So, we wrote about that “four-barrel” rifle last week and posed a few questions to the inventor, Martin Grier, in an email. He got back to us that day with our initial query and has now responded to some more of the questions we posited in the original article. His answers make us even more excited about the weapon’s promise, assuming that everything holds true through testing in Army labs and the field.

The FD Munitions L5 rifle prototype has five bores and few moving parts. The Army has requested a four-bore version for testing.

(YouTube/FD Munitions)

First, a bit of terminology. The weapon is a rifle. Most people have described it as having four barrels, but it’s really a barrel with four bores (the original prototype had five). The inventor prefers to call it a “ribbon gun,” which we’ll go ahead and use from here on out.

Just be aware that “ribbon gun” means a firearm with multiple bores that can fire multiple multiple rounds per trigger squeeze or one round at a time. The bullets are spinning as they exit the weapon, stabilizing them in flight like shots from a conventional rifle.

If you haven’t read our original article on the weapon, that might help you get caught up. It’s available at this link.

So, some of our major questions about the rifle were how the design, if adopted, would affect an infantryman’s combat load, their effective rate of fire, and how the rounds affect each other in flight when fired in bursts. We’re going to take on those topics one at a time, below.

Weight

How much weight would an infantryman be carrying if equipped with the new weapon? Grier says it should be very similar, as the charge blocks which hold the ammunition are actually very light

“In practice, Charge Block ammo, shot-for-shot, is roughly equivalent to conventional cartridge ammo,” he said, “depending on which caliber it’s compared to. It’s lighter than 7.62 and slightly heavier than 5.56. It outperforms both.”

Since the weapon fires 6mm rounds, that means the per-shot weight is right where you would expect with conventional rounds. The prototype weapon weighs 6.5 pounds. That’s less than an M16 and right on for the base M4.

The L4m ammo blocks feature four firing chambers and their rounds, stacked vertically. The blocks can clip together in stacks and be loaded quickly. Excess blocks able to be snapped off and returned to the shooter’s pouch easily.

(Copyright FD Munitions, reprinted with permission)

And those blocks of ammo provide a lot of benefits since they can withstand 80,000 PSI. That lets designers opt for higher muzzle velocities if they wish, extending range and increasing lethality. For comparison, the M4 and M16 put out about 52,000 PSI of chamber pressure.

Even better, the blocks snap together and can be loaded as a partial stack. So, if you fire six blocks and want to reload, there’s no need to empty the rifle. Just pull the load knob and shove in your spare stack. The weapon will accept six blocks, and you can snap off the spares and put them back into your pouch.

Rate of fire

But what about effective rates of fire?

Well, the biggest hindrance on a rifle’s effective rate of fire is the heat buildup. Grier says that’s been taken care of, thanks to the materials used in the barrel as well as the fact that each chamber is only used once per block.

“In the L4, … the chamber is integral with the Charge Block,” he said. “Every four shots, the Block is ejected, along with its heat, and a new, cold one takes its place. The barrel is constructed with a thin, hard-alloy core, and a light-alloy outer casing that acts as a finned heat sink. In continuous operation, the barrel will reach an elevated temperature, then stabilize (like a piston engine). Each bore in the L4 carries only a 25 percent duty cycle, spreading the heat load and quadrupling barrel life.”

FD Munitions expects that the military version of the L4 would have a stabilized temperature during sustained fire somewhere around 300-400 degrees Fahrenheit, but they took pains to clarify that it’s a projected data point. They have not yet tested any version of the weapon at those fire rates.

But, if it holds up, that beats the M16 during 1975 Army tests by hundreds of degrees. The M16 barrels reached temperatures of over 600 degrees while firing 10 rounds per minute. At 60-120 rounds per minute, the barrels reached temperatures of over 1,000 degrees. That’s a big part of why the military tells troops to hold their fire to 15 rounds per minute or less, except in emergencies.

All of this combines to allow an effective rate of fire somewhere between 60 and 100 shots per minute. That’s about five times more rounds per minute than a M4 or M16 can sustain. And that’s important; paratroopers in a 2008 battle died as their weapons malfunctioned. One soldier had three M4s fail while he was firing at an average rate of 14 rounds per minute.

The guts of the weapon feature very few moving parts, a trait that should reduce the likelihood of failures in the field.

(YouTube/FD Munitions)

Do rounds affect one another mid-flight?

Sweet, so the combat load won’t be too heavy, and the weapon can spit rounds fast AF. But, if rounds are fired in volleys or bursts, will they affect each other in flight, widening the shot group?

Grier says the rounds fly close together, but have very little effect on each other in flight, remaining accurate even if you’re firing all four rounds at once.

And, four rounds at once has a special bonus when shot against ceramic armor, designed for a maximum of three hits.

“The projectiles do not affect each other in flight,” he said. “Even when fired simultaneously, tiny variations in timing because of chemical reaction rates, striker spring resonances, field decay rates, electric conductor lengths etc., ensure that the projectiles will be spaced out slightly in time along the line of sight. The side effect is that the impacts will be likewise consecutive, defeating even the best ceramic body armor.”

Meanwhile, for single shot mode, each bore can be independently zeroed when combined with an active-reticle scope. With standard mechanical sights, Grier recommends zeroing to one of the inside bores, ensuring rounds from any bore will land close to your zeroed point of impact.

Some other concerns that have arisen are things like battery life, which Grier thinks will be a non-issue in the military version. It’s expected to pack a gas-operated Faraday generator that not only can power the rifle indefinitely, but can provide juice for attachments like night vision scopes or range finders.

There’s also the question of malfunctions, which can happen in any weapon. Failure to fire will be of little consequence since you’re going to eject that chamber quickly anyway. If a barrel becomes inoperable due to some sort of fault, the fire control can simply skip that barrel, allowing the shooter to still fire 75, 50, or 25 percent of their rounds, depending on how many barrels are affected.

So, if everything goes well, this weapon could shift the balance of power when the U.S. goes squad vs. squad against other militaries. Here’s hoping the final product lives up to the hype and makes it into the hands of service members.