Best 4-Inch AR Pistol Uppers for CQB from GorillaMachining.com 

Why a 4-inch AR pistol upper for CQB?

In close-quarters battle (CQB) or home-defense contexts, maneuverability is king. A 4-inch AR pistol upper delivers ultra-compact handling, rapid presentation in hallways/vehicles, and plenty of modularity for optics and weapon lights—without the bulk of a rifle-length system. GorillaMachining.com curates short-barrel uppers and components designed for micro builds (AR-15 and AR-pattern pistol calibers), with options and landing pages specifically aimed at the 4? format. 

What to look for (fast checklist)

• Caliber for CQB: .300 BLK and 9mm are the most practical at 4?. .300 BLK is optimized for short barrels (especially subsonic/suppressed); 9mm keeps recoil low and pairs well with duty pistol ammo. Gorilla Machining highlights caliber options across pages and product families. 

• Thread/mount compatibility: Most .30-cal short barrels use 5/8×24; pistol calibers often use 1/2×28 or model-specific threads. Confirm thread pitch before buying muzzle devices or suppressor mounts. Gorilla’s .300 BLK suppressor-ready guidance calls out thread and QD systems (ASR, KeyMo). 

• Gas & reliability: Short barrels mean high pressure and back-pressure when suppressed; prefer uppers built/tuned for micro gas systems or with adjustable gas blocks. Gorilla notes the importance of short-barrel gas tuning and suppressor-rated parts in its .300 BLK content. 

• Receiver & handguard: Look for 7075-T6 forged/billet uppers with slim free-float M-LOK or Pic rails to keep weight down and allow light/optic placement. Gorilla’s short-barrel pages outline those construction details and layouts.

Top CQB-oriented Gorilla Machining options (4? class)

Below are the best picks and close alternatives at GorillaMachining.com for a 4? CQB build. Some are 4? directly; others are micro 5? in the same family (a great fallback when a 4? spec is out of stock or limited in a given caliber).

1) 4? .300 Blackout AR Pistol Upper (suppressor-ready concept)

If you want a 4? upper that still hits hard at CQB distances—and plays nicely with subsonic/suppressed use—.300 BLK is the sweet spot. GorillaMachining.com’s dedicated pages call out why short-barrel .300 BLK shines for compact platforms and how to pair it with QD systems (KeyMo, ASR) or direct-thread cans on 5/8×24 barrels. Expect guidance around adjustable gas and reliable cycling with subsonic or mixed loads. This is the #1 CQB pick for extreme compactness + terminal performance. 

Why it’s great for CQB:

• Designed to retain performance in very short barrels

• Excellent with subsonic ammo + suppressor (once legally obtained and rated)

• Easy to keep compact with a micro handguard + mini red dot + 300-specific muzzle device

Watch-outs:
Verify your suppressor’s minimum barrel rating and confirm thread pitch before mounting. Keep in mind the legal/NFA requirements where you live (see Legal Notes below). Gorilla’s .300 BLK content highlights these considerations. 

2) 4? 9mm (AR-9) Upper (and components)

For low recoil, budget-friendly ammo, and straightforward indoor use, 9mm in a 4? format is an outstanding CQB choice. Gorilla offers AR-9 barrels and parts that underpin a compact upper, with site copy emphasizing black nitride finishes and precision manufacturing for reliability. Pair with a micro free-float handguard and a compact red dot for a light, quick CQB tool.

Why it’s great for CQB:

• Soft recoil and quick follow-ups

• Pistol-caliber magazines (often Glock-pattern with the right lower)

• Indoor blast is more manageable than 5.56 in very short barrels

Watch-outs:
Confirm feed geometry (mag block vs. dedicated AR-9 lower) and thread pitch for your chosen muzzle device or comp. Gorilla’s AR-9/9mm pages provide the barrel/upper foundations to build on. 

3) 4? .45 ACP Upper — Gorilla Machining ‘BEAST’ 4? .45 ACP (when available)

If you like big-bore pistol calibers in a tiny package, Gorilla’s AR-15 ‘BEAST’ 4? .45 ACP Pistol Semi-Complete Upper Build is a beastly CQB option when in stock. It’s showcased on their .45 ACP upper category with pricing and availability indicators; it’s a direct nod to micro CQB with a large-caliber, sub-1,000 fps profile that minimizes over-penetration versus rifle rounds (ammo/structure-dependent). If you see it available, it’s a top CQB candidate for pistol-caliber fans. 

Why it’s great for CQB:

• Heavy-hitting .45 ACP performance in an ultra-short format

• Pairs well with subsonic loads for sound/flash management

• Simple optic + light setup keeps it fast and controllable indoors

Watch-outs:
.45 ACP uppers can be heavier up front and demand caliber-specific mags/lowers. Check in-stock status and compatibility details on Gorilla’s product page. 

4) Micro 5? 5.56 NATO Upper (close alternative when 4? 5.56 is impractical)

If your heart is set on 5.56, Gorilla’s AR-15 “THUMPER” 5? 5.56 NATO micro upper is a near-4? option that preserves AR-15 magazine/commonality with a hair more barrel for gas reliability. The listing describes a Mil-spec compliant micro series build suitable for shooters who want the lightest, smallest 5.56 upper possible from the brand. It’s a practical substitute when a true 4? 5.56 is limited or too gassy for your use case. 

Why it’s CQB-friendly:

• Standard AR-15 ecosystem (magazines, bolts, parts)

• Compact, slick-side micro configuration for indoor maneuvering

• Works with common 5.56 comps/flash devices (mind the blast indoors)

Watch-outs:
At micro lengths, blast/concussion is significant; prioritize flash mitigation and hearing protection. Reliability is tightly linked to gas tuning and ammo selection. Gorilla’s “micro upper” listing frames it as a quality, U.S.-made solution. 

Recommended configurations (by use case)

Home defense (quiet, controllable):

• 4? .300 BLK upper + subsonic defensive ammo

• Low-profile direct-thread or QD muzzle device rated for 4? .300 BLK

• Compact 1× red dot + 500–1,000-lumen weapon light

• Adjustable gas (if available) tuned for your load + can, per Gorilla’s suppressor-ready notes. 

Indoor training & PCC-style drills:

• 4? 9mm upper path (using Gorilla AR-9 barrels/components)

• Compact compensator (pistol-caliber)

• Micro dot + light; dedicated AR-9 lower or mag block as required. 

Big-bore CQB fun / subsonic focus:

• ‘BEAST’ 4? .45 ACP (when available)

• Subsonic .45 JHP defense loads; dedicated mags/compatible lower

• Minimalist handguard + light; consider a can rated for .45 ACP (legal considerations apply). 

“Small but 5.56” approach:

• 5? 5.56 “THUMPER” upper

• Flash-forwarder or hybrid device to reduce side blast

• Ammo that cycles reliably in micro gas systems (test thoroughly). 

Build quality, materials, and parts ecosystem

GorillaMachining.com emphasizes precision-machined components, black-nitride barrels in pistol calibers, and 7075-T6 receivers/handguards across many of its short-barrel pages. Expect M-LOK/Pic micro rails, compact slick-side uppers, and options that keep total weight and length down—ideal for CQB. Their AR-9 barrel page and premium upper receiver page highlight the material choices and finishing that matter in hard-use builds. 

Buying tips (so you pick the right 4? upper the first time)

1. Start with the mission: Indoor CQB? .300 BLK or 9mm in 4? makes sense. Want rifle-cal ballistics? Consider the 5? 5.56 micro. 

2. Match your lower: AR-15 lower for .300 BLK/5.56; dedicated AR-9 lower (or proven mag block) for 9mm; .45 ACP may need caliber-specific lowers/mags. Gorilla lists components to support these routes. 

3. Confirm threads & devices: Ensure your muzzle device/suppressor mount matches the thread pitch and caliber bore (e.g., 5/8×24 for .300 BLK). Gorilla’s .300 BLK suppressor-ready page reiterates this. 

4. Plan the front end: Short free-float handguard (enough rail for a light), durable mount, and a rugged micro red dot. Gorilla’s short-barrel product pages outline compact rail setups. 

5. Test your ammo: Micro gas systems are finicky; confirm reliable cycling with your chosen load—especially subsonic .300 BLK or hollow-point 9mm/.45 ACP. Gorilla’s .300 BLK page encourages tuning for your exact load/can combo. 

Legal notes (high-level)

• Pistol configuration: A 4? upper should be paired with an AR pistol lower (no shoulder stock) unless you’ve taken the steps to configure and register an SBR under applicable law.

• Suppressors: If you plan to run suppressed, follow NFA and state laws. Gorilla’s suppressor-ready guidance is technical; legal compliance is your responsibility. 

Availability and close alternatives

Short-barrel SKUs—especially 4?—can go in and out of stock quickly. If a specific upper is sold out, check:

• Another caliber in 4? (e.g., 9mm vs .300 BLK)

• The 5? micro upper in the same family (e.g., the 5? 5.56 “THUMPER”)

• Semi-complete kits (e.g., .45 ACP ‘BEAST’ 4?) and add the last few parts you prefer

Gorilla’s category/landing pages often point to adjacent options or semi-complete builds that get you 90% there with brand-matched parts. 

Quick FAQ 

What’s the best 4? AR pistol upper for CQB from GorillaMachining.com?
For maximum CQB performance, start with a 4? .300 BLK upper concept (suppressor-ready, tuned gas). If you want minimal recoil and cheap training, consider a 4? 9mm path. If you need 5.56, Gorilla’s 5? “THUMPER” is an excellent near-4? alternative. 

Does Gorilla sell a true 4? upper in .45 ACP?
Yes—look for the ‘BEAST’ 4? .45 ACP Pistol Semi-Complete Upper in the .45 ACP category (availability varies). It’s a compelling CQB option for big-bore fans. 

What accessories should I add for CQB?
A durable micro red dot, high-candela weapon light, compact sling, and (where legal) a suppressor or blast-forward device. Use a slim M-LOK rail to keep weight and bulk down; Gorilla’s short-barrel pages show these layouts. 

Any reliability tips for micro uppers?
Test your exact ammo (especially subsonic .300 BLK) and tune gas/buffer as needed. Confirm muzzle device thread pitch and torque. Gorilla’s suppressor-ready .300 BLK page highlights these points. 

The bottom line

If you’re building a CQB-first AR around the 4? form factor, GorillaMachining.com has the calibers and micro configurations to make it work:

• Best all-around CQB: 4? .300 BLK (suppressor-friendly; strong subsonic options)

• Best low-recoil trainer: 4? 9mm (PCC ergonomics with AR handling)

• Best big-bore fun/defense: ‘BEAST’ 4? .45 ACP (when in stock)

• Closest 5.56 alternative: 5? “THUMPER” micro upper (near-4? footprint)

Match the upper to your lower, verify threads and mounts, keep the front end minimal (optic + light), and test/tune with your chosen load. For current availability or semi-complete kits, check the Gorilla Machining product and category pages referenced above.