In my ‘Building a .450 Bushmaster AR-15 Hunting Rifle’ article, I built a very lightweight and powerful hunting rifle with the help of Brownell’s. This rifle had some issues with feeding and ejection, and I was determined to solve its problems.
These issues centered around the magazines.
In this article, I will be talking about tips and tricks to ensure reliable feeding with the .450 cartridge in an AR rifle.
The main issue with the .450 is that it is an inferior cartridge for the AR rifle. It is an oddball in that it has a .308 case head which is actually rebated, meaning that the case body is wider than the rim and has a case body that is tapered. The problems associated with the AR rifle and its magazines are well known, and the .450 cartridge cruelly exacerbates them.
How did I figure out that the magazines were causing the problems in the .450 Bushmaster AR?
The gun fired single rounds and locked back on an empty mag, which meant that the reliability issues were being caused somewhere in the mag and how it interacted with the gun.
When I finished the rifle, I was expecting it to be a walk in the park when it came to range time. Instead, I experienced problem after problem with standard GI-style magazines, even Bushmaster brand mags with special followers. The main issue was hard to determine. The .450 case has to single-stack feed from the mag because it is too wide to feed double-stack. This alone shouldn’t be a problem, but it found a way to be.
The central issue regarding feeding is that the .450 is wider at the base than the case mouth. The case is, therefore, not equally supported along its length in an AR magazine. The upward pressure from the magazine spring can cause the cartridges to tilt point-up, which in turn can cause failure to eject. The bullet tip can get lodged behind the bolt lugs, thus causing the fired case to be stuck or the next bullet to be damaged. Double feeds are also a common problem, as the magazine lips often fail to retain the cartridges. The more rounds in a mag, the worse these problems got.
Why not use a commonly available mag like the Magpul PMAG? Well, for starters, the .450 BM won’t work in a PMAG due to the central rib that runs down the inside of the magazine body. In my research on this topic, I found that some people successfully modified a PMAG to work, but it was sketchy at best and not a solution that I could present to you to buy.
I decided that I would intentionally limit my capacity to what is allowed for hunting in Michigan, where I will be using the rifle as a way to research reliability in the .450. There was so little information out there about reliably feeding these big-bore rounds that I had to contact my friend Tony Rumore at Tromix Lead Delivery Systems for some much-needed advice.
Tony is a leader in the industry regarding anything above .223 in AR rifles. Tromix is a great, innovative company that makes some elegant stuff for the discerning shooter. My talk with Tony revealed that they were also aware of the issues associated with the .450 in an AR mag, and they had some unique solutions.
In the course of my conversation, Tony explained to me that they had been puzzling over the .450 problem for some time and had developed a modification of a particular magazine that ensured reliability. Tony sent me two special, single-feed followers for Lancer L5AWM magazines and an example of a modified mag. They are made of machined billet aluminum and are extremely well made. I contacted my friends at Brownell’s to see about getting some more Lancer mags to modify and mess with. These products can be seen under these numbers at Brownell’s: 100-022-876WB, 100-022-879WB.
Why go with 10 round mags instead of bigger ones?
As stated, this will be for hunting, and in Michigan, I can’t have more than a 5-round magazine in a semiautomatic. The Lancer mags hold four rounds plus one in the chamber. This is plenty good for hunting and is legal in my woods. I received the mags and followers and went about testing them. Unlike the PMAG and GI mags, the Lancer has steel feed lips and a polymer body. It combines the best of many worlds and has no internal rib like the PMAG, so there isn’t much to modify.
I installed the Tromix followers into two mags and left the stock follower in the other two. Tony told me that the stock Lancer mags would reliably feed .450, and he was right. The only issue I noticed was that I had the same tip-up issue as with GI mags due to the shape of the follower. I had no serious issues with feeding or ejection with stock mags, but I noticed that the bolt carrier moved slowly compared to the modified mags. This has to do with the way the cartridges sit in the magazine with the dedicated single-feed follower. They lay much flatter across the top of the magazine, which I noticed allows the bolt to move more freely.
Live Inventory Price Checker
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Lancer Systems L5AWM Magazine Translucent Smoke 5.56 / .223 Rem 10-Rounds | GrabAGun | $ 17.49 |
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Lancer Systems L5AWM Magazine Flat Dark Earth 5.56 10-Rounds | GrabAGun | $ 16.79 |
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Lancer Systems L5AWM AR-15 Magazine 5.56 / .223 Rem. 10-Rounds | GrabAGun | $ 15.39 |
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Lancer Systems L5awm Gen 2 5.56x45 Nato Rifle Magazine | Brownells.com | $ 17.99 |
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Lancer Systems L5AWM 30-Round AR-15 Magazine - 5.56 NATO - Black | Primary Arms | $ 13.99 |
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I noted at this time that there are significant differences in case taper across brands of ammo that went unnoticed earlier. This can lead to problems with unmodified Lancer mags and, as you can see in the photo below, even lead to ‘tipping up’. The ‘tipping up’ problem goes away instantly when the new followers are installed. I will be talking more about case taper and individual ammo types in my next article.
Feeding with the Tromix followers resulted in 100% reliability.
I tested the mags with just the follower installed and a Tromix recommended cutout in the front of the magazine. Tony told me that this helps ensure the case mouth doesn’t get caught during feeding. I had no problems with either version, modified or not. I’m sure that I’ll eventually modify all my mags to have this little cutout, as it doesn’t hurt anything and can only help. This can be done with a Dremel tool in just a couple seconds.
It took me a while to get my Brownell’s .450 Bushmaster rifle running reliably. The magazine is the biggest problem with the .450 in a semiautomatic. It has no issues in a bolt action magazine and obviously no issue in a single-shot rifle, but it is frustrating that there is no industry-wide solution to the semiautomatic problem. Tromix is a company that is quick to identify and fix problems, and their mag followers are a must-have for the .450 AR rifle.
If you think I got out of the woods with this rifle when I fixed the magazine situation and got it running well, you’d be wrong. Stay tuned for my adventures with .450 Bushmaster ammunition, a very cool Ruger No. 1 rifle, and my thoughts on handloading this cartridge.
Visit www.tromix.com to take a look at their must-have followers and other interesting products. The 10-round Lancer mags can be found at www.brownells.com.
About Josh Wayner
Josh Wayner has been writing in the gun industry for five years. He is an active competition shooter with 14 medals from Camp Perry. In addition to firearms-related work, Josh enjoys working with animals and researching conservation projects in his home state of Michigan.
I had extraction issues with mine from day 1 even with factory Bushmaster mag. I fixed my extraction issues about 10 years ago, not by using different followers or mags, but by finishing the manufacturers job. They should have cleaned up the lips of the extractor. During manufacturing of the extractor, while cutting the lip on the extractor that holds onto the edge of the casing, they left little burrs on the outer ede of the extractor lips that need cleaned up in order for it to properly “let go” of the casing upon extraction. If you take your bolt… Read more »
What kinds of rifles do you guys have? I bought a Windham “Thumper” .450 BM and I have only used the mags it came with but I have had absolutely zero issues with it! I even used my Dremel to cut the head off of the rivets that prevented the follower from moving too far down the magazine, which was preventing them from holding more than 6 rounds……after removing the rivets the mags now hold 8 rounds each and I still haven’t had a single issue! Also, the bullet tips just barely sit higher than the rear, they sit damn… Read more »
I read the entire build story and don’t understand why you had so many problems. I built my 450BM with Palmetto State lower, and J5 Tactical complete upper(this included the bolt). Uppers were hard to get then – 2020 – and had to try a number of different retailers, but it works perfectly. Think you just bought from the wrong company. Only problem is finding who has 10 Round mags for this. Lancer is supposed to make them but nobody seems to have them. Lancer L5 4 rounders work fine!
I researched what big bore I wanted not only by availability but also what quirks were involved in building one. Full Disclosure: I researched it for over ten years – building a 6.8 SPC first, and paying attention to reports on the larger cartridges. One of the things I was read was how the .458 case body was set up to work with original GI mags with no issues – and that is the track record over the years. Apparently the .450 didn’t get as much scrutiny. There are other cartridges which suffered the same, and 6.8SPC was one –… Read more »
I bought a Tromix mag in 2019, and it was a complete waste of time and money. It auto-ejected all but the last round if I tilted or bumped the mag, and sometimes without moving the mag at all rounds would fly all over the place. I advise everyone to stay away from Tromix mags. I’ve had great experiences with surplus GI mags.
Mike bought pre-production prototype mags that worked fine when they were tested in the winter time, but when it got warm, the feed lips spread apart. Those were on Tapco mag bodies. Only a handful were sold and everyone was refunded, and those magazines never went to production. The magazines Tromix sells now are not the same as the few prototypes Mike was talking about.
Thank you for your article and diligence I had the same problem and you were right on the money. No my 450 Bushmaster runs great fun to shoot as a little kick to it but great for Michigan. I couldn’t find anyone that had a lancer follower so I had to buy the whole magazine which was 50 bucks but I needed it in 5 days so it was my own fault but it worked great!! No more misss feeds or stove pipes just nice smooth operation I had to touch up the ramp a little but after that it… Read more »
someone designed a single-shot follower for shooting the slow fire stages of NRA Highpower matches and It was adopted for use as the .450 caliber follower until they began producing one of their own. You can use a magazine with this follower either as a multi-round .450 magazine, or as a single-shot magazine in a .223 rifle. It locks the bolt back just like a standard magazine. It works well in most any of the good quality aluminum and most steel magazines. He sells them on ebay for less than 10 bucks and works perfect, you can switch them back… Read more »
I have a factory Bushmaster 5 round magazines. I have read on several sites that the tensions, and the fact the lips prevent the round from tipping up just slightly are the main cause for the failure to feeds. I just took my mag apart and low and behold, I found an ASC .223 follower attached to the end of the spring on the bottom. I took it off. Opened up the feed lips ever so slightly and will go shoot the rifle tomorrow or Saturday but the rounds definitely SEEM to eject from the mag much better without all… Read more »
I think I have purchased and tried about every mag and follower there is available. Several have “worked” but scarring of brass was great and I load and found this unacceptable for me and focused on the 10 round PMAG’s. Removing rib in front wall and running a .500″ ball mill down follower and radius cut installed on front edge has 100% three rounds for me in several platforms including a SOCOM. You can push four and load and have three below but this is a hunting rifle, no fire fights anticipated. Two more in pockets and one would think… Read more »
Tengo problema en ocasiones con mi rifle .450 bushmaster con el magazine
Your feed issues could have been easily resolved by just squeezing the front of the feed lips in the bushmaster mag to make the round stay more level. This can be done with Lancer mags also, since they have metal feed lips. Single feed in a semi-auto makes little sense in a hunting situation.
I have a 50 Beowulf, not too different from a 450 Bushmaster. My experience is the exact opposite. Bullet tip up is not a bad thing to overcome. Bullet tip up is mandatory to get it to chamber. Without bullet tip up the bullet just T-bones the barrel extension. I bend the metal feed lips out to increase bullet tip up.