Words and pics: Arthur Dent

Having read too many books on the US involvement in the Indochina war, plus decent availability of RS parts, I decided to build a retro PTW: an XM177E1.

This would be a late version, with full ‘fence’. The fence is the barrier contour we now take for granted in modern M4 carbines, which is located in the mag release area of the lower receiver.

First, research.

There are many so called XM177 pics out there, but it took me some time to find ones of the real version.


Retro Black Rifle was a great help. It showed the small differences that are unique to the real XM177E1.

The XM177E1 – or Colt Model 609 – was the first Colt carbine length version used by US forces on a larger scale. It came into production in late 1966 and lasted until early 1967. It turned out that the 10” barrel used in the E1 had some flaws.

The new XM177E2 – or Colt Model 629 – therefore had a 11.5” barrel and other improvements.

The XM177E1 was an assault carbine, developed to fulfill the need for a portable, close-in weapon system to be used by special forces. It was based on the classic ArmaLite AR-15. It chambered a intermediate 5.56x45mm NATO cartridge, the same as the M16.

Despite its assault rifle roots and its carbine functionality, the U.S. military classified the weapon as a Submachine Gun (SMG).

Colt was supposed to supply each XM177E1 with seven 30-round mags, but where unable to build a reliable 30-round curved mag that would fit in the M16 magwell at that time; so most XM177E1s were shipped with 20-round mags.

Later, operators used to buy the 30-round mags on the civilian market by themselves.

Enough history…back to the build.

A PTW obviously needs:

  • Gearbox
  • Motor
  • Electronics
  • Cylinder
  • Hop
  • Barrel

Pretty straightforward.

The XM177E1 related items, on the other hand, are a pain in the ass – especially when you live in Germany, like I do.

The first thing I managed to source was the RS upper. The upper included the charing handle and dust cover.

It had to be milled to accept the Systema cylinder.


It’s not that hard to do. You must mill out a hole ~2cm deep and ~2.6cm wide at the rear of the receiver.

There also has to be a notch at 6 O’clock for the cylinder’s locating tab. Then there has to be some relief on the right inner side of the upper, to make room for the gear box’s sun gear (which I managed to miss on my first attempt).


The first difficult to obtain part came along shortly after that.

Other than the newer M4s which exhibit a tapered delta ring, the XM has a flat version.

After some days of unsuccessfully searching the net for an EU-based store, I finally found a shop that could supply the flat delta ring assembly required.


The 4.5” flash hider was the first Airsoft part I had to use. Real ones are illegal in Germany.

The XM177 uses a unique flash hider, sometimes called a flash or sound moderator for its 10” barrel. It was designed as a counterbalance measure, as the shorter barrel makes the weapon unwieldy. It reduces flash signature greatly and sound signature slightly, making the XM carbine sound like a longer barreled M16A1

Edit: on 23/05/20 the blog received this submission by email:

I was with the Small Arms Weapons System test (US Army) 1965 and we had most of the “Colt Weapons System” firearms. In Vietnam in 1967 I used the XM177E1. The muzzle blast and flash suppressor performed a critical function. It provided enough restriction to allows the gas system to recock the weapon. Remove it and most of the time the weapon will not cycle even with a rifle flash suppressor in its place. The E2 longer barrel was to allow the mounting of the M148 grenade launcher. No doubt the longer barrel helped subdue flash & report, but my experience is limited to the E1.

Frank Camper

My choice was the G&P Airsoft XM177 Flashider (14mm clockwise).


The front sight was an easy thing too. A look in my spares box unearthed many unused Systema A-frame sights. All I had to do was remove the bayonet lug. The XM was not supposed to be used as a “charge weapon”, so there was no need for the attachment.


The outer barrel was a bit of a compromise.

I do not like steel barrels, because they make the gun too front heavy. So I used an aluminium version. The length had to be 10” and the thread 14mm CW to accept the moderator.

The real XM barrel had a 0.625” or ~1.58cm outer diameter. The aluminium barrel I used is thicker, at around 1.9cm. Perhaps a milled barrel and an RS front sight will be fitted in future.

The upper was almost done and the hand guard should not be that much of a problem, right?

Wrong. They are if you are after a 100% correct version.

At a glance, the hand guard looks like a standard PTW one. But the latter’s is more oval and the correct one is round and skinny. After a few days I was happy to find something almost correct, from Brownells: AR-15/M16/M4 SKINNY STYLE CARBINE HANDGUARD.

It’s a similar version – a round, skinny hand guard – but it’s not 100% XM-style. The original version exhibits only six cooling holes – Brownells’ has seven.


The next impossible to get part was the stock and the receiver extension tube (buffer). The XM had a two position receiver extension tube with an aluminium-bodied stock, coated with vinyl-acetate.

The real deal is extremely rare and expensive too, here in the EU. So, again, I had to compromise with an Airsoft version.

Again, I chose G&P. This time a GBB G&P Airsoft WA XM177 Stock. The problem was, mine came without the old style castle nut.


With the E1 there is a castle nut that does not look like the newer, more turret-like units. The old one was just a round thing, with a hole in the side to fasten it.

So I had to get the Viper XM177 Receiver Extension Castle Nut.


I wanted to add a recoil pad to my gun, because it would cover the area where the PTW’s battery would be. Again, Brownells was a good source with the JOHN MASEN SEMI-AUTO RIFLE RECOIL PAD.


The RS grip is easy to get, but hard to rework to accept the motor. So I thought I’d make it easy with the FCC Custom XM177 A1 Grip.


It turned out to be not so easy, however. The grip had to be reworked with a Dremel to accept the motor end plate.


The lower I used was a G&P with Colt M4A1 trades – again from my spares box. Yes, I was forced to compromise, but at least it had a full ‘fence’ like all modern M4 lowers.

But what about those trades?

There are XM177E2 or M16A1 lowers, but the correct markings are:


To make my lower a bit more fitting, I decided to remove some of the markings. I did not want “M4A1 Carbine” and “For Military and Law Enforcement” on my XM.

The first step in reworking the lower’s markings was to fill in the unwanted text. The next step was to paint it.


As for paint, why not cerakote it?

I got myself some Graphite Black Cerakote but after filling, sanding and then soaking the lower in acetone I had a problem. G&P’s marvellous receiver coating started to come off with the acetone. I had to sandblast it, to get rid off the paint that had begun to peel off.

After doing this I had to fill and sand it again. I was then able to Cerakote it.

Like so:


To complete the look I added a NOS M1 Sling dated from 1967.


In the end, the whole thing came out like this:


Here are some Pictures of the XM177E1 in Vietnam, for reference.


Further resources:


http://www.militaryfactory.com/smallarm … arms_id=39


https://bpullignwolnet.dotster.com/retr … e/609.html



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