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Viper Tech Colt LMG / Diemaco C7 LSW


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It's ya boi, poor financial decisions.

I've been thrilled with the GHK AUG H-BAR LMG. I enjoy machine gunning in airsoft and I enjoy GBBRs, and bringing the two together has been a ton of fun.

So, after receiving a Viper Tech M733 and being very impressed with it, I've decided to pursue another 'grail gun' project. Originally I was going to build this as an AEG using an old G&P M16 as a base, but now I feel I have to go all-in.

In real steel, the Colt LMG was a product concept that Colt brought back over and over again for about thirty years but never had any success with the US military. A collaboration with Diemaco to answer a Canadian RFP, however, resulted in the gun being adopted by Canada as the Diemaco LSW:


Despite looking like a cosmetically modified M16A2, it has little commonality with an A2. It features an extremely heavy barrel (1") profile, and a unique bipod and handguard arrangement (with carry handle and foregrip clamped to the barrel). The upper and lower receivers are both unique, with the upper receiver omitting the forward assist, the lower receiver having a redesigned trigger pack to fire full auto only from an open bolt, and the bolt modified to function with it. It uses a hydraulic buffer to reduce the rate of fire to about 700 RPM.

At some point in the 2000s- reference info is hard to find- Diemaco updated the weapon to modernize it.


The upper receiver has been updated to a flattop, the carry handle removed from the handguard and replaced with a rail-mounted equivalent, the bipod relocated to a barrel clamp under the handguard, the standard C7A1 tri-rail added to the front sight block, and a new flash hider.

However, a slightly different version was produced for export to Denmark and the Netherlands, using a standard bolt and standard upper and lower receivers- basically just a heavy barrel and handguard/bipod setup for an otherwise normal AR.


This makes things much easier for me since removing the forward assist and converting to open bolt are not in the cards for the immediate future. Now, I do like the flattop, but I prefer the old-style bipod, which is an issue.

After much research I stumbled across this image of what appears to be a transitional model that has the exact combination of features I like:


I have no context for when and where this was photographed, but it looks to be a real configuration so I'm using it as a reference. I don't believe I'm the first person to have this idea, as I also found a real steel civilian replica with very similar parts:


You may notice if you look closely that this person used a M249 bipod, which was the last piece of the puzzle. It's not exactly correct, but close enough for me.

So! While I wait for an order on a Viper Tech M16A3 to process, I got started on the rest of the gun.

First, I ordered a steel M249 bipod from an HK shop, which I am still waiting for.

Second, I got my engineer buddies together and CAD'd up the handguard, carry handle, and foregrip, and sent them off to a 3D print farm to be produced in multijet fusion nylon. We'll see how those turn out- with any luck they should require minimal post-processing. The bracket which secures the foregrip and carry handle will be made from sheet steel cut and bent to shape, and the carry handle itself made from steel rod.


Next up: The magazine.

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The magazine is the most critical element of the project, as a machine gun is only as good as its feed system. A standard 30-45 round GBBR mag won't cut it for machine gunning, so building a hicap or drum of some sort was a necessity. I considered several options:

  1. Cut the gas reservoir short on a few gas mags, tap for HPA, and insert hicap internals to the bottom, for a capacity of 200-300 BBs. This is basically replicating how I did the GHK AUG mags, but in a STANAG form factor.
  2. Build some dual-clamped HPA hicaps- an HPA-tapped GBB mag, and a hicap clamped to it, with the hicap feeding into the side of the GBB mag. This gets a full hicap capacity and is easy to build, but is bulky and annoying to carry several.
  3. Convert a TM C-mag, which is non-motorized and functions essentially like a pair of hicaps stuck together, to HPA-tapped GBB. This has a capacity of around 1500 BBs but needs to be re-wound about halfway through.
  4. Find and convert a 2500-3000 round motorized C-Mag to HPA-tapped GBB.

Conveniently, while I was planning this project ICS drums came back into stock in the US, so option 4 won out. The airsoft copies of the Beta C-Mag use a standardized double drum with interchangeable feed stacks, allowing them to easily swap between different guns. So I figured it'll be easy, just splice the feed stack onto the upper half of a GHK mag, tap the internals, good to go.

Well, turns out that had some issues.

First, the plastic feed stack is fine as a solid injection-molded piece, but after cutting it to fit to the GHK mag, it lost its structural integrity and could easily warp. I did not trust it to stand up to field use. Second, the GHK STANAG mag shells (note that the actual AR C-Mags use a polymer feed stack, but the steel mag shells are more conducive to this sort of mod) are made of thin steel and warp easily. Third, on the GHK mags the upper surface of the mag catch is part of the magazine internals- which means that the entire weight of the drum would hang off of whatever I used to secure the GHK mag internals to the shell.

So I ended up cannibalizing the shell of a EB-Tech STANAG mag (thicker steel, much more solid, and mag catch is entirely contained to the shell) and building a new base for it out of 16ga sheet steel. The STANAG shell splays out under the base, so it was structurally sound even before I added JB Weld to fill in the gaps and hold it securely. And then a simple spring tube connects the drum's feedway to the feed lips. All in all it was more work than expected, but it functions properly.



And here's a demo video of the drum in the 733:


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2 hours ago, hitmanNo2 said:

That drum mag looks and sounds amazing.  Would you consider building them to order/small batches?

I'd love to, but cannibalizing two different gas mags and cutting the bracket out of sheet steel with a Dremel aren't decisions that lend themselves to affordable production.

If this project works out I might try making another out of a GHK Gmag. If that can be made to work out as a more reproducible process then I definitely would consider making them for sale.

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Two more pieces of the puzzle.

The 3D printed furniture was done in nylon using multijet fusion as the process. I am very impressed- it has the same feel as cast nylon parts, and much less surface texture than I'm used to on FDM prints. I'm unsure whether I'll just sand it or use some kind of paint over top.

The handles are solid, and are designed to use off-the-shelf hardware. My intent is to bend 8mm steel rod to shape for the carry handle, and thread the end for a nut that will slip into a hexagonal recess on the handle. The foregrip is intended to use a M8 bolt. Both of these will secure to a barrel clamp that I will need to make from scratch.

And additionally, I got a steel 249 bipod- in spite of Tiger111HK sending it to my old apartment even after they verified for me that they would change the shipping address on file and send to my new address. Annoying. Anyways, because it's a 249 bipod it seems the locking mechanism will need some work to function properly with the handguard here, but that's certainly doable.

This is all I can do until I actually receive the host rifle- which shouldn't be more than a few weeks away.



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  • 2 weeks later...


The rifle has been acquired, so now the real work can begin.

First up is getting the rifle up to spec:

  • Filing out the magwell to accommodate the wide tops of GHK magazines
  • Removing the bolt catch detent, inserting a spacer to limit bolt catch travel, and reprofiling the bolt catch (not strictly necessary for this project as the HPA mag will never lock open, but these help with normal GHK mags, which have weak springs that can't reliably actuate the bolt catch otherwise)
  • Installing a real-steel buffer (debounce helps reliability on FA) and charging handle (stock one is blued, which doesn't look quite right IMO)
  • Cleaning out all the factory oil and relubricating parts with grease

Once all that's sorted, then I can get to the real task of fabricating the foregrip/carry handle mount.

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Well, getting the magwell and bolt catch sorted took less time than anticipated, so I threw the drum, handguard, and bipod on the gun to see how it looks.


Still a lot of work but this is a good start.

You may notice the barrel seems weedy in comparison to the bipod. The A2/A4 uses a 0.7" barrel diameter forward of the gas block, while the real Colt LMG appears to be ~0.94", and the M249 bipod yoke is intended to fit on a tube 1" in diameter.

Originally I was planning to use tubing of the correct diameter to bulk up the barrel, and fit it to an A2-style front sight block intended for bull barrels, but it seems those have been out of production for years and are now impossible to find. So, that means sticking with the current FSB, and a 0.94" barrel would actually be wider than the FSB itself. I'm thinking instead I'll split the difference and do roughly 0.88" forward of the gas block, leaving the barrel under the gas block at 0.75".

The side effect of this decision is that the bipod yoke needs to be reworked. Fortunately, this provides an opportunity to more closely copy the design of the original bipod. The plan is to saw off the circular yoke, fabricate a semicircular yoke out of sheet steel, and pin/epoxy it to the remainder of the bipod.

Additionally, real Colt LMGs did not have bayonet lugs; however as I would prefer not to saw off the one on this rifle, I may simply cut a slot in the barrel sleeve and use the bayonet lug to index it. I'll reconsider if it's really glaringly bad.

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It's actually going faster than expected.


First off, the barrel sleeve and bipod yoke. This is not the cleanest work I've ever done, but it does hold. The barrel sleeve fits very snugly over the barrel, and indexes via a cutout to accommodate the bayonet lug. The birdcage is then torqued down in front of it and that holds it in place.

On the underside of the barrel sleeve, I've filed a cutout. There's a grub screw protruding up from the bipod yoke which fits into this cutout and can move from side to side. That keeps the bipod on the gun and allows about 30 degrees of rotation to either side. Simple.

Tentative plan is to epoxy the U-shaped steel plate to the bipod yoke, then cut these ugly screws short to act as pins and peen them in place.

Next up, the foregrip.


After a lot of brainstorming and hemming and hawing I realized that it would be easiest to just get one of the thirteen million rail mounts intended to clamp onto a barrel and work from that. So, here we are.

On the underside, I've drilled a hole through the middle of the rail mount, and tapped it for M8x1.25. An M8 bolt then runs up through the foregrip, threads through the rail mount, and stops when it hits the barrel. A piece of aluminum tubing serves as a spacer to stand the foregrip off the right distance, and that's all there is to it.

So, I still need to work out the carry handle, and that's looking like it might be A Whole Big Thing. I also need to add hooks to the bipod feet so they can clip into the handguard, and shim the front of the handguards to keep them tight as they're ever-so-slightly undersized. Otherwise, getting pretty close to completion.

Current state glamor shot:


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Cleaned up the bipod mount:


And fabricated the carry handle mount, plus reattached the gas tube (which required cutting the actual rail off the top of the clamp, as you can see below):



The carry handle bracket needs cleanup and reinforcement to prevent bending, but it's at the correct height now and holds the 8mm steel rod, so should be good once that's complete.

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Got the carry handle done- I reinforced the mount bracket further, then bent the rod (not the easiest thing with hand tools). I filed a notch on the rod which a grub screw on the handle indexes into, and cut grooves for E-clips to keep it attached to the mount.


And then on the bipod, I epoxied small strips of steel, then bent the ends, and reinforced with screws. These hook into the handguard and provide the retention mechanism to keep the bipod legs folded up.


And that about completes the project. Here's some glamor shots of the full thing:





I might go back and either work out a clamp-on barrel sleeve for the stub of barrel under the front sight, or just clamp on that tri-rail after all, but I'm undecided. In the meantime I need to break in the internals and get some test footage.

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  • 2 months later...
45 minutes ago, Aaron Hyry said:

SO I was just wondering where did you find the dimensions for the handguard and sorts 

I just found some side-on blueprints through Google and extrapolated dimensions from it. I cheated a bit on the width of the handguard by copying it from a set of handguard files someone else made.

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  • 2 months later...
18 hours ago, paranoiddroid said:

that's an incredible piece of work and much more  impressive than the typical attempts. 


I actually think how you mounted that bipod is better than the original idea. 

Thank you, pretty pleased with how it came out, although I suspect based on reference images that the handguard is actually slightly oversized compared to the real thing.

But it's held up pretty well in gameplay, so can't complain. I actually have some updates that I haven't posted here.

After trying it out in-game, I did some further experimentation with other configurations. Using an Elcan seems to work pretty well for long-ranged fire (although the replica isn't spectacular), but the reduction in FOV and lack of backup irons is problematic when I don't know where targets will be appearing or at what range.

So, I threw an A2 upper on instead, and attached a Norinco copy of a 3x Colt scope. The scope doesn't take up nearly as much of my field of view (at the cost of lower FOV through the scope), and I can still use the irons underneath:


There are two further configurations I want to try- same thing but ditch the scope and see how well I get by on just A2 irons (or possibly a carry handle mounted red dot), and go back to the flattop but use DD irons ahead of the Elcan (can't get Diemaco BUIS so it's the next best thing), and take advantage of the Elcan's QD mounting screws to remove it from the gun and use the irons for closer-in shooting.

Anyways, here's footage of the gun in use:


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  • 2 months later...


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