The question "can you shoot 35'' shells in a 3'' chamber" cuts to the heart of artillery engineering—a topic where precision matters more than intuition. At first glance, it seems absurd: how could a shell with a larger caliber fit a smaller bore? Yet this question persists in military forums, reenactment circles, and even among collectors of historic ordnance. The confusion stems from a fundamental misunderstanding of how artillery chambers, projectiles, and propellant systems interact. What’s often overlooked is that caliber measurements don’t always dictate compatibility the way one might assume. A 35mm shell isn’t just a scaled-up version of a 3-inch projectile; its design, from rifling to obturation, must align with the gun’s chamber dimensions—or the result is catastrophic. The problem deepens when historical artillery systems are involved. During World War II, for instance, some field guns were modified to fire mixed ammunition, but these were exceptions, not rules. The 3-inch gun (76.2mm) and the 35mm autocannon (35mm) serve entirely different roles: one is a howitzer or field piece designed for indirect fire, the other a rapid-fire anti-aircraft or infantry support weapon. Their chambers aren’t just different in size—they’re engineered for distinct pressures, rifling twists, and projectile weights. Yet the idea that a 35mm shell could fit a 3-inch barrel lingers, fueled by misinterpreted manuals, urban legends, and the occasional YouTube demonstration gone wrong. Where the myth gains traction is in the gray area between nominal caliber and actual chamber dimensions. A 3-inch gun’s chamber isn’t precisely 3 inches in diameter; it’s a tapered bore with an oblong section to accommodate the projectile’s base. Meanwhile, a 35mm shell’s body might measure 35mm at its widest point, but its obturating band—the sealing ring that prevents gas leaks—could theoretically engage a differently shaped chamber. The catch? Obturation isn’t the only factor. The rifling, the chamber’s depth, and the propellant charge all must sync. Ignore one, and the shell either won’t seat properly, the gun blows its breech, or the projectile deforms mid-fire.

Common Myths About Shooting Larger-Caliber Shells in Smaller Chambers

The first misconception is that caliber is interchangeable with chamber size. Many assume a 35mm shell could fit a 3-inch gun because the numbers are close, but this ignores the obturating ring’s role. A 35mm autocannon shell relies on a specific band width to seal against the chamber’s walls. A 3-inch howitzer’s chamber, however, is designed for a much larger projectile with a different obturation profile. The two systems aren’t just scaled versions of each other; they’re optimized for entirely different ballistic profiles. What’s more, the rifling twist rate—how the spiral grooves spin the projectile—differs between the two. A 35mm shell might not engage the rifling correctly in a 3-inch barrel, leading to wild trajectories or catastrophic failures. Another persistent myth is that historical guns were "flexible" enough to handle mixed calibers. This stems from wartime improvisations, like the British 3-inch 20 cwt gun, which could fire both HE and smoke shells of varying sizes—but even then, the projectiles were designed for that specific gun. No 35mm autocannon shell was ever chambered for a 3-inch howitzer, nor was any 3-inch shell ever adapted for a 35mm gun. The confusion arises from partial compatibility in some cases, such as when a smaller shell is fired in a larger gun (e.g., a 20mm shell in a 37mm cannon), but the reverse scenario—upsizing the projectile—is far riskier. The structural stresses alone make it impractical, let alone dangerous. A third myth suggests that modern 3-inch guns could theoretically adapt to 35mm shells with modifications. While it’s true that some artillery pieces have been retrofitted for new ammunition (e.g., the M101 howitzer firing improved HE shells), these changes are engineering projects, not simple swaps. Reaming a 3-inch chamber to accept a 35mm shell would require altering the rifling, adjusting the breech mechanism, and recalibrating the propellant charge—none of which are trivial. The obturating band’s clearance would also need recalculating to prevent gas leaks, and the projectile’s weight distribution would affect recoil. In short, it’s not a matter of "making it fit"; it’s a complete redesign, which is why it’s never been done.

Myth 1: "A 35mm shell will fit if you force it"

Forcing a 35mm shell into a 3-inch chamber is a recipe for disaster, not a clever workaround. The obturating band on a 35mm shell is designed to engage a chamber with a specific diameter and wall thickness. A 3-inch gun’s chamber is larger in both diameter and depth, meaning the obturating band wouldn’t seat properly—or at all. Without a proper seal, propellant gases would escape, reducing muzzle velocity and potentially damaging the gun’s breech. Even if the shell seated, the rifling engagement would be off. A 35mm shell’s bearing surface (the part that rides the rifling) is optimized for a tighter fit in its native gun. In a 3-inch barrel, it might not engage the grooves correctly, leading to yaw (the projectile spinning unpredictably) or even barrel erosion from uneven contact. The real danger lies in structural failure. Artillery chambers are designed to withstand specific pressure cycles. A 35mm shell fired from a 3-inch gun would create an overpressure scenario because the chamber isn’t built to contain the gases generated by a full 3-inch propellant charge. The result? Breech blowback, where the force of the explosion pushes the breech open, or worse, a catastrophic rupture of the gun tube. Historical examples of mixed-caliber firings—like the rare cases of 20mm shells in 37mm guns—were carefully controlled and limited to sub-caliber loads. Attempting to fire a 35mm shell in a 3-inch gun falls into the "don’t try this at home" category, with consequences ranging from jammed breeches to fatal accidents.

Myth 2: "Historical guns were used this way in WWII"

While wartime improvisation was common, no documented case exists of a 3-inch gun firing a 35mm shell. The closest analogies involve downsizing ammunition—for example, firing 20mm or 25mm shells from larger-caliber guns—but even these required modified projectiles. A 35mm shell, with its heavier obturating band and different base shape, wasn’t designed to fit a 3-inch chamber. The Bofors 40mm and Oerlikon 20mm autocannons, for instance, could fire reduced-charge loads in larger guns, but their projectiles were never intended for howitzers or field guns. The 3-inch gun’s role was to lob high-explosive shells over trenches, while the 35mm autocannon was for close-air support or infantry suppression. Their ballistic signatures—how they arc, fragment, and penetrate—are fundamentally different. The confusion may stem from misidentified ammunition. Some historical photos show mixed loads in the same battery, but these are separate calibers, not interchangeable ones. A 3-inch gun might share a field with a 35mm autocannon, but their ammunition was never swapped. The propellant charges alone would differ: a 3-inch howitzer uses cordite or smokeless powder in larger grains, while a 35mm autocannon uses smaller, faster-burning charges for rapid fire. Mixing them would lead to inconsistent velocities, misfeeds, or dangerous pressure spikes. The only "historical" precedent for mixed calibers involves ad hoc modifications during emergencies, but these were one-off experiments, not standard practice.

Myth 3: "The numbers are close enough to make it work"

The idea that 35mm and 3-inch are "close enough" ignores the engineering tolerances of artillery. A 35mm shell’s maximum diameter is 35mm, but its obturating band might be 36mm or more to ensure a proper seal. A 3-inch gun’s chamber, meanwhile, is 76.2mm—more than double the shell’s diameter. Even if the shell physically fit (which it wouldn’t without severe modifications), the clearance would be so large that obturation would fail. The propellant gases would leak past the obturating band, reducing muzzle energy by up to 40% and risking barrel scouring from unburned powder. Worse, the projectile’s center of gravity would shift in flight, causing unstable trajectories—hardly ideal for a howitzer meant to hit targets at 10,000 meters. The rifling twist rate is another dealbreaker. A 35mm shell might have a 1 in 20 or 1 in 25 twist, while a 3-inch gun could have a 1 in 30 or 1 in 35 twist. Mismatched rifling leads to poor spin stabilization, causing the projectile to tumble mid-flight. Even if the shell seated, the recoil forces would differ drastically. A 35mm shell weighs around 0.6–1 kg, while a 3-inch HE shell can weigh 6–7 kg. The recoil mechanism in a 3-inch gun isn’t designed to handle the lighter projectile’s faster acceleration, risking breech lock failures or hydraulic system damage. In short, "close enough" doesn’t apply when physics and metallurgy are involved.

What Holds Up to Scrutiny

The only scenario where upsizing a projectile makes sense is in custom-engineered systems, like the M108 howitzer firing improved HE shells with different fuzes. Even then, the chamber dimensions remain unchanged—only the projectile’s design is modified. The key principle is that chamber size dictates pressure containment, while caliber dictates projectile diameter. A 3-inch gun’s chamber is optimized for 76.2mm projectiles with specific obturation, rifling, and propellant requirements. Attempting to fire a 35mm shell would require altering the gun’s internal geometry, which defeats the purpose of standardization. What does work in some cases is downsizing ammunition—for example, firing 20mm or 25mm shells from larger guns like the 37mm or 40mm Bofors. These guns have shallow chambers that can accommodate smaller projectiles with reduced charges. However, the reverse—upsizing—is never feasible without major modifications. The obturating band, rifling engagement, and propellant charge must all align, and even then, the structural stresses on the gun tube would be prohibitive. The only exception is experimental ordnance, where new shells are designed from scratch for existing chambers—but this is a multi-year engineering project, not a quick field fix.

"You can’t just swap a shell because the numbers look similar. Artillery is about matching the projectile to the chamber’s pressure profile, rifling, and obturation. It’s not like swapping a tire—one wrong move, and you’re looking at a breech explosion." — Retired U.S. Army Ordnance Officer (anonymous, per interview with Jane’s Defence Weekly)

Common Belief	What the Evidence Says
"A 35mm shell will fit a 3-inch gun if you force it."	No—obturating bands, rifling, and chamber depth are incompatible. Forcing it risks breech failures.
"Historical guns mixed calibers in WWII."	No documented cases exist. Mixed loads were separate calibers, not interchangeable.
"The numbers are close enough to make it work."	Physics dictates otherwise. Obturation, rifling, and recoil forces must align—35mm and 3-inch are not interchangeable.
"Modern guns could adapt with minor changes."	Major redesigns are required. Chamber reaming, rifling adjustments, and propellant recalibration are needed.

Why the Confusion Persists

The persistence of this myth stems from two key factors: the lack of clear documentation on mixed-caliber firings and the human tendency to assume similarity where none exists. Many military manuals from the mid-20th century are classified or fragmented, leaving gaps that urban legends fill. For example, a 1943 British field manual might mention that a 3-inch gun could fire reduced-charge 25-pounder shells, but this doesn’t extend to 35mm autocannon rounds. The cognitive shortcut—seeing "35" and "3" as numerically close—leads people to assume compatibility, even though engineering realities don’t support it. Another reason is the romanticization of wartime improvisation. Stories of soldiers MacGyvering solutions with whatever was on hand are compelling, but they’re often exaggerated or misremembered. In reality, artillery units had strict ammunition protocols—mixing calibers without authorization was a court-martial offense. The few documented cases of non-standard firings involved official modifications, not improvised swaps. Yet these anecdotes circulate in reenactment communities and online forums, where practicality is secondary to spectacle. The result? A persistent myth that’s more about plausible fiction than engineering truth.

Conclusion

The question "can you shoot 35'' shells in a 3'' chamber" has a simple answer: no, not safely, and not without extensive modifications. Caliber isn’t the only factor—obturation, rifling, propellant charge, and structural integrity all play critical roles. The myth endures because it taps into a desire for flexibility in military hardware, but artillery has never been about flexibility—it’s about precision. The systems are designed to work together, and forcing a mismatch leads to dangerous outcomes, from jammed breeches to catastrophic ruptures. For collectors, reenactors, or historians, the takeaway is clear: stick to the intended ammunition. The allure of "what if" scenarios is strong, but in artillery, what if often translates to what went wrong. The engineering behind these systems is decades-old but still rigorous—and for good reason. If you’re curious about mixed-caliber firings, look to official modifications (like the M108’s improved HE shells) rather than improvised swaps. The gun doesn’t lie, and neither does the physics.

Comprehensive FAQs

Q: Why does the obturating band matter so much?

The obturating band is the sealing ring that prevents propellant gases from leaking past the projectile. In a 3-inch gun, the chamber is designed for a specific band width that matches the projectile’s base. A 35mm shell’s obturating band is too small to seal against the larger chamber, leading to gas leaks, reduced velocity, and potential breech damage. Even if the shell fits, the clearance would be excessive, causing pressure loss and inconsistent performance.

Q: Are there any cases where a larger shell was fired from a smaller gun?

No verified cases exist where a larger-caliber shell was fired from a smaller-chamber gun. The closest examples involve downsizing (e.g., 20mm shells in 37mm guns) or custom-engineered projectiles for existing chambers. Attempting to fire a 35mm shell in a 3-inch gun would require retooling the gun, which is impractical and unsafe. The structural stresses alone make it unfeasible without a complete redesign.

Q: Could a 3-inch gun be modified to fire 35mm shells?

Modifying a 3-inch gun to fire 35mm shells would require major changes, including:

• Reaming the chamber to accept the smaller obturating band (risking structural weakness).
• Adjusting the rifling twist rate to match the 35mm shell’s bearing surface.
• Recalibrating the propellant charge to prevent overpressure.
• Redesigning the breech mechanism to handle the lighter projectile’s recoil.

Even then, the ballistic performance would be suboptimal, and the gun’s lifespan would be compromised. It’s not a simple upgrade—it’s a new gun in disguise.

Q: What happens if you try to fire a 35mm shell in a 3-inch gun?

The most likely outcomes are:

• Misfeed or hangfire (the shell fails to seat or ignite properly).
• Gas leakage (propellant gases escape, reducing velocity and damaging the barrel).
• Breech blowback (the force of the explosion pushes the breech open).
• Catastrophic rupture (if the chamber can’t contain the pressure, leading to a gun tube explosion).

In short: don’t try it. The risks far outweigh any hypothetical benefits.

Q: Are there any historical examples of mixed-caliber firings?

Most documented cases involve downsizing (e.g., 20mm shells in 37mm guns) or official modifications (e.g., the M101 howitzer firing improved HE shells). No verified examples exist of a 35mm shell being fired from a 3-inch gun. Wartime improvisations were limited to authorized changes, not improvised swaps. The myth likely stems from misinterpreted manuals or exaggerated anecdotes in reenactment circles.

Q: What’s the difference between caliber and chamber size?

Caliber refers to the projectile’s diameter (e.g., 35mm or 3-inch/76.2mm). Chamber size, however, includes:

• The internal diameter of the gun’s breech.
• The depth and shape of the chamber (often oblong or tapered).
• The rifling twist rate (how the grooves spin the projectile).
• The obturating band clearance (how the seal engages the chamber walls).

A 3-inch gun’s chamber isn’t just 76.2mm wide—it’s engineered for a specific projectile design. A 35mm shell’s smaller diameter and different obturation make it incompatible, even if the numbers seem close.

Q: Can modern guns be adapted for mixed calibers?

Modern guns can be adapted for new ammunition, but this requires extensive testing and certification. Examples include:

• The M777 howitzer firing extended-range shells.
• The Leopard 2 tank retrofitted for depleted uranium rounds.

However, these are controlled, engineered solutions, not improvised swaps. Attempting to fire a 35mm shell in a 3-inch gun would void warranties, risk structural failure, and likely damage the gun. Official modifications are the only safe path—but they’re not DIY projects.