nathan13n
AF-UK patch owner
- Nov 13, 2017
- 87
- 15
Building the Ultimate Lightweight HPA LMG: My S&T M249 MK2 SAW Project
Going through my airsoft collection, I realized I was missing one key piece — an LMG. Naturally, I thought: why not? I didn’t expect it to become one of the most frustrating builds of my airsoft life. From the start, I knew I was going to HPA it, especially since I had a Backdraft Phoenix engine lying around — a solid engine, though a bit inconsistent in DMR roles.
Why I Picked the S&T M249 MK2 SAW
Let’s be real — the S&T M249 is just a rebranded A&K Featherweight (also sold under Cybergun), but with a few advantages:
At first, the V2 gearbox and light weight seemed like a win. But that big proprietary nozzle and body flex quickly turned into a massive headache.
Out-of-the-Box Performance
Running a 7.4V battery, it gave me:
Not bad for £200. Decent compression, a metal rack piston, basic MOSFET locked to full auto. It worked — but it wasn’t good.
Goals for the Build
The Build Process
Body Flex & Rail Replacement
Because the gun is so light, body flex was a huge issue — even minor misalignment can cause major problems with HPA. I replaced the plastic top and bottom rails with metal ones and added bracing plates. This significantly stiffened the gun and improved alignment.
Engine & Electronics
Installed the Backdraft Phoenix HPA engine and wired it to a Gate Aster II for better control.
Problems started immediately:
Hop-Up & Feed Ramp Hell
The stock hop-up is garbage — cheap, flimsy plastic with lots of flex.
Swapped to:
Tried:
Final Solution:
Bucking & Barrel Testing
Bucking Results:
Tip: With Bullgear hop-up, use high elasticity buckings. Stiffer ones perform worse.
Barrel Results:
Magazines: A Comedy of Errors
The stock S&T mag (9V battery) fed only 12 RPS — upgraded to 7.4V LiPo, gained speed, then shredded the gears (poor plastic internals).
Tried:
Stock Upgrade
The original SAW stock is huge and ideal for batteries, but looks awkward and feels cheap.
Didn’t want to pay £50 for a CNC adapter, so:
Final Build & Cost Breakdown
Part
Price
S&T M249 MK2
£200
PDI 6.05mm Barrel
£60
Backdraft Phoenix HPA Engine
£250 (already owned)
Gate Aster 2
£65 (already owned)
A&K 2000rd Box Mag
£75
Bullgear Hop-Up Unit
£60
Custom Feed Ramp & Stock Adapter (PA12)
£50
Total: ~£760
Final Verdict
View attachment 150877
What I have now is a lightweight LMG, feeding 0.30g–0.32g BBs reliably, with:
Great for suppressive fire, fun skirmishing, and surprising people who think it can’t reach them.
Would I do it again? Probably not.
Am I happy with it? Hell yes.
Little video of the bad boy shooting
Going through my airsoft collection, I realized I was missing one key piece — an LMG. Naturally, I thought: why not? I didn’t expect it to become one of the most frustrating builds of my airsoft life. From the start, I knew I was going to HPA it, especially since I had a Backdraft Phoenix engine lying around — a solid engine, though a bit inconsistent in DMR roles.
Why I Picked the S&T M249 MK2 SAW
Let’s be real — the S&T M249 is just a rebranded A&K Featherweight (also sold under Cybergun), but with a few advantages:
- V2 gearbox (sounds great at first… more on that later)
- Lightweight — only 4kg, compared to 5.5–6kg for most other M249s
- Cheap — cost me just £200
At first, the V2 gearbox and light weight seemed like a win. But that big proprietary nozzle and body flex quickly turned into a massive headache.
Out-of-the-Box Performance
Running a 7.4V battery, it gave me:
- 12 RPS
- 1.1J
- 30m accuracy with a torso-sized spread
Not bad for £200. Decent compression, a metal rack piston, basic MOSFET locked to full auto. It worked — but it wasn’t good.
Goals for the Build
- Fix the weak plastic stock
- Tighten up the shot spread
- Increase rate of fire
- Get consistent mag feeding
The Build Process
Body Flex & Rail Replacement
Because the gun is so light, body flex was a huge issue — even minor misalignment can cause major problems with HPA. I replaced the plastic top and bottom rails with metal ones and added bracing plates. This significantly stiffened the gun and improved alignment.
Engine & Electronics
Installed the Backdraft Phoenix HPA engine and wired it to a Gate Aster II for better control.
Problems started immediately:
- Sporadic shots
- Inconsistent feeding
- Laser shot followed by a 2-meter fart
Hop-Up & Feed Ramp Hell
The stock hop-up is garbage — cheap, flimsy plastic with lots of flex.
Swapped to:
- Bullgear Featherweight M249 Hop-Up — improved consistency, but feeding issues remained
Tried:
- Drilling stock feed ramp — BBs jammed
- Anttech feed ramp — flimsy 3-part design, reintroduced flex and made things worse
Final Solution:
- Found a 3D model of the original feed ramp by Gazadonf77
- Printed in PA12 nylon (MJF) via 3D People
- Game changer — fixed all feeding issues
Bucking & Barrel Testing
Bucking Results:
- Prometheus Purple: Good but had flyers every 5 shots
- Quantum: Best range (50–60m), but overhopped everything and grouping was meh
- Maple Leaf 60°: Winner — torso-size grouping at 50–60m with 0.30g BBs, great price-to-performance
Tip: With Bullgear hop-up, use high elasticity buckings. Stiffer ones perform worse.
Barrel Results:
- XT 6.00mm: Tight bore, low PSI, bad consistency
- ZCI 6.02mm: Better, but still not quite there
- Stalker Morpheus 6.03mm: Extended crown = more range (70m), but groupings were wild
- PDI 6.05mm: Winner — 55–65m range, best consistency, 2–3 inch spread. Uses more air but totally worth it.
Magazines: A Comedy of Errors
The stock S&T mag (9V battery) fed only 12 RPS — upgraded to 7.4V LiPo, gained speed, then shredded the gears (poor plastic internals).
Tried:
- Lonex 200rd & EPM1: Fed great, but too low capacity for an LMG
- Classic Army M249 mag: Worst of all — inconsistent, weak, loud motor
- Novritsch M249 mag: Winner — actually a rebranded A&K 2000rd box mag, fed up to 30 RPS on both 7.4V and 9V. Responsive mic, auto shutoff, and only £75.
Stock Upgrade
The original SAW stock is huge and ideal for batteries, but looks awkward and feels cheap.
Didn’t want to pay £50 for a CNC adapter, so:
- Found an STL file by CitrusPers - Stock adapter
- Got it printed by 3D People
- Now using M4 buffer tube + compact stock = lighter and more ergonomic
Final Build & Cost Breakdown
Part
Price
S&T M249 MK2
£200
PDI 6.05mm Barrel
£60
Backdraft Phoenix HPA Engine
£250 (already owned)
Gate Aster 2
£65 (already owned)
A&K 2000rd Box Mag
£75
Bullgear Hop-Up Unit
£60
Custom Feed Ramp & Stock Adapter (PA12)
£50
Total: ~£760
Final Verdict
View attachment 150877
What I have now is a lightweight LMG, feeding 0.30g–0.32g BBs reliably, with:
- 55–65m effective range
- Consistent grouping (~2–3 inch spread)
- 30 RPS at 1.1J
- 3,500–4,000 rounds per 48ci tank @ 70 PSI
Great for suppressive fire, fun skirmishing, and surprising people who think it can’t reach them.
Would I do it again? Probably not.
Am I happy with it? Hell yes.
Little video of the bad boy shooting
