cases each
turned on a different state’s law and a different factual record.
The Tenth Circuit’s holding rested largely on the plaintiff’s
failure to testify that he never pulled the trigger. Id. at *7 n.4.

-- 15 of 16 --

16
By contrast, both Slatowski and the Sixth Circuit plaintiff did
testify to that. App. 184–85; Davis, 126 F.4th at 1232–33.
Whether Slatowski is telling the truth is now for the jury to
decide. We will thus reverse the District Court’s grant of sum-
mary judgment, affirm its exclusion of expert testimony on
causation, and remand to let the case to proceed to trial.

-- 16 of 16 --