AP51 + F5-ACR G3 Stock/Cyberarm Light Brace and 50rd Drum Mag Combo
LIMITED QUANTITIES
The HK51, Finally Built the Way It Should Be
The Century Arms AP51 is the closest thing on the market to a factory HK51 — a chopped-down G3 in full-power 7.62x51mm NATO, built by MKE in Turkey on the same tooling that supplies the Turkish military. It’s a roller-delayed .308 that fits in a backpack. What it doesn’t come with is a rear end worth keeping.
That’s what this package fixes. We’ve paired the AP51 with F5 Manufacturing’s F5-ACR Gen 3 stock system, the skeletonized Cyberarm Light brace, and a 50-round G3-pattern drum. One order, one box, no parts hunting.
What’s in the Package
Century Arms AP51 PDW — 7.62x51mm NATO
- 8.3″ cold hammer-forged, chrome-lined barrel, threaded 5/8×24
- Roller-lock delayed blowback action
- 23.3″ overall length, 7.9 lbs unloaded
- Navy-style trigger group with ambidextrous selector
- Fully adjustable rear diopter sight, fixed front post
- Birdcage flash hider, MP5-compatible wide handguard
- Tested to NATO environmental standards — salt, fog, humidity, temperature extremes, sand and dust
F5 MFG F5-ACR Gen 3 Stock System — G3 / HK91 / HK51 Billet aluminum, left-side folding, multi-position adjustable length of pull. This is the turn-key version, so the correct recoil rod and spring are in the box — you’re not sourcing a buffer assembly separately or guessing at fitment. The folding hinge takes the AP51 from a PDW to something that disappears in a pack, and it locks up tight enough that your zero doesn’t care.
F5-ACR Skeletonized Cyberarm Brace, Gen 3 The lightweight configuration, with F5’s patent-pending retractable cheek riser. Drops meaningful weight off the back of a 7.9 lb gun and gives you a repeatable cheek weld with a red dot or LPVO — a problem most brace-equipped large-format pistols never solve.
F5 MFG 50-Round Drum Magazine — G3 Pattern Fifty rounds of .308 in a compact housing that drops straight into the AP51’s G3 magazine well. Steel and aluminum internals, straightforward to load and strip down. Also a meaningful contributor to 922(r) parts count if you’re tracking compliance on your build.
Why These Parts Belong Together
The AP51 is a genuinely great action wrapped in a 1960s ergonomic package. The stock is a rifle-length A2 relic or nothing. The sights are excellent but low. The trigger group is proven. The result is a gun most owners immediately start modifying.
F5’s ACR Gen 3 platform solves the rear end in a single part. Because it uses the same chassis for both the stock and the Cyberarm brace, you can run either configuration on the same firearm and switch as your paperwork status changes — no second purchase, no new adapter. The drum takes the AP51 from a 20-round range toy to something that earns its weight.
Stock vs. Brace — Read This Before You Order
The AP51 ships as a pistol. Installing a brace keeps it a pistol. Installing a stock creates a short-barreled rifle under the National Firearms Act, which requires an approved ATF Form 1 and a $200 tax stamp before installation. State law may restrict this further or prohibit it outright.
We include both so you’re covered either way — run the Cyberarm brace today, file your Form 1, and swap to the stock when your stamp clears. Check your state and local law before ordering. This isn’t legal advice.
Configuration Notes
- 5/8×24 muzzle threads accept most .308 suppressors and muzzle devices
- Included Picatinny rail accepts red dots, LPVOs, and clamp-on claw mounts
- Accepts standard G3/HK91/PTR-91 pattern magazines — 20-round steel and polymer, plus the included drum
- Folded with the ACR stock, overall footprint drops significantly for transport and storage
FAQ
Is the AP51 a real HK51? No. It’s an MKE-manufactured semi-automatic pistol built on the G3 pattern, imported by Century Arms. MKE holds a Turkish military production license for the design, which is why the fit and finish run well above typical clone territory.
Will the F5-ACR stock fit without gunsmithing? The turn-key G3/HK91/HK51 version includes the recoil rod and spring needed for installation. Basic hand tools and standard HK-platform familiarity are enough for most owners.
Does the 50-round drum work with the AP51? Yes. The AP51 uses G3-pattern magazines, which is what the drum is built for.
Can I suppress it? The barrel is threaded 5/8×24, the standard .308 pattern.
Do I need a tax stamp? Only if you install the stock. The brace configuration keeps the firearm a pistol under current federal law.
