Knights Armament KS1 Review | Royal Marines New L403A1 Rifle
The Knight’s Armament Company (KAC) KS-1 is the latest evolution in the SR-15/SR-16 series. It serves as a high-end, mil-spec AR-platform rifle optimized for modern combat demands, including heavy suppressor use, sustained fire, and full ambidextrous operation. Selected by the British Armed Forces as the L403A1 Alternative Individual Weapon (AIW), it now equips elite units such as the Royal Marines Commandos and British Army Ranger Regiment.
Built on KAC’s decades of expertise with the Stoner system, the KS-1 emphasizes extreme durability, reliability in harsh conditions, minimal point-of-impact shift under heat, and seamless integration with modern optics, lasers, and low-backpressure suppressors.
History and Development
Unveiled around 2022, the KS series represents a major update to the proven SR-15 platform with features tailored to international special operations needs. In 2023, after intense competitive trials (Project Hunter), the UK Ministry of Defence selected the KS-1 as the L403A1. Initial orders exceeded 1,600 units with options for several thousand more under a multi-year contract.
Key Specifications
| Specification | Details |
|---|---|
| Manufacturer | Knight’s Armament Company (KAC) |
| Model | KS-1 (L403A1 in UK service) |
| Caliber | 5.56x45mm NATO |
| Barrel | 13.7″ Cold Hammer Forged, Chrome-Lined, 1:7 Twist, Dimpled Heavy Profile (32CrMoV12-10 steel) |
| Gas System | Proprietary Mid-Length, optimized for suppressed use |
| Handguard | 13″ URX 6 Free-Float M-LOK (self-indexing) |
| Bolt | E3.2 Enhanced Bolt (dual ejectors, lobster-tail extractor) |
| Weight (Unloaded) | 6.88 lbs (3.12 kg) |
| Overall Length | 30.3″ – 33.5″ (stock collapsed to extended) |
| Action | Direct Impingement, Rotating Bolt |
| Controls | Fully Ambidextrous |
| Muzzle Device | KAC QDC 3-Prong Flash Eliminator |
Design Features and Innovations
- Dimpled Heavy Barrel: Reduces weight while improving rigidity and heat dissipation, resulting in minimal POI shift during sustained fire.
- E3.2 Bolt: Dual ejectors for superior reliability (especially suppressed), lobster-tail extractor, and significantly extended service life.
- URX 6 Handguard: Slim, extremely rigid, self-indexing M-LOK system with excellent anti-rotation properties.
- Fully Ambidextrous Lower: Mirrored safety, magazine release, and bolt catch for intuitive use by all operators.
- Optimized Gas System: Tuned for low recoil, reliable function suppressed or unsuppressed, and reduced gas blowback.
Pros and Cons
✅ Pros
- Exceptional reliability and durability in sustained fire and harsh environments
- Outstanding accuracy with minimal POI shift when hot
- Fully ambidextrous controls
- Excellent suppressor performance and compatibility
- High-quality components throughout (chrome-lined barrel, E3.2 bolt, match trigger)
- Proven in demanding military trials
- Low maintenance for its performance class
❌ Cons
- Premium pricing (typically $3,000+)
- Heavier than many lightweight modern ARs
- Limited civilian availability and KAC-specific parts
- Expensive replacement components
Performance and Real-World Use
The KS-1 excels as a versatile fighting rifle from CQB to 600+ meter engagements. It offers smooth recoil, positive extraction/ejection (even when suppressed), and maintains accuracy and reliability during high round counts. In British service it is commonly paired with a 1-10x LPVO, offset red dot, and KAC flow-through suppressor.
Who Should Buy the KS-1?
Best suited for professional users, serious tactical shooters, collectors, and anyone who values top-tier reliability, suppressor performance, and longevity over cost.
Frequently Asked Questions
What is the main difference between the KS-1 and previous SR-15 models?
The KS-1 adds the E3.2 dual-ejector bolt, fully ambidextrous lower, improved gas system, dimpled heavy barrel, and URX 6 handguard for better reliability, ergonomics, and suppressor performance.
Is the KS-1 available for civilian ownership in the US?
Yes. 16″+ models are available as standard rifles. The popular 13.7″ version is sold as a pistol or SBR (NFA rules apply).
How does the KS-1 perform when suppressed?
Extremely well. The E3.2 bolt and tuned gas system provide reliable cycling with minimal gas to the face when used with KAC QDC suppressors.
What optics and accessories pair best with it?
1-10x LPVOs (e.g. Vortex) with offset red dots like the Aimpoint ACRO. The URX 6 handguard accepts a wide range of lights, lasers, and grips.
Is the KS-1 worth the high price?
Yes — for users who need proven military-grade performance, suppressor excellence, and a rifle built to last decades under hard use.
How does the KS-1 compare to the HK416 or SIG SPEAR-LT?
It offers lighter weight than the HK416, excellent suppressor performance, and was selected over both in rigorous UK military trials.
Full Video Transcript
Full Video Transcript – Knights Armament KS-1 Review
0:00 Hey everybody, welcome back to the Armed Scotsman. Today on the table we have a Knights Armament KS-1. Let’s get this going.
0:59 Alright folks, as I said, this is the Knights Armament KS-1. This is the new rifle adopted by the UK military, the Royal Marines, and so forth. So it seemed fitting that I would go and buy this being from Scotland. Who am I kidding? I was going to buy it anyway.
1:13 We’re going to go over the features of the gun, the pros, the cons, 100-yard accuracy, and I’ll briefly compare it to an older Knights SBR with some side-by-side images.
1:38 Starting at the back, this is a Magpul stock. I knew it was coming with this, but I was surprised they didn’t include the Magpul CTR that the UK military uses. Magpul makes great stocks, but this one isn’t perfect for me.
2:05 I usually run B5 SOPMOD stocks because I like to keep the sling on this side and out of the way. So this stock would be something I personally change. It does come with a mil-spec buffer tube with that gray dry film lubrication Knights is known for, a castle nut, and a three-position QD sling attachment.
3:03 Moving down, this is a Magpul K2+ grip. It didn’t come with this — it came with a standard A2 grip which I swapped out immediately. Nice rubber overmold, excellent grip angle, and it has a storage compartment.
3:28 You have an ambidextrous charging handle with extended levers — that’s a nice touch. The upper and lower are 7075-T6 aluminum.
3:46 One of the big upgrades is the ambidextrous bolt catch and release. The previous versions only had the release, not the catch. This makes locking the bolt back much easier and faster. Huge fan of this change.
4:44 They’ve also upgraded the safety levers. My old one actually cracked on the right side, so this is a welcome improvement. You still have the enhanced mag release fencing, a curved one-piece trigger guard (which I prefer), and a nice two-stage trigger.
6:37 The included flip-up iron sights are excellent — all metal and some of the best on the market. If you bought them separately they’d easily cost over $300.
7:02 The handguard is the new URX 6. It’s very rigid, has great ergonomics for a C-clamp grip, and full M-Lok slots. They removed the built-in QD points, but they include a rail-mounted one.
8:03 Underneath is an ArmaLite barrel (made in France) with aggressive dimpling. It’s a heavy barrel that’s chrome-lined, 1:7 twist, and designed to minimize POI shift when hot. Very durable — Knights rates them for 25,000+ rounds.
9:55 Knights uses a proprietary mid-length gas system that’s very soft shooting. I have a Knights CRS-3 suppressor on here with their 3-prong flash hider.
11:36 The bolt is another proprietary part — rounded design with dual ejectors and a lobster tail extractor. Excellent reliability, especially suppressed.
13:01 Range Results (100 yards):
– PMC: ~2 MOA
– AAC 77gr: Under 1 MOA on good groups
– Black Hills: Around 1–1.5 MOA on good groups
15:45 Overall, the gun is accurate (capable of 1 MOA with good ammo), extremely soft shooting, and very reliable. I put over 400 rounds through it with zero issues.
16:17 Price-wise, these are expensive (MSRP around $3200, street price higher). They’re not for everyone — there are excellent rifles for half the price. But if you want one of the best AR-15s on the market and have the budget, the KS-1 is fantastic.
18:20 That’s it for the review. Any questions, drop them below. Until next time, catch you later.
