THE THIRD AND FINAL RIFLE in our Range 365 Ultimate AR Builder’s guide proved the most difficult—notably due to our lack of experience with big bore firearms. If you want to build a .450 Bushmaster, .458 SOCOM, or any other thumper of an AR, learn from our mistakes.

None other than Col. Jeff Cooper popularized the “thumper” concept in the early 1980s. That is, a large bore semi-automatic rifle of .44 caliber or greater capable of one-shot kills on big animals to 250 yards.

Tim LeGendre of LeMag Firearms delivered with a round he called the .45 Professional, that was later licensed to the gunmaker Bushmaster and further improved upon by Hornady. Thus the .450 Bushmaster cartridge was born.

450 bushmaster ammunition
(top) Three popular 450 Bushmaster hunting loads, from left to right: the Hornady 250-grain FTX load in Black and Custom Shop packaging; the Hornady American Whitetail 245-grain InterLock, and the Remington Premier 260-grain AccuTip. (bottom) A 450 Bushmaster cartridge and a group from the finished rifle shot at 100 yards. Michael R. Shea

The round has experienced a recent resurgence in popularity as a hunting load. Many states have changed their game laws to allow the use of rifles chambered for straight-walled rifle cartridges, like the .450, during primitive weapons or muzzleloader seasons.

That makes a very not-primitive AR-15 chambered in .450 Bushmaster legal for deer in many places. Compare that to a slow-to-load smoke pole and it’s easy to see why hunters have rushed toward straight-walled cartridges in AR-15s.

We set out to build just that: a .450 Bushmaster hunting rig, light, fast, and accurate—perfect for hogs in the southeast or an old fashioned deer drive in the upper Midwest.

Big bore ARs present a unique set of challenges. Here’s what we learned, and what you should know if you plan to build one.

450 bushmaster rifle
Cerakote pro Nick Kesserling inspects our 450 Bushmaster build at Valley Armament in Sayre, Penn. Soon after Nick took the rifle to his shop, broke it down, and turned it into a work of functional art. Michael R. Shea


  • Orientation. For our purposes, “left” and “right” refer to sides of the rifle from the shooters perspective when the rifle is pointed forward.
  • Work Surface. Make sure you have a clean well-lit area to work. Try our Yoga Mat trick. A drop cloth under your work area saves time hunting for dropped roll pins, too.
  • Tools. It doesn’t take tons of tools to build an AR, but these tools are worth the investment.
  • Safety. Eye protection is highly recommended. Springs especially like to fly.


Lower Receiver Bushmaster A2 Lower
Extra Parts Bushmaster Lost Parts Kit
Pistol Grip Luth-AR Chubby Grip
Selector Radian Talon Ambidextrous Safety Selection 2-Lever Kit
Bolt Catch Luth-AR Paddle Oversize Bolt Catch
Magazine Release Odin Works XMR 3 Extended Magazine Release
Trigger Timney Triggers AR-15 Competition Trigger
Buffer Tube Bushmaster
Buffer Bushmaster
Buttstock Luth-AR MBA-2 w/ .223 Buffer Assembly
Upper Receiver Bushmaster 450 Bushmaster Flat Top
Forward Assist Radian Forward Assist Aluminum Black Anodized
Barrel 16″ Tromix .450 Bushmaster Barrel & Bolt Combo
Gas Block Yankee Hill Machine Low Profile Gas Block
Gas Tube Tromix Gas Tube
Muzzle Device Tromix Linear Compensator
Bolt Tromix .450 Bushmaster Barrel & Bolt Combo
Bolt Carrier Group Bushmaster Complete BCG
Charging Handle Radian Raptor Ambidextrous
Forends Omega Mirage 16.5-inch Carbon Fiber M-LOK
Sights FAB Defense FBS & RBS Iron Sights
Engraving & Cerakote FSG Customs
Accessories Lancer Magazine, 4 Round w/ Tromix Aluminum Center-feed Follower


450 bushmaster rifle parts
Where we started and where we ended. A few of our original parts were swapped out for better big bore options. Michael R. Shea

The .450 Bushmaster cartridge works with AR-15 patterned parts, so long as the barrel, bolt, and magazines are tailored to the round.

The build procedure for this .450 Bushmaster Hunter was identical to the .223 Wylde video build, which can be found here.


Barrel & Buffer

When building a 450 Bushmaster, ask your barrel manufacturer for a gas block and buffer recommendations. Our 16″ Tromix barrel worked best with a non-adjustable gas block and a stock buffer and spring. At the top is the KynSHOT Hydraulic Buffer with JP Spring we initially used, and a stock buffer spring and buffer below it. KynSHOT and Aero Precision

For this build, we went with country’s best Big Bore specialist for barrel and bolt, Tony Rumore of Tromix Lead Delivery Systems, creator of the .458 SOCOM.

Our first range trip with the rifle highlighted a few problems. As far as accuracy, the gun was hitting all over the place at 50 yards with a couple different types of ammo, which we thought was the main problem.

It turned out that the scope we’d initially mounted was a bit too diminutive and the internals were damaged beyond repair by the .450’s stout recoil. That was the first problem. We decided to go with a good set of adjustable iron sights for the time being and used a beefier scope and mounts for our bench test once the rifle was running.

Additionally, our rifle would not cycle at all. Our first culprit to blame for this issue was the adjustable gas block—and no amount of adjusting did anything to get the rifle going.

We swapped the adjustable gas block for a simple, wide-open Yankee Hill block.

However, it didn’t solve our cycling problem. The gun was still effectively a single shot rifle.

When we finally reached out to Tony, the first thing he asked about was the buffer we were using along with the 16-inch Tromix barrel.

The gas port on Tromix barrels are drilled for a standard AR-15 buffer and spring, not the more expensive hydraulic buffer assembly we had originally installed on the gun. So we removed it and went with a stock buffer and spring. The rifle has cycled perfectly ever since.

Handguard & Barrel Nut

ar-15 handguard
Our original pressure-fit handguard from Luth-AR (top) walked off the barrel nut under the heavy 450 Bushmaster recoil. We switched it out for an inexpensive Omega Manufacturing “Mirage 15” carbon fiber M-Lok handguard that attached directly to the barrel nut. Michael R. Shea + Omega Mfg.

The handguard on our original build installed by pressure fitting to a knurled barrel nut. That is, screws under the handguard squeeze the handguard to the nut, rather than a direct tie-in. The 450 Bushmaster recoil was too much for this kind of attachment, and the forend walked right off the nut after half a dozen shots.

We replaced it with a light, inexpensive carbon fiber handguard from Omega, with a direct tie-in with the forend screwing right into the barrel nut via a set of Torx screws. It’s not going anywhere. Problem solved.


450 Bushmaster magazines
The Tromix 450 Bushmaster magazines with the company’s center-feed follower solved all our feeding issues with the rifle. Tromix

We used a matched Tromix bolt for our Tromix barrel, and even though we solved our cycling problem, we still had some feed issues. The problem was the magazines we were using.

The .450 Bushmaster was intended to fit in a standard .223 double-stack STANAG magazines and effectively turn them into a single stack mag, and this will work in many rifle builds. However, there have always been feed issues because of the double-stack follower, especially for the last couple rounds.

Also, the .450 BM doesn’t work with most polymer AR magazines, like the Magpul P-Mag, and what we had on hand couldn’t offer the kind of reliable feeding we were looking for in our hunting rifle.

When we switched to Tromix Lancer mags with the company’s center-feed followers, that problem went away. The gun has run flawlessly since.

The Tromix mags and followers are specifically designed for the .450 Bushmaster round and keep the cartridges lined up correctly from the first round in the mag to the last.


450 bushmaster grouping
The business end of a 450 Bushmaster load is a heavy .452-inch bullet. Common weights run around 250- to 300-grains.
Our 450 Bushmaster Hunter build printed its best 100 yard groups, by far, with Hornady Black 250-grain FTX ammunition. Michael R. Shea

We set the bar for this rifle at 1-inch at 100 yards—a tall order for an AR, let alone a big bore, heavy bullet caliber like the .450 BM.

We shot every factory .450 Bushmaster load on the market and were roundly disappointed, except for one: the Hornady Black 250-grain FTX. (This load is also packaged as Hornady Custom 250-grain FTX.)

In our rifle, the 250-grain Hornady’s did what others could not, and brought MOA performance to our little thumper. To achieve this, we shot off sandbags on a concrete bench with a Nikon Black FX1000 6-24×50 optic in a Nikon Black mount on top of the rifle.

450 Bushmaster Hunter rifle
The finished 450 Bushmaster Hunter rifle on the firing line. Cosmo Genova

So, if you’re planning a big bore AR build, remember:

  • Get a quality barrel and matched bolt
  • Start with a stock AR-15 buffer• Skip the adjustable gas block
  • Make sure your handguard attaches directly to the barrel nut
  • Get caliber-specific magazines and followers if possible
  • When looking for the best performing .450 BM factory ammo for your gun, start with Hornady
tiger stripe cerakote finish
The final touch to tie the whole rifle together was an extraordinary tiger-stripe camo Cerakote finish done by Nick Kesselring at Freedom Sporting Goods in Sayre, PA. Cosmo Genova
450 Bushmaster Hunter rifle
Another shot of the 450 Bushmaster Hunter blending in nicely with its natural surroundings Cosmo Genova

For the rest of our AR-Builds, Step-by-Step Video and Photo instructions, plus a list of Necessary Tools, go here