Windham Weaponry RMCS-4 Multi Caliber AR15 System Review

Windham Weaponry RMCS-4 Multi Caliber AR15 System Review The Windham Weaponry RMCS-4 Multi Caliber System is a pretty slick exercise in engineering. It is a tool less caliber conversion system for […]

Windham Weaponry RMCS-4 Multi Caliber AR15 System Review - documentationWindham Weaponry RMCS-4 Multi Caliber AR15 System Review

The Windham Weaponry RMCS-4 Multi Caliber System is a pretty slick exercise in engineering. It is a tool less caliber conversion system for the AR platform. Each caliber switch can be performed in just a few minutes.


Let’s get the obvious question out of the way… why buy a complicated multi-caliber AR system when you could buy a budget AR in each caliber for about the same amount? For many folks the economics may well swing towards buying separate ARs in each caliber. However, if you live in a state with restriction, permits, or some other barrier(s) to acquiring and owning modern sporting rifles, then the ability to purchase a single serialized lower that can be configured to shoot multiple calibers could be a true godsend. I suspect there are also a variety of specialized missions where it would be advantageous to leave the base with one modest sized rifle case, but the ability to reconfigure in the field without special tools for four different cartridges, depending on fluid mission requirements.

For most of us, the above likely doesn’t apply. To be honest, I’m pretty good at justifying firearms purchases. In this case, the Windham Weaponry RMCS-4 system may make my short list on “cool factor” alone.

Where to get Windham Weaponry products?

Windham Weaponry does not sell via their website. However, you can find Windham Weaponry products online at Brownell’s or Cabela’s.

First Impressions

On first look the system is pretty sweet and well put together, which is to be expected from a company with such a strong heritage in the AR market. Windham Weaponry, if you didn’t know, is a great American success story as the brand rose from the ashes of the former Bushmaster after the later was purchased by the Freedom Group and moved from Maine to New York. There is pride evident in the fit and finish of the finished product.  The Windham Weaponry RMCS-4 system arrived in a heavy duty Pelican Gun Guard case with cutouts for each conversion caliber and one for the assembled rifle.  The lone lower receiver has the ability to change magazine wells from the standard  allowing use of standard AR magazines, 7.62x39mm caliber AK magazines and 9mm Colt type magazines at the appropriate times.  .223/5.56mm, .300 Blackout, and 7.62x39mm barrels are all gas operated while the 9mm conversion is a standard blowback operation.

Windham Weaponry RMCS-4 Specs

Brand Windham Weaponry
Model # Tested RMCS-4
Finish/Stock Black Manganese Phosphate
Barrel Length 24” w/QLA
Caliber 9mm/.223/.300 Blackout/7.62 x 39mm
MSRP $2971.00

Caliber conversion

The Caliber conversion process consist of 3 steps (well… and a bunch of sub-steps):

Windham Weaponry RMCS-4 Multi Caliber AR15 System Review - barrel connectionChange the barrel

  • Separate the Upper and Lower Receivers.
  • Remove the Barrel Retaining Block & Bail by sliding it forward on the lower portion of the quad rail mount to expose the Barrel Retaining Arms.
  • Rotate the Barrel Retaining arms back towards the Upper Receiver to release the barrel.
  • Grasp the barrel and pull straight away from the Upper Receiver to remove it.
  • Install the new barrel by reversing the process.

Change the bolt

  • Bolt changing is not necessary when switching between .223/5.56mm and .300 Blackout or vice versa.
  • The bolt change is done the same way as it is with any standard AR platform.  Pull the charging handle about 2/3rds of the way out of the Upper Receiver while holding the bolt.  It will fall away from the Upper Receiver into your hand.
  • Store it appropriately and replace with the correct caliber specific bolt for the barrel being replaced.
  • All barrels and bolts are marked so you can positively know you are installing the right one!

Change the magazine well

  • Mag well change is not necessary when switching between .223/5.56mm and .300 Blackout or vice versa
  • The magazine well is released by pressing the magazine release button.  This allows the magazine well to slide away from the trigger portion of the Lower Receiver.
  • Simply slide the caliber appropriate magazine well into place and you are ready to reconnect the Upper and Lower Receivers.

See Roger from Windham Weaponry change calibers in the RMCS-4 in this video…

Test Firing the Windham Weaponry RMCS-4

The Windham Weaponry Multi Caliber System is capable of the accuracy expected from any quality AR platform firearm. With all of the custom parts I expected at least some level of hiccups in operation, but the Windham Weaponry RMCS-4 system ran like a top throughout testing.

The system, as you should expect, does not maintain zero from caliber to caliber (which should be self-evident when you take into account that all 3 calibers have vastly different ballistic properties), but it will group after any caliber change making re-zeroing a relatively easy task. It also doesn’t hold zero if you remove a barrel and then immediately reinstall it (again, not surprising) though the zero does remain relatively close as demonstrated in the above target (look at the bullseye labeled 223 and the one to the right of it).  In this case the barrel was removed and then reinstalled.  The point of aim shifted down slightly and to the left ~2 MOA.  Close enough for a quick re-zero, but far enough off to preclude extended range sniping immediately after a barrel change without reconfirming your zero.


The RMCS-4 rifle handles as expected. It is not unusually heavy and has standard AR ergonomics.  It is capable of the accuracy one expects out of an AR platform in each of the respective calibers.

The Windham Weaponry system is the 2nd caliber conversion system Modern Sporting Rifle (MSR) that I’ve reviewed. I have to admit that I greatly prefer the Windham Weaponry RMCS-4 system to the other system I tested.  Why? Because, I consider it to be essential that the caliber conversions are straight forward and could be (and were) easily performed while at the range without tools or fuss. The JRC multi-cal platform (review video link) is lots of fun to shoot and easier on the wallet, but changing calibers involves special tools and is far more difficult than the Windham Weaponry RMCS-4 system.

What’s not to love?

The only downside to the system was the cost.  At an MSRP of nearly $3000, you have to seriously look at purchasing caliber specific uppers (to which you could then mount dedicated sights/optics which would keep their zero) and which, depending on the caliber, might require either special magazines or magazine well inserts.  It is largely a matter of personal preference, mission requirements or political/legal situation which would be better for both your wallet and your needs.

Gears Rating:

Gear Score
If a multi-caliber AR is meets your needs and/or desires, the Windham Weaponry RMCS-4 deserves serious consideration as a robust, yet easy to use system.

About Chris

The Gear Report team has more than our share of practical experience built over decades of shooting, hunting and spending time outdoors. We even have a bit of formal training here and there. However, we are thrilled to formally bring Chris of Three Pillars Shooting, LLC (firearms and archery training services) into the Gear Report ranks. Chris’s impressive resume includes various NRA instructor certifications, Boy Scouts of America firearms and archery certifications, as well as being a US Army veteran. While Chris’s credentials are impressive, more impressive is how he chooses to invest his time. Chris has been a Boy Scout leader for years, volunteering countless hours supporting the local Cub Scout pack before moving up to adult leadership roles with the local, district and council as his son’s advance through Boy Scouts. Chris founded a Boy Scout Venture Crew that focuses on shooting sports and is passionate about teaching our youth firearms safety and effectiveness. All of the Gear Report team are good people who make positive impacts in their communities. So, Chris is a natural fit on the team.