.450 Bushmaster Magazine Problems – The Tromix Solution

Gun Gear

.450 Bushmaster Magazine Problems – The Tromix Solution

GRAND RAPIDS, MI USA-( In my last 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 the problems it had. 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 we have with the .450 is that it is a very poor 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 they are cruelly exacerbated by the .450 cartridge. How did I figure out that the magazines were causing the problems? 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 there were some people that successfully modified a PMAG to work, but it was sketchy at best and not a solution that I could present to you to buy.

The .450 BM won’t work in a PMAG due to the central rib that runs down the inside of the magazine body.
The .450 BM won’t work in a PMAG due to the central rib that runs down the inside of the magazine body.

I decided that I would intentionally limit my capacity to what is allowed for hunting in Michigan, where I will be using the rifle this fall 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 when it comes to anything above .223 in AR rifles. Tromix is a great, innovative company that makes some really neat 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.

Single-feed followers for Lancer L5AWM magazines
Single-feed followers for Lancer L5AWM magazines

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.

I noted at this time that there is 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.

There is significant differences in case taper across brands of ammo that went unnoticed earlier.
There is significant differences in case taper across brands of ammo that went unnoticed earlier.

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.

Feeding with the Tromix followers resulted in 100% reliability.
Feeding with the Tromix followers resulted in 100% reliability.

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 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 to take a look at their must-have followers and other interesting products. The 10-round Lancer mags can be found at

About Josh WaynerJosh 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.

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