The mystery in HBO’s 2021 hit limited series Mare of Easttown hinges on the details of the gun used to murder a teenage girl. Drawing on my experience as a forensic firearms examiner and fiction author, I explain what the creators got wrong about the Mare of Easttown gun and assess how they blended fact and fiction in the show’s crucial scenes.
Warning: this article contains spoilers for the final episodes of Mare of Easttown.
Forensic Firearms Examination
In my day job as a forensic scientist, I analyze guns and fired evidence from crimes in and around Chicago. This field of forensic science is often termed “ballistics,” though practitioners prefer the term “forensic firearms examination” or “forensic firearms analysis.” Recently, I found my work as an author and a forensic firearms examiner intersecting as I watched HBO’s Mare of Easttown.
In the sixth episode of the show, a forensic firearms examiner (played by Christopher Mann) explains that a bullet recovered at the crime scene must have originated from a Colt Detective Special revolver.
“In most guns, the rifling goes to the right,” he tells Detective Mare Sheehan (Kate Winslet). “But Colt guns are unique. They have a left-hand barrel twist. Like we see on the [bullet recovered from the crime scene].” He adds, “The weapon that killed Erin is a Colt Detective Special revolver,” and surmises that Mare’s father, also a police officer, probably carried this model of firearm.
But how would a firearms examiner arrive at such a conclusion? And is it possible to say a fired bullet could only have come from this specific firearm make and model? I answer both questions below.
Class Characteristics
Forensic scientists group fired bullets and other evidence items based on what we call class characteristics. Class characteristics associate one item with a group of similar items but do not distinguish it from the other items in that group. The color of a car is an example of a class characteristic. The fact that a car is red associates it with other red cars and distinguishes it from non-red cars. But the color alone does not distinguish one red car from other red cars.
When it comes to guns and ammunition, class characteristics are determined by the manufacturer before the manufacturing process. They include the caliber of a cartridge and the caliber and rifling characteristics of a gun.
Caliber
Caliber means slightly different things for a gun, a cartridge (a single unit of ammunition consisting of a cartridge case and a bullet) and a fired bullet. But it essentially refers to the size of bullet that a particular firearm can fire. So a 38 Special caliber Colt Detective Special is designed to fire a 38 Special caliber cartridge. And a 38 Special cartridge contains a bullet with a diameter of 0.357 inches, the same diameter as the barrel of a 38 Special caliber gun.
Knowing the caliber of a bullet limits the pool of guns that could have fired it. A 38 caliber bullet could have originated from a number of different cartridges and could have been fired by a number of different firearms (for example, 38 Special or 357 Magnum). But it could not have been fired by many other guns. A 38 caliber bullet is too large to fit through the barrel of a 22 Long Rifle caliber firearm. And a 38 Special cartridge is too narrow for the chamber of a 45 Auto caliber firearm.
Rifling
Knowing the rifling of a bullet limits the pool of potential guns even further. Rifling refers to the set of helical strips cut or impressed into the interior of a firearm barrel. Rifling causes a bullet to spiral as it travels down the barrel. This spiral rotation improves the stability and accuracy of the bullet in flight (similar to a well-thrown football).
The cut-out or impressed areas in the barrel are called grooves, and the uncut or unimpressed areas are called lands. When a bullet passes through the barrel, these lands and grooves become impressed into the surface of the bullet to form land and groove impressions on the bullet. We can characterize barrels (and the bullets fired through them) by the number of lands and grooves and the direction of the rifling—described as either left-hand or right-hand twist.
The evidence bullet in Mare of Easttown had six-left rifling, which means it had six land and groove impressions with a left-hand twist. A six-left bullet could only have been fired by a gun with six-left rifling. Guns with five or seven lands and grooves and guns with right-hand twist rifling could not fire a six-left bullet.
Lands and Grooves
We never learn the caliber of the evidence bullet in Mare of Easttown, only the rifling. Colt manufactured Detective Specials models to fire two different calibers of bullet: 32 and 38. And the forensic firearms examiner on the show would surely know the bullet caliber. As he explains to Mare, Colt Detective Specials do have six-left rifling. However, it is not the case that Detective Specials are the only guns with six-left rifling. There are many other 32 and 38 caliber firearms, including different models of Colt firearms, which also have six-left rifling.
But fired bullets have another class characteristic: the widths of the land and groove impressions on the bullet. A firearm manufacturer also determines the width of the lands and grooves for a particular firearm model. And it is possible to measure the widths of the corresponding land and groove impressions on a fired bullet.
So the firearms examiner on Mare of Easttown would have determined the caliber and rifling of the evidence bullet and measured its land and groove impressions. In some cases, these class characteristics can limit the pool of possible guns to a single make and model. But that is not the case with the Mare of Easttown bullet.
The Mare of Easttown Gun and Bullet
Let’s take a hypothetical bullet fired by a 32 caliber Colt Detective Special, a fact which is unknown to the firearms examiner analyzing it. The examiner would determine the caliber and rifling of the bullet and measure its land and groove impressions. They would then enter this information into the FBI’s General Rifling Characteristics (GRC) database. Based on this database search, they would discover that the bullet in question could have been fired by a 32 caliber Colt Detective Special or by several different models of 32 caliber Colt firearms or by a least one firearm made by a different manufacturer.
A similar situation applies to a hypothetical 38 caliber bullet fired by a Colt Detective Special (again, unbeknownst to the firearms examiner). Such a bullet would have similar rifling characteristics to several other Colt firearms, as well as firearms made by five different manufacturers. In short, given the available evidence, there would be no way for the firearms examiner on Mare of Easttown to attribute a single fired bullet to a Colt Detective Special.
Weighing Story and Factfulness (Part 1)
So why do it? The Mare of Easttown creators likely received input from someone with knowledge of firearms. Why say the bullet could only have come from one make and model of firearm if that’s not the case?
Because it makes for a better story. When another Easttown resident, Glen Carroll (Patrick McDade), tells Mare his gun went missing and that it was a Colt Detective Special, that’s a huge bombshell. Suddenly, Mare and the audience realize that the suspect she has in custody might not be the killer. And it all hinges on the fact that only one type of gun could have (supposedly) fired the fatal bullet.
In the interest of accuracy, the show’s creators could have had the firearms examiner tell Mare that the murder weapon was most likely a Colt. But that simply isn’t as dramatic as naming one specific gun and then referencing that same gun at a pivotal moment later in the story. However, it would have been possible to maintain the one-gun dramatic effect and remain factually accurate had the show creators chosen a different make and model of gun.
Other Candidates for the Mare of Easttown Gun
Using the GRC database, I found two firearms that would have fit the show creators’ requirements: 1) that a bullet fired from this type of gun could have only originated from that make and model and 2) that the gun in question has some larger connection to the story (e.g. the Colt Detective Special was a popular police officer gun that was used by Glen Carroll and likely by Mare’s father during their employment).
Charter Arms Undercover
The first gun is a 38 caliber Charter Arms revolver. According to the GRC database, many newer models of Charter Arms revolvers have eight-left rifling, which is relatively rare. A 38 caliber bullet with eight-left rifling and certain land and groove impression widths could have originated from two different models of Charter Arms revolvers. One of those models, the Undercover, was used by Mark David Chapman to murder John Lennon.
Colt New Service
The second gun is a 44 caliber Colt New Service revolver. Colt made this model of revolver in several different calibers. But the combination of a 44 caliber model, Colt’s standard six-left rifling and the specific land and groove widths of this model would make it possible to say that a bullet with these class characteristics was fired by this model of firearm. The New Service is a slightly older model than the Detective Special but was also popular with law enforcement.
So when it comes to this detail, the Mare of Easttown creators could have maintained the drama of the story and factual accuracy by simply choosing a different weapon. But there is another moment in the show that stretches forensic truth for the sake of the story. And I believe this moment actually works as it is.
Weighing Story and Factfulness (Part 2)
In the show’s seventh episode, before Mare learns of Glen Carroll’s missing Detective Special, she arrests John Ross (Joe Tippett) and gets him to confess to the murder. But afterward, something doesn’t sit right with Mare. She watches a video of the confession in which Ross says he doesn’t remember the details of the murder weapon.
Mare asks her chief (John Douglas Thompson) what he thinks of the firearms expert from the previous episode. The chief responds, “He’s the best. I’d use him every time if we could.”
Now it is possible that the police officers my lab services talk about my colleagues and me in this way. But I suspect they usually gloss over the names on our reports and focus on our conclusions. Furthermore, discerning the caliber and rifling characteristics of a fired bullet and reporting a list of possible firearms is a relatively simple examination. Any firearms examiner should be able to perform this task. Those who provide such information are not exemplary. Rather, a firearms examiner who could not perform this type of analysis should be prevented from doing casework.
Heightened Drama Over Perfect Accuracy
But in this case, I believe the demands of the story outweigh the lapse in factual accuracy. The point of this scene is to cast doubt on what seems to be convincing evidence of Ross’s guilt. Stretching the truth here is part of what fiction creators do. There isn’t a significantly better way to convey Mare’s doubt, and the factual stretch isn’t egregious enough to warrant serious complaint.
This brief exchange reminds the audience of the import of the firearms evidence in this case. It sets up the scene between Mare and Carroll and the discovery of the murder weapon and the true killer. If only the murder weapon were a New Service instead of a Detective Special.