XM177E2 clone build: the Vietnam CAR-15, part by part

In April 1967 the Army ordered 510 short Colt carbines for MACV-SOG. John Plaster, a SOG veteran, later wrote that the order gave every recon team member his own CAR-15. Colt called it the Model 629. The Army called it the Submachine Gun, 5.56mm, XM177E2.

We rebuilt it from the Army's 1968 test reports, two museum originals and Plaster's account, then priced every slot in our feeds. You'll see what matches the issued part and what nobody sells.

What an XM177E2 clone copies

An XM177E2 is an M16A1 with a different barrel, flash suppressor and stock. The Army's June 1968 technical report lists no other differences.

So your clone takes an A1 upper and lower and adds an 11.5 inch barrel, Colt's 4.25 inch moderator, a two position aluminum stock and round carbine handguards.

XM177 serial 901635, the 10 inch forerunner of the E2, with the moderator, handguards and stock the E2 kept. Photo: Springfield Armory NHS, National Park Service. Public domain, via Wikimedia Commons.

XM177E2 specs from the 1968 Army tests

Aberdeen Proving Ground measured 11.6 inches from bolt face to muzzle on its test gun, and 15.4 inches to the front of the moderator. The technical report gives 6.1 pounds empty and a Royal Armouries original weighs 6.4.

Who carried the XM177E2 in Vietnam

SOG recon teams, Special Forces, LRRP and Ranger companies, Navy SEALs and the Air Force carried the XM177 series. Period photos show line units with it too. A 1968 Army survey quoted on Wikipedia found 85 percent of respondents wanted the M16 or the XM177E2.

A 1st Cavalry Division rifleman extends his CAR-15's stock at An Khe in January 1967, months before the first E2s. Photo: SP5 Robert C. Lafoon, U.S. Army. Public domain, via Wikimedia Commons.

MACV-SOG recon teams and the 1967 order

The April 1967 order bought 510 Model 629s for SOG, and Wikipedia puts delivery of all of them by that September. Plaster's account gives you period touches to copy.

Some men fitted a reversed M16 grip under the handguard. Some taped the muzzle, and some swapped the moderator for a birdcage.

The Son Tay raid carbines of 1970

Wikipedia, citing Schemmer, says the 56 Son Tay raiders of November 1970 carried 48 CAR-15s. The ARSOF museum caption says their night sights came from the civilian market.

Plaster calls the sight the Singlepoint Night Sight, while a JFK Special Warfare Museum label calls it a Sears Roebuck dot sight sold for $59.95. Both museum Son Tay rifles have fixed A1 rifle stocks, so the raid guns may not have been XM177E2s.

A Son Tay raid CAR-15 variant at the ARSOF museum, with a fixed rifle stock and a civilian night sight. Photo: U.S. Army, ARSOF History. Public domain, via Wikimedia Commons.

XM177 variants over its service life

Four Commandos matter to a cloner. The barrel, forward assist and slip ring tell them apart.

Colt's 14.5 inch carbines took over the job, a line that runs to our Black Hawk Down CAR-15 build and Gordon carbine clone build, then to the M4 in our M4A1 Block I clone build. SOCOM's 10.3 inch guns get the Mk18 Mod 0 clone build and CQBR Block I clone build.

XM177E1, Colt Model 609: the 10 inch Army gun

The Army ordered 2,815 Commandos on 28 June 1966. Wikipedia counts them all as Model 609 XM177E1s, but Stoner says the buy included Air Force guns.

The E1 had a 10 inch barrel, an unplated chamber, a 0.072 inch gas port and a flat slip ring. The Firearm Blog photographed one in a Hanoi museum with a short moderator, so whether early E1s had the 4.25 inch device is disputed.

XM177, GAU-5/A and GAU-5A/A: the Air Force guns

The Model 610 is the slick side Commando, designated XM177 and adopted by the Air Force as the GAU-5/A. The Royal Armouries example is stamped SMG GAU-5A, with a 10 inch barrel and a flat slip ring.

The GAU-5A/A is the 11.5 inch Air Force counterpart of the E2, also without a forward assist. For either one you'll want a slick side upper.

XM177E2, Colt Model 629: what changed besides the barrel

Aberdeen's report lists more than the 1.5 extra inches of barrel. The E2 also got a chrome plated chamber, an XM148 grenade launcher spacer, a Delrin charging handle latch, an angled slip ring, shot peened receivers and a nylon coated stock.

Aberdeen tested the E2 from August 1967 to April 1968 and rejected the Delrin latch after one broke at minus 65 degrees F. The longer barrel added 229 fps with M193.

XM177E2 parts list: the issued part and what to buy

Here's every slot, what Colt fitted and the closest part you can buy. Exact means the period pattern, equivalent a legal or available stand in, and missing a part we don't stock yet.

Product photography supplied by the listed merchants through their affiliate feeds.

The E2 shares most of its upper and lower with the M16A1, so this list overlaps our M16A1 clone build.

The lower receiver group

Every part here is semi auto.

Lower receiver: A1 profile and the XM177E2 rollmark

The Royal Armouries' Model 629, serial 906200, is stamped COLT AR-15, PROPERTY OF U.S. GOVT., XM177E2 and CAL. 5.56 MM, with SAFE, SEMI and AUTO at the selector. Every card here is a semi auto equivalent, and none of the H&R listings states its fence or full rollmark.

The H&R XM177E2 and BRN-16A1 lowers were out of stock on 27 September 2026. If you'd rather engrave your own marks, the plain BRN-16A1 is your blank.

Fire control group: semi auto only

The issued M16A1 group had an auto sear, and you can't use it. Fit a standard semi auto group, and treat an AUTO mark on your lower as cosmetic. Both cards are equivalents.

Stock: two position CAR-15 aluminum

Plaster credits the two position aluminum stock to Colt engineer Robert Roy. Aberdeen calls the E2's coating nylon, while collectors and Colt's reissue say vinyl acetate.

On Google it's our most visible XM177 part. The H&R retro CAR15 stock page drew 228 impressions in the 90 days to 24 September 2026.

Only the ACE kit is coated aluminum, and it was out of stock on 27 September. The H&R stock is a polymer lookalike.

Buffer tube, carbine buffer and action spring

Aberdeen weighed the buffer at 2.98 ounces with three weights and a urethane end cap. It measured the spring at 10.1 inches free with 36 working coils.

Pistol grip: M16A1

Use the standard M16A1 grip. For a SOG touch, Plaster describes recon men mounting a second one, reversed, under the handguard.

The upper receiver group

The upper is an M16A1 upper with a short barrel.

Upper receiver: A1 with a teardrop forward assist

You want the fixed carry handle, a teardrop forward assist, an ejection port cover and no case deflector. For an Air Force build, use a slick side upper.

H&R's listing doesn't say whether this equivalent ships stripped, so ask before you order.

Barrel: 11.5 inch lightweight, 1 in 12 twist

Colt's barrel was 11.5 inches with a 1 in 12 twist, a chrome plated chamber and carbine gas. Pick 1 in 12 for period 55 grain M193 or 1 in 7 for heavier bullets.

No feed we track sells a 1 in 12 barrel of this length alone, so every card below is a 1 in 7 equivalent. Length sets your legal route; see our AR-15 barrel length guide.

An F marked front sight base sits too tall for a carry handle, so check the front sight base first. Then add a carbine gas tube.

Sights: A1 front sight base and flip rear aperture

The A1 front sight has an adjustable post, 2.8 cm per 100 m a click. The Royal Armouries 629's bayonet lug was cut off at the factory. Gun Digest says H&R's repros copy that, but we haven't found a lugless base for sale.

The rear is the A1 flip aperture, set for 0 to 300 m or 300 to 500 m.

Handguards: short round carbine with a delta ring

Colt's handguards are round and ribbed, with identical halves and heat shield liners, about 6 5/8 inches long per Plaster. Only enthusiasts document six vent holes, and we don't carry H&R's six hole set, so the handguard card is an equivalent.

Use the delta ring for an E2 and the flat Brownells ring only for an E1.

Bolt carrier group and charging handle

The issued group was the M16A1 automatic carrier. Skip chrome, because an owner notes Colt's reissue has chrome parts the originals didn't.

Aberdeen rejected the E2's Delrin charging handle latch. Nobody we read says whether production guns kept it and no feed sells one, so a standard metal latch, the E1 pattern, is what you'll fit.

The moderator and grenade ring

The long muzzle device is the XM177's signature. Because ATF treats an original as a silencer, your clone needs a replica.

Moderator: 4.25 inches, and why an original is a silencer

Colt's standard moderator was 4.25 inches with six slots and an expansion chamber. Two Royal Armouries originals, one auctioned original and Plaster all agree on that length.

It cut sound only slightly, and Aberdeen measured the E2 at 155.0 dB. ATF still treats an original as a silencer, and Poulin Auctions listed one as a transferable NFA silencer. We couldn't find the ruling itself.

Replica moderators sold as flash hiders

Buy a replica its maker markets as a non NFA flash hider, and don't modify its internals. KAK's 4 inch retro hider is sized to bring a 12.5 inch barrel to legal length when pinned and welded. Its XM177 Flash Hider, Clone Correct adds the grenade ring, but we don't carry it yet.

An A1 birdcage copies the later look our Colt 733 build from Heat covers.

The 4 inch hider was out of stock on 27 September. KAK's 6 inch version runs far longer than the original.

Grenade ring: separate piece or built in

Aberdeen lists the XM148 spacer as its own part, and Plaster describes a ring behind the moderator. Wikipedia and The Firearm Blog describe a boss on the moderator itself. Sources disagree. Brownells sells a separate ring, and KAK builds it in.

Magazines, sling and period accessories

Aberdeen's April 1967 test directive calls the sling the only accessory.

Magazines: 20 round aluminum, then 30 rounds

The issued magazine was a 20 round aluminum box. Production 30 rounders arrived in November 1967, and Plaster describes carrying twenty 20s plus a 30 in the gun. The DuraMag and D&H cards are new production aluminum 20s.

Sling, optics and the XM148 launcher

No source we read says whether the sling was cotton web or nylon, and we don't stock a GI sling. Springfield's cotton web sling and GrovTec's nylon one are stand ins, not issue items. No optic was issued either.

The E2 was built to take the XM148 40mm launcher. Aberdeen judged firing it feasible only for experienced shooters with the stock extended. The launcher is military only.

The civilian legal path to an XM177E2 clone

An original XM177E2 is a select fire machine gun, and only transferable machine guns registered before 1986 can change hands. Your clone needs a semi auto lower and fire control group. The barrel decides the rest.

Short barreled rifle: 11.5 inches with a stock

An 11.5 inch barrel with a stock makes a short barreled rifle. ATF announced on October 9, 2026 that it would stop enforcing SBR registration and approval, but some states ban SBRs outright and some only allow registered ones.

An original moderator is a silencer on top, and cloners called that a two stamp gun. Our suppressor tax and paperwork guide covers where that stands now.

12.7 inch barrel pinned and welded to 16 inches

A muzzle device counts toward the 16 inch minimum only when it's permanently attached, and you measure from the bolt face to its end. So the rifle route pins and welds a replica moderator to a 12.5 to 12.75 inch barrel. Brownells and H&R both use 12.7 inches. H&R's Commando style 12.5 inch upper has no live offer in our feeds yet.

11.5 inch pistol: why adding a stock makes it an SBR

H&R sells an 11.5 inch XM177E2 pistol. Add a stock and you've built a short barreled rifle. Braces follow their own rules, covered in our AR-15 pistol brace guide.

State limits on stocks, flash hiders and SBRs

Several states restrict telescoping stocks, flash hiders or SBRs. Colt listed its own reissue as restricted in California, Colorado, Connecticut, Massachusetts, Maryland, New Jersey, New York and Vermont, so check yours before you order.

What you can buy today and what is unobtainable

Brownells uppers and H&R lowers are your shortcuts, and every H&R offer in our feeds comes from Palmetto State Armory. On 27 September 2026 the Brownells 11.5 inch XM177E2 upper and H&R's complete Commando lower came to $1,065.26, before magazines and a sling.

Brownells XM177E2 and GAU-5A complete uppers

The XM177E2 upper comes with an 11.5 inch 1 in 12 barrel at $645.27. The 12.7 inch 1 in 7 version, with its hider pinned and welded, was out of stock at $687.99. Owners report the 2024 upper has the correct tapered ring, and the slick side GAU-5A upper was $736.69.

Both are semi auto equivalents.

H&R XM177E2 and Commando lowers

H&R's XM177E2 stripped lower was out of stock, and its in stock blem and pistol lowers aren't matched to a live offer in our feeds yet. The complete Commando lower comes with the tube and an aluminum stock, so this one equivalent covers three slots.

Colt's CRXM177E2 reissue and its fixed long flash hider

Colt sold a semi auto XM177E2 Retro Carbine, SKU CRXM177E2, at $2,599 list. Its 11.5 inch barrel wears a permanent extended hider shaped like the moderator, and an AR15.COM owner measured 16 3/16 inches from the bolt face to the hider's end.

It's in no feed we track.

Parts no retailer can sell you

The select fire lower and auto parts are machine gun parts. An original moderator sells only at auction as a registered silencer, the XM148 is military only, and the Son Tay sight is long gone.

We don't yet stock a GI sling, a six hole handguard, KAK's clone correct hider or Colt's reissue. Nobody we found sells a lugless front sight base or a Delrin latch.

XM177E2 builds on GPP: what they show

Not many, and we'd rather say so. None of our named carts from 10 December 2025 to 27 September 2026 carries an XM177, CAR-15, Commando, SOG or Vietnam name.

What GPP builders do with the H&R Commando lower

From 18 December 2025 to 20 September 2026, 41 carts picked up an H&R Commando lower, and only 2 added another retro part. None holds a Commando style upper, retro barrel, moderator style hider or A1 sight. Instead they hold parts like Geissele triggers, Radian charging handles and SIG Romeo 5 optics.

So builders buy the retro lower and build a modern rifle on it. Only 2 of the 41 carts created 18 December 2025 to 20 September 2026 pass our complete build filter, and 24 predate our bot check, which began 22 April 2026.

What the internet gets wrong about the XM177

Here's what gets called one thing when the records show another.

Barrel length: 10 and 11.5 inches, not 10.5

Wikipedia's CAR-15 article opens with 10.5 and 11.5 inch barrels, but its body, Aberdeen and Plaster give the XM177 and E1 10 inches. Shooting Illustrated's 2018 review gives the original E2 10.5. The technical report and the Royal Armouries 629 say 11.5.

Weights, lengths and velocities that mix E1 and E2

Military Factory lists 5.36 pounds, against Aberdeen's 6.20 for an E2 and 6.0 for an E1. Shooting News Weekly's 28.3 and 32.5 inch lengths pair an E1 closed figure with an E2 open one.

The Firearm Blog gives the E2 a heavier buffer and half a pound more weight, where Aberdeen found the same buffer and a 0.2 pound gap. Stoner's 2,995 fps compares with the technical report's 2,770.

The COMMANDO rollmark and the survival rifle story

The Royal Armouries 629 is marked XM177E2. The COMMANDO lower on Retro Black Rifle carries serial 900077, below Aberdeen's E1 test guns, so it's probably an earlier gun.

The Firearm Blog also calls the GAU-5A an aircrew survival rifle. Colt's survival rifle was the Model 608, and the Air Force reused the GAU-5A name for a 2018 to 2020 aircrew weapon. No source we read says whether Vietnam era GAU-5/As rode with aircrew.

Sources and photo credits

Every figure above traces to a source here.