FCU Spells Freedom: Testing the Sig Sauer P365 Fire Control Unit

The Sig P320 changed the way we see handguns with its modular construction. Unlike other pistols, the trigger components, configured in a self-contained chassis, is the serialized part. This revolutionary concept means that for military/service use, the stock can be easily replaced in case of wear or damage. For civilian use it opened the floodgates of aftermarket possibilities. Now the Sig Sauer P365 Fire Control Unit (FCU) provides even more options.

The Sig Sauer P365 FCU

Gone are the days of chancing a stippling or sculpting job on the “firearm.” Here is the idea that for about $40, you can replace the entire grip. Fortunately, Sig applied the same construction system to their popular P365 line of micro-compact concealed carry pistols.

Fire Control Unit is the term used by Sig to describe the bundle of parts the ATF considers the firearm. These handy units are the only part of a Sig P365 you need to go to a gun shop to receive. Likewise, it is the only part requiring a 4473 to be filled out.

The rest of a complete “gun,” including barrel, slide, recoil assembly, and grip module, can all be sourced either directly from Sig or one of the many aftermarket companies and shipped directly to your home.

Build It Your Way

The freedom the FCU brings is the ability to start from scratch if you want to. There’s no need to buy a complete pistol, then replace it part by part until you’ve got the gun you want. And with it, a pile of leftover parts almost making an entire gun. By starting with a serialized FCU, you can take your pick of readily available Sig or aftermarket options.

To exercise this, we started with a Sig Custom Works FCU and built out a complete pistol using Sig parts. As a fan of muzzle energy and getting my whole hand on the gun, I opted for a 365 XL build. However, with the FCU, it’s easy enough to convert to a standard 365.

Installing the FCU requires no tool for the tough-handed. Everyone else can use a blunt object to drive in the one pin mating the FCU to the grip module. As expected, the gun ran and handled just like any other Sig P365 XL would. But it had a touch of that nice “my way” feel to it.

With a baseline established, we set out to find some of the aftermarket options and see how they change the feel and performance of the pistols. Our goal was to take an already fine pistol and elevate it to the next level. Hopefully, we could tame some of that snappiness that small pistols naturally have.

Barrel Selection

Chances are you’ll never shoot-out a factory pistol barrel, but what about upgrades or if you’re starting from scratch? What other options are out there? We tried samples from Zev Technologies and True Precision.

The Zev Technologies Z365XL Pro barrel offers a DLC coating, dimpling, and a slightly different hood profile. No information is posted about the rifling method or type. The DLC coating is a major attraction feature both for practicality and aesthetics. Diamond-like-carbon coatings are named so because they impart diamond-like characteristics to the surface of the treated material.

The characteristics we enjoy in firearm parts include corrosion and abrasion resistance and a slickness that serves almost like a dry lubricant. On a pistol barrel, this means reduced and easier maintenance as filth wipes off easily. Not to mention potentially smoother operation as the slide glides over the barrel hood during cycling.

The dimples provide a nice aesthetic and increase surface area for enhanced cooling. They also reduce the weight of the Sig P365 XL barrel from the factory 2.6 ounces to 2.5 ounces. Not a huge weight savings, but it’s there.

In our testing the Zev barrel provided excellent accuracy and did smoothen out the cycling of the pistol a little. It’s not something that can be measured without a laboratory, but racking and recoil felt smoother. The coating on our barrel did show wear after a couple of dozen cycles.

True Precision Options for the P365 XL

True Precision offers a variety of options for the Sig P365 XL. These include threaded and standard lengths, as well as seven coating options. We opted for the threaded model, not to use the threads, but because it bumps the 365 XL’s barrel length up from 3.7 inches to about 4.5 inches.

Thanks to the flutes, this added only three-tenths of an ounce in weight. However, the fluting also bumped up ballistic performance to meet or exceed many duty-sized guns. True Precision’s barrels are made from certified 416R match-grade stainless steel. Additionally, the six-groove rifling is broach cut using a proprietary method to be compatible with all bullet types, including cast lead.

The feed ramp is also enhanced and leads to a fully supported chamber. Our sample was titanium-nitride coated, giving it a gold color, and enhanced corrosion, and wear resistance. The aesthetic also tied in well with the Sig Custom Works FCU.

Super Slides

The aftermarket hasn’t quite caught up yet for 365 XL slides. There are a couple of shops offering work to be done to your existing slide. However, there isn’t much in the way of complete slides. You can choose from eight options on the Sig Custom Works website, including our choice of the Spectre slide.

The Spectre offers a different aesthetic but, more importantly, drops the reciprocating mass from 10.1 ounces of a standard slide without optic to 9.3 ounces with a Romeo Zero mounted! Less reciprocating mass makes for a quicker-cycling gun and helps reduce felt recoil.

Recoil Assembly

Sig, of course, offers the standard spring option, but experience with DPM Systems’ products sent me immediately after their 365 XL kit. These tunable recoil systems are comprised of three springs. These work together to progressively decelerate the slide after firing, then progressively accelerate the slide back into battery.

As an added bonus, in some firearms, they also prevent the biggest source of felt recoil when the slide’s travel bottoms out against the frame. Since discovering the Greek company years ago at SHOT Show, I’ve enjoyed their solutions in ARs, AKs, and several pistol models. Not every pistol has shown much difference, yet with others, the DPM System completely transformed the shooting experience.

We’re pleased to report that for the 365 XL, the DPM System makes a remarkable difference. The kit includes three user-changeable springs to tune the action for the ammunition you’ll be running. This is important with a carry gun. Check that the gun will function with your defensive ammunition! For our build the longest spring worked fine and greatly reduced the recoil impulse.

Frame Jobs

The frame, or grip module, versatility is a real treat with Sig’s FCU concept. Unlike most other pistols, attempting modifications of the frame does not risk ruining the entire “firearm.” For less than the cost of dinner for two, you can take risks and have alternates.

At the time of writing this, the Sig Custom Works shop offers three color options for the 365 XL and one laser-stippled option in black. Thanks to Sig’s encouragement of aftermarket options, there are some interesting options out there. These include some brilliant work in aluminum from Icarus Precision.

The Icarus Precision A.C.E. (Accuracy Control Enhanced) X Elite grip module brings the assembled weight from 4.4 ounces (factory Sig polymer) to 7.1 ounces. This increase brings the balance of the pistol lower into the hand, further enhancing recoil control.

Beyond just weight, the X Elite grip module offers better traction all the way around the grip and a higher hold. This higher hold is accomplished with an aggressive double undercut of the triggerguard. The first undercut allows more room for the firing-hand middle finger. Correspondingly, the second undercut lets the support hand grip higher on the gun.

The X Elite and PRO Elite models also add a “gas pedal” extension of the frame on the left side. This allows the support thumb to rest and provide a downward force to counter muzzle rise while firing.

The icing on the cake for those looking to get the most out of their 365 FCU is the inclusion of two rail slots on the dust cover; opening the pistol up to the wide world of light and laser attachments.

Holstering the X Elite Grip

Between the weight and undercuts, the X Elite grip module seats the pistol deeper in hand. One thing to note if you opt for the gas pedal is the holster fitment. The Warcat Tactical holster had been home to the 365 XL until the frame swap. It simply will not fit with the A.C.E. X Elite grip module.

I next tried a Crossbreed holster hoping the leather would stretch enough to fit. But Crossbreed makes their holsters too well for that. The good news is there are specialty holsters for this particular grip module. A quick search will help you find the right one for you.

The Upshot

Our build exemplifies the advantage that the Sig P365 FCU provides. We were able to “build” a complete pistol to our liking at a much lower total cost than purchasing a complete pistol, then purchasing aftermarket parts.

Having built a micro compact that is now tame enough for extended range sessions but still small and light enough to carry, we selected two full metal jacket range loads and three defensive loads for the range test. All five loads were run through each of the three barrels.

As a point of interest, the muzzle energy was calculated for each of the loads fired. This explains the extra punch you feel with some loads. Testing was conducted free-standing at 10 yards as a practical demonstration of a defensive pistol.

All three barrels performed well and certainly provided more mechanical accuracy than most shooters can take advantage of, with an average group size across the five loads and then across the three barrels only differing by 0.27 inches. Just like with rifle barrels, each barrel seemed to have a pet load.

Now that we know how different the same FCU can turn out, how will you build yours?

For more information, please visit SigSauer.com, ZEVTechnologies.com, True-Precision.com, IcarusPrecision.com, and DPMSystems.com.

Performance

Bullet weight measured in grains, velocity in feet per second (fps) by chronograph, energy in foot-pounds (fpe) and accuracy in inches for best five-shot groups at 10 yards.

This article was originally published in the Combat Handguns May/June 2022 issue. Subscription is available in print and digital editions at OutdoorGroupStore.com. Or call 1-800-284-5668, or email subscriptions@athlonmediagroup.com.