Building and Feeding One — The Bolt, the Barrel, and the Magazine Problem

A .450 Bushmaster AR-15 is a calibre conversion of an existing rifle, not a different rifle. That is the cartridge’s commercial advantage and the source of every mistake made building one.

I built mine at home on a Black Rain Ordnance lower; the parts list in Volume 1 §1.8 is the worked example throughout. Volume 7 explains how the rifle works part by part and how to tell .450 parts from 5.56 ones; Volume 8 covers stripping, maintenance and repair. This volume is the build.

⚠ Update, 19 October 2026. When this volume was first published (17 September 2026) no maker documentation on gas, buffer or muzzle threads for a .450 AR had been read. Three makers’ own pages have now been read, Bear Creek Arsenal (my barrel’s maker), Tromix and Ruger, and §4.1, §4.4, §4.5, §4.6 and §4.7b now quote them. One earlier statement is reversed: §4.5 said heavier buffers are commonly fitted to big-bore AR conversions; Tromix, the longest-established big-bore AR maker, says its uppers want a standard buffer and spring, and that a heavier one may cause short-stroking.

4.1 What carries over, and what does not

Table 1 — 4.1 What carries over, and what does not

Component	Carries over from a 5.56 AR-15?
Lower receiver	Yes, entirely. No modification of any kind.
Trigger, safety, grip, stock, buffer tube	Yes
Buffer and spring	Yes, standard, per Tromix (§4.5)
Bolt carrier, firing pin, cam pin, firing pin retaining pin	Yes, per Tromix’s bolt listing (§4.2)
Charging handle	Yes
Handguard	Yes, subject to barrel profile and gas block size (§4.4)
Upper receiver	Usually works, but makers enlarge the ejection port (§4.7)
Barrel	No: .450 Bushmaster specific
Bolt (with its extractor and ejector)	No: .473 bolt face, not .378
Magazine and follower	No: single-column follower (§4.6)
Muzzle device	No: it must be bored for a .452 bullet (§4.7b)

Tromix’s own summary for the .458 SOCOM, which uses the same bolt: “The only unique parts to the 458 SOCOM uppers are the barrel, bolt, upper receiver, and muzzle attachment.”¹ Add the magazine follower for the .450 and that is the list.

The lower receiver carrying over completely is the whole point. The lower is the serialised, regulated part; everything above it is ordinary commerce. A .450 Bushmaster upper on an existing lower is the same rifle with a different barrel and bolt, which is why mine sits on the same make of lower as my 5.56 builds.

4.2 The bolt is the non-negotiable part

A 5.56 bolt will not work and cannot be made to work. A 5.56×45 has a .378-inch case head; a .450 Bushmaster has a .473-inch rim. The bolt face must be cut for the larger dimension, and the extractor and ejector must suit it (Volume 7 §7.2 draws the two faces to scale).

Two makers’ listings make the boundary exact:

• Tromix: its bolt “for use in 458 SOCOM, 375 SOCOM, and 450 Bushmaster uppers … Installs in standard 5.56 bolt carrier with standard 5.56 firing pin, cam pin, and firing pin retaining pin. Bolt includes gas rings, extractor, and ejector. Extractor and Ejector are specific to the big bore calibers. They are not 5.56 parts.”²
• Bear Creek Arsenal: its “450 Bushmaster / 458 SOCOM Bolt Assembly” (SKU BA-450-458, $52.99 on 19 October 2026) comprises the bolt, extractor, ejector, extractor pin, ejector roll pin, ejector spring and extractor spring, E9310 steel, parkerized, MPI inspected.³

So the bolt head with its extractor and ejector is the calibre part. The carrier and the small pins around it are 5.56 parts.

My rifle uses a black nitride .450/.458 bolt carrier group with an MPI-marked bolt, as my inventory records it. The record does not name the BCG’s maker, and that is not guessed here.

“.450/.458” is not a compromise. The .458 SOCOM shares the .473 rim, so one bolt serves both cartridges, and the pool of available bolts is larger than a .450-only market would support (§4.7a).

“MPI” means magnetic particle inspection, a non-destructive test for surface and near-surface cracks. On an AR bolt the lugs are the most highly stressed small part in the rifle, and the failure mode is a lug cracking at its root; the Army’s armorer manual tells the inspector to look for cracks “especially around bolt cam pin hole and locking lugs.”⁴ On a big-bore conversion an MPI-marked bolt is worth paying for.

🔴 The bolt is also where both big-bore makers put the blame for feeding trouble. Tromix’s barrel listing: “If you experience feeding problems with your upper, 99% of the time, the issue is due to the customer using a sub-standard bolt from an alternate manufacturer. The construction of the bolt, extractor, and ejector are all extremely important for proper feeding of the 450 Bushmaster cartridge.” Its FAQ, of the .458 SOCOM: “90% of feeding issues … are attributable to an incorrectly machined bolt.”¹⁵ A maker selling bolts has an interest in saying so, and these percentages are its own, not measurements; but it is the one maker in this market that has been building big-bore uppers since 2001, and it points at the bolt rather than the barrel.

4.3 The barrel, and the twist that is already right

My barrel is a Bear Creek Arsenal .450 Bushmaster, 16-inch, 4150 chrome-moly vanadium, parkerized, heavy profile, 1:24 twist, per my inventory.

Bear Creek’s own listing for its current barrel of exactly that description, read on 19 October 2026:⁶

Table 2 — 4.3 The barrel, and the twist that is already right

Item	BCA listing
Name	BC-15 .450 Bushmaster, Parkerized 16” Heavy Barrel, 1:24 Twist, Carbine Length Gas System
SKU	1817-B450CHB16124(P).875
Price	$72.93 (shown “Out of stock”)
Material	4150 Chrome Moly Vanadium
Profile / finish	Heavy Barrel / Parkerized
Twist	1:24
Gas system	Carbine
Gas block size	0.875
Feed ramp	Single
Thread pitch	5/8 x 32
Listed weight	2.5 lb
Includes	”Does not include gas block or gas tube”
Recommended ammunition	”We recommend Hornady Black 250 gr. ammo.”

⚠ That is the maker’s description of the barrel it sells now. It is not proof of what my barrel is. Every attribute in my inventory matches the listing, and the 0.875 gas block and single feed ramp are consistent with it, but the barrel was bought before 19 October 2026 and a maker can change a specification between batches. The thread is the one that matters (§4.7b) and it stays an open question until it is gauged. (The page’s first heading also reads “.450 18” Parkerized Heavy Barrel”, a stale title that contradicts the product name, URL and specification table, which all say 16.)

The 1:24 twist is SAAMI’s reference twist (Volume 2 §2.4) and stabilises every supersonic factory load with a wide margin. It is not, as this volume and Volume 2 said in September, what “the industry has converged on”: Ruger’s whole current .450 line is 1:16, Savage’s is 1:24, and Tromix’s 12.5-inch barrel is 1:20. The difference matters only for the heavy subsonic bullets (Volume 2 §2.4).

4150 chrome-moly vanadium is the ordinary quality steel for AR barrels and is sound at the .450’s 38,500 psi maximum average pressure. Barrel life is not a practical concern for a hunting rifle in this chambering; the price of the ammunition limits round counts long before the throat does.

4.4 The gas system

The mechanism is ordinary. The rifle is a direct-impingement autoloader: gas is tapped from the bore through a port, runs back through a tube into the carrier key, and drives the carrier rearward (Volume 7 §7.1). Whether it cycles correctly depends on port size, port position, barrel length and the cartridge’s pressure curve.

What the makers publish about gas length is now on record, and they agree:

Table 3 — What the makers publish about gas length is now on record, and they agree

Maker	Barrel	Gas system	Gas block
Bear Creek Arsenal	16 in heavy, 1:24	Carbine	0.875 in
Tromix	16 in lightweight, 1:24	Carbine	.750 in at the block
Tromix	18 in lightweight, 1:24	Mid-length	.750 in
Ruger AR-556 MPR (2018)	18⅝ in cold hammer-forged	Carbine	0.875 in

Sources: the makers’ own pages.⁵⁶⁷

Two things follow. A 16-inch .450 barrel is normally a carbine-gas barrel, so a carbine gas tube and a gas block of the barrel’s journal diameter are what an owner of a BCA 16-inch barrel should expect to find; the barrel ships without either.⁶ And the gas block must match the journal: a 0.875 heavy barrel takes a 0.875 block, a Tromix lightweight a .750.

⚠ No maker publishes a gas port diameter, and none is printed here. My inventory does not record the gas block fitted to my rifle.

Tromix on adjustable gas blocks: “You only need an adjustable gas block if you are shooting full power supersonic ammo, suppressed. You don’t need an adjustable gas block for shooting subsonic suppressed, subsonic unsuppressed or supersonic unsuppressed.” For its 12.5-inch barrel it says one is required for supersonic and suppressed use.¹⁵ That is advice about Tromix’s own port sizes, but it is the clearest published statement of when an adjustable block earns its place.

What can be said without a source is diagnostic:

• Short-stroking (the carrier not travelling far enough to pick up the next round, or the bolt not locking back on an empty magazine) indicates undergassing, or too heavy a buffer (§4.5).
• Brass thrown violently, with heavy recoil impulse, indicates overgassing. Note that a .450/.458 ejecting forward is not by itself a symptom (§4.7).
• Both are tuned at the gas block or the buffer, one variable at a time.

4.5 The buffer

The September version of this section said heavier buffers are commonly fitted to big-bore AR conversions and recommended treating buffer weight as one of two tuning variables. That was reasoning, not a source, and the one maker who addresses it says otherwise. Tromix’s FAQ:¹

“Q: Do I need a heavier buffer or spring in my lower? A: No. The gas port is sized to blow the bolt carrier completely rearward with a standard GI buffer and spring. Adding a heavier buffer or spring may cause the bolt to short stroke.”

and, on the extractor:

“Q: Do I need a heavier extractor spring or insert? A: No. Too much extractor tension will cause pieces of brass to be chipped off the rim of the case.”

The maker’s position, then, is that a correctly ported big-bore barrel runs on the standard carbine buffer and spring, and that the 5.56 owner’s instinct to add a heavy buffer and an extractor-boost kit is wrong for this bolt. Tromix’s FAQ also advises against “goofy/Gucci 5.56 upgrade parts” on its uppers, which it says are “designed and built to run with standard GI type parts.”

⚠ That statement is about Tromix’s gas ports. Bear Creek’s listings say nothing about buffers. A rifle with a different barrel can behave differently, and my inventory does not record the buffer fitted to my rifle. The practical rule is the same either way: start with the standard carbine buffer and spring, and change one thing at a time.

4.6 The magazine problem, in full

4.6.1 Why standard AR magazines need a different follower

A 5.56 AR-15 magazine takes two staggered columns and delivers them alternately into a single feed path. The follower is shaped to do that: asymmetric, presenting rounds from the left and right columns in turn.

A .450 Bushmaster case is .500 inch across the body and cannot stagger. The rounds stack in one vertical column, centred, and a double-column follower presents that column at the wrong angle and with the wrong support. The results are the classic .450 feeding complaints: rounds nosediving into the front of the magazine well, rounds tipping, the bolt riding over the top round, and failures to lock back. The handloading side of the same problem, and the overall-length limit the magazine sets, is in Loading the .450 Bushmaster, Volume 4.

4.6.2 My configuration

My rifle uses Lancer four-round magazines fitted with Tromix followers, and my inventory marks the followers “must use.” That note is mine, from building the rifle, and it agrees with everything the geometry predicts. The four-round capacity is also from my inventory.

⚠ Neither maker could be confirmed on this point. Lancer’s site still refuses the shared fetch engine (HTTP 403, re-tested once on 19 October 2026), so no Lancer specification is quoted. And no magazine follower appears anywhere in the Tromix shop as read on 19 October 2026 (five pages of listings); whether Tromix still sells the follower in my magazines is not known.

Two published magazine warnings, both from Tromix and both written about the .458 SOCOM, which feeds a same-diameter column through the same magazine bodies:¹

• “All 5.56 magazines will work with the .458 SOCOM other than Magpul P-Mags. Those have a center divider in the front that can block the bullet path.”
• “It appears that Lancer has inadvertently made a change to the feed lips on magazines date coded 2021 and newer. 2020 and earlier magazines feed fine.”

The second bears directly on my rifle, which runs Lancer magazines. Whether mine predate 2021 is not recorded (owner question 5 in this dive’s CLAUDE.md). Neither warning was written about the .450, and neither is a measurement; they are the maker’s field experience, quoted as such.

4.6.3 Capacity, and the Michigan rule

Magazine bodies yield roughly:

Table 4 — Magazine bodies yield roughly

AR magazine body	.450 Bushmaster rounds
10-round	4
20-round	5 to 7
30-round	9

These are the figures in general circulation, repeated in Loading the .450 Bushmaster; none was measured for this dive except the four-round figure for my own magazines.

🔴 Volume 3 §3.4 is the reason to care. Michigan’s order prohibits taking an animal with, or possessing in an area frequented by animals, a semi-automatic rifle “capable of holding more than six shells at one time in the magazine and barrel combined.” A four-round magazine plus a chambered round is five: compliant. A nine-round magazine plus a chambered round is ten, and the test is capacity, not what is loaded.

The four-round magazine is therefore the right magazine for this rifle’s actual job.

4.7 Feeding and reliability

The ejection port. A .450 case is wider and shorter than a 5.56 case, and has to leave through the same window. Ruger described its 2018 .450 MPR’s upper as having “a large window for easy ejection”; Tromix sells a “Big Bore” upper with the “Ejection port enlarged” and a billet version whose port is “.665″ wide.” Tromix’s FAQ explains why the shape matters as well as the size:¹⁷⁸

“Many manufacturers of large bore upper receivers only raise the ejection port at the front half of the opening, and leave the rear portion in the same position as a 5.56 upper. This can cause random failures to eject since the casing is usually ejected at the rear portion of the port.”

My rifle’s upper is an Aero, per my inventory; whether it is a standard 5.56 upper or one with an enlarged port is not recorded. Volume 7 §7.4 shows the two ports side by side.

Where the brass goes. Tromix: a .458 case ejecting forward, around 1 to 2 o’clock, “is not indicative of a problem”, because “the .458 casing has a rebated rim which causes the bottom of the case body to strike the nose of the extractor as it rolls out.”¹ The .450 has the same rebated rim in the same bolt, so the same forward pattern is to be expected; a 5.56 owner reading ejection angles should not treat it as overgassing on its own.

Feed ramps. Bear Creek lists its 16-inch barrel’s feed ramp as “Single”.⁶ Tromix’s upper listing adds a builder’s tip: “To enhance feeding reliability with blunt nose bullets, it is recommended to blend the barrel feed ramp into the upper receiver after barrel installation … (Warning: DO NOT Modify the steel locking lugs on barrel extension.)”⁸ That is the maker’s tip for its own receiver; anyone acting on it is removing aluminium from the receiver, permanently, and the warning about the extension is not optional.

The round is short and fat and has almost no neck, which makes bullet setback a real concern: a round chambered repeatedly can have its bullet pushed deeper, raising pressure. Rounds that have been chambered and extracted repeatedly should be retired from the magazine, not cycled indefinitely (Loading the .450 Bushmaster, Volume 4 treats setback and crimp in full).

It headspaces on the case mouth (Volume 2 §2.2). Trim length is a headspace dimension, and a heavy roll crimp deforms the datum surface. The method for measuring headspace rather than inferring it from gauges is in the Headspace dive.

4.8 4.7a The parts pool, and why it is bigger than it looks

The .458 SOCOM shares the .450 Bushmaster’s .473-inch rim. That is why bolts are catalogued “.450/.458”, and why Bear Creek’s .450 BCG is “compatible with both all Rear Charging 450 Bushmaster and .458 SOCOM uppers” and Tromix’s bolt serves “.458 SOCOM, 375 SOCOM, and 450 Bushmaster.”²³ Two cartridges from different designers draw on one bolt market.

The .50 Beowulf does not share it. Its rim is .445, matched to the 6.5mm Grendel, and it needs a different bolt.

And the .350 Legend, the other straight-wall AR cartridge, uses the 5.56 bolt. Bear Creek sells one BCG “compatible with Rear Charging 5.56, 223, 300 Blackout, and 350 Legend” uppers.⁹ That matters for identification (Volume 7 §7.5): a “.350 Legend” upper and a “.450” upper look alike from the outside and take different bolts.

Everything below the upper receiver is an AR-15 parts market of essentially unlimited depth.

4.9 4.7b Muzzle devices, threads and suppressors

⚠ Update, 19 October 2026. This section published no thread specification in September, because none had been read from a maker. Three makers now have been, and they publish three different threads:

Table 5 — ⚠ Update, 19 October 2026. This section published no thread specification in September, because none had been read from a maker. Three makers now have been, and they publish three different threads

Maker	.450 barrel	Muzzle thread (maker’s own page)
Bear Creek Arsenal	BC-15 16 in parkerized heavy, 1:24	5/8 x 32
Tromix	16 in and 18 in lightweight	5/8-24
Ruger (bolt guns; 2018 MPR brake)	American Gen II, Hawkeye Hunter; AR-556 MPR	11/16”-24
Savage (bolt guns)	110 Trail Hunter Lite V2, Ridge Hunter, Scout V2	11/16x24

Sources: the makers’ pages, read 19 October 2026 (Ruger and Savage via the cartridge dive’s Volume 8).⁵⁶⁷

🔴 That table is the argument for gauging, not a substitute for it. Bear Creek’s listing for the barrel that matches mine says 5/8 x 32, and the cartridge dive found the same figure on a retailer’s listing. My barrel has not been gauged and the maker’s listing describes its current product, not mine. The thread on my barrel is recorded here as unconfirmed (owner question 3).

🔴 A second trap sits inside the first. A 5/8-inch thread is the size a great many .30-calibre muzzle devices are made for. A device that screws onto a 5/8 x 32 thread is not thereby bored for a .452-inch bullet. A .30-calibre brake or flash hider on a .450 barrel is a bore strike on the first shot. A muzzle device or suppressor for this rifle must match the thread and be rated for a .45-calibre or larger bore, and both must be confirmed before it is fired.

On suppressors generally, three of this hub’s dives own the subject:

• How Suppressors Work for the mechanism.
• Suppressor Law for the NFA framework and, in its Volume 7, Michigan’s position.
• Suppressors by Caliber for how a given bore rates.

Tromix’s advice that an adjustable gas block is needed for supersonic suppressed use (§4.4) is the build consequence: a suppressor raises back-pressure and gas flow, and a port sized to cycle unsuppressed will overgas suppressed.

4.10 4.7c Troubleshooting

Change one thing at a time. Volume 8 §8.5 sets out the diagnosis in more detail; this is the build-stage short form.

Table 6 — 4.7c Troubleshooting

Symptom	First suspect	Then
Failure to feed, round nosed into the ramp	Magazine follower (§4.6)	Magazine (Lancer date code; not a P-Mag), then the bolt (§4.2)
Round tips or jams sideways	Follower or feed lips	Magazine body
Bolt does not lock back on empty	Follower not lifting; undergassed	Then buffer too heavy (§4.5)
Random failure to eject	Ejection port too small at the rear (§4.7)	Extractor, chamber
Brass thrown hard, harsh impulse	Overgassed	Gas block; suppressed supersonic needs an adjustable block
Brass lands forward, 1–2 o’clock	Normal for a rebated rim (§4.7)	Nothing, if function is otherwise good
Chips off the case rim	Too much extractor tension (§4.5)	Remove any extractor-boost insert
Bolt will not close on a handload	Case too long: mouth headspacing	Trim length, then crimp
Groups open up suddenly	Loose optic mount or barrel nut	Torque check before anything else

⭐ Three rows are specific to this cartridge, and all three trace to the rebated rim and the mouth datum: forward ejection, chipped rims and long cases. None has an equivalent on a 5.56 build.

4.11 4.7d First range trip

A deliberate first outing is worth more on a big-bore build than on a 5.56, because the failure modes are less familiar and the ammunition costs $2.00 to $3.95 a round on the makers’ own list prices in October 2026 (Volume 6 §6.3).

1. Check torque on everything before firing: barrel nut, gas block, optic mount, stock.
2. Verify headspace, or have it verified. Tromix sells a cartridge case gauge for its calibres and specifies that a correct round drops “BELOW flush” with the top of the gauge;¹ for the .450, a case gauge cut to SAAMI’s 1.700–1.710 mouth datum serves the same purpose. See the Headspace dive.
3. Single-load the first rounds to separate a chambering problem from a feeding problem.
4. Then feed from the magazine, one round, then a full magazine.
5. Watch the brass: where it lands and how hard it is thrown, remembering that forward is normal (§4.7).
6. Inspect fired cases for mouth damage, chipped rims and extractor marks.
7. Only then zero (Volume 5 §5.10).

4.12 What a builder must check

In order of consequence:

1. The bolt is a .450/.458 bolt, with its own extractor and ejector. Not a 5.56 bolt, and not a .350 Legend bolt (which is a 5.56 bolt).
2. Headspace. A mouth-headspacing cartridge in a self-loading action; check it.
3. The magazine has a single-column follower, and it is not a P-Mag.
4. The magazine capacity suits the use: five in the rifle for Michigan deer hunting.
5. The muzzle thread is gauged and the device is bored for .45 calibre before anything is screwed on.
6. The ejection port clears the case, rear half included.
7. Start with a standard buffer and spring; change gas or buffer, not both.
8. Function-fire before hunting, across the full magazine, from a cold rifle, with the hunting ammunition.
9. Chronograph it. My barrel is 16 inches, shorter than every barrel the published velocities come from (Volume 5 §5.5).

4.13 References (Vol 4)

Footnotes

1. Tromix Corporation, “FAQs”, tromix.com, read 19 October 2026. Questions on unique parts, buffer and spring, extractor spring, adjustable gas block, magazines (P-Mag divider; Lancer date codes 2021 and newer), feeding, failures to eject, ejection pattern, and case gauge. Written about the .458 SOCOM. URL: https://tromix.com/faqs/. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸

2. Tromix Corporation, “Bolt – 458 SOCOM, 375 SOCOM, 450 Bushmaster”, read 19 October 2026. URL: https://tromix.com/product/bolt-458-socom-375-socom-450-bushmaster/. ↩ ↩²

3. Bear Creek Arsenal, “BCA 450 Bushmaster / 458 SOCOM Bolt Assembly” (SKU BA-450-458) and “BCA .458 SOCOM / .450 Bushmaster Bolt Carrier Group” (SKU BCG-450-458, $74.99), read 19 October 2026. URLs: https://www.bearcreekarsenal.com/bca-450-458-bolt-assembly.html; https://www.bearcreekarsenal.com/bca-458-socom-450-bushmaster-bolt-carrier-group.html. ↩ ↩²

4. Departments of the Army and Air Force, TM 9-1005-319-23&P, Unit and Direct Support Maintenance Manual, Rifle, 5.56mm, M16A2, May 1991 with Change 5 (9 April 1997), §2-14 Bolt assembly, inspection. Approved for public release. Read from the Internet Archive copy (item milmanual-tm-9-1005-319-23-m16a2-maintenance—repair-manual), 19 October 2026. ↩

5. Tromix Corporation, “.450 Bushmaster Barrel Only – Lightweight”, read 19 October 2026: 12.5, 16 and 18 inch barrels, gas lengths, 5/8-24 thread, twists, profile; and the feeding statement quoted in §4.2. URL: https://tromix.com/product/450-bushmaster-barrel-only/. ↩ ↩² ↩³ ↩⁴

6. Bear Creek Arsenal, “BC-15 .450 Bushmaster, Parkerized 16” Heavy Barrel, 1:24 Twist, Carbine Length Gas System”, SKU 1817-B450CHB16124(P).875, read 19 October 2026. URL: https://www.bearcreekarsenal.com/450-bushmaster-16-parkerized-hb-1-24-twist-carbine.html. ↩ ↩² ↩³ ↩⁴ ↩⁵

7. Sturm, Ruger & Co., news release “AR-556 MPR Now Chambered in .450 Bushmaster”, 26 September 2018, read 19 October 2026. No current Ruger .450 AR was found on ruger.com in October 2026 (cartridge dive Volume 8). URL: https://ruger.com/news/2018-09-26a.html. ↩ ↩² ↩³

8. Tromix Corporation, “M4 Style Standard Big Bore Upper Receiver – Right Handed” ($149.00) and “Tromix Big Bore Billet Upper Receiver 7075”, read 19 October 2026. URLs: https://tromix.com/product/upper-receiver/; https://tromix.com/product/big-bore-billet-upper-receiver/. ↩ ↩²

9. Bear Creek Arsenal, “BC-15 .223/5.56 NATO/.300 Blackout/.350 Legend Rear Charging AR-15 Bolt Carrier Group” (SKU BCG-15, $68.99), read 19 October 2026. URL: https://www.bearcreekarsenal.com/bca-223-5-56-nato-300-blackout-bolt-carrier-group.html. ↩