Bolt Catch Depressing Cartridge

Talk about the AR15 style rifles chambered in 450 Bushmaster.

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Bolt Catch Depressing Cartridge

Postby Pathfinder » Thu Oct 25, 2018 1:46 pm

First, Thanks so much to all here who take their time to share their experience. This forum is truly a wealth of information for those who take the time to look for it.
I couldn't find any reference to what I am experiencing, so here goes.
I'm struggling to keep a build extracting & ejecting properly. I'm wondering if this might have any effect on proper ejection.

When inserting a loaded magazine with the bolt locked back, I am seeing the front edge of the bolt catch pressing down on the rim of the top cartridge which causes that round to pivot up to the point it will not chamber by releasing the bolt. The pic shows an empty brass in this position. I was trying to reproduce this inside and didn't want to be fooling with a live round. The result with a live round is the same whether factory or my reloads.
I get the same result with a Stoner (Midway) 450BM magazine, or a metal 20rd Colt mag with single follower inserted.
I get the same result when switching lowers as well.
The only way I have found to avoid this is to make sure the top round is pushed slightly forward in the magazine when it's inserted.
Is this just to be expected, or will adjustment of the magazines be required ??

Thanks again for your input.

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Re: Bolt Catch Depressing Cartridge

Postby Hoot » Thu Oct 25, 2018 8:47 pm

Just when you think you've seen anything that can possibly go wrong with a 450b, up pops a new one!

The first thing that comes to mind is whether you have too much upward tilt at the nose when the feed lips stop the case from rising any further. That would allow the rim to position itself more rearward, to the point where the hook is catching it. The rounds should ride almost parallel to the lips with maybe a couple of degrees upward tilt at the nose, at the most. If they don't, you need to "tweak the lips" closer together. None of the case should slip between the lips along its entire length that's under the lip. Sometimes you just need to narrow the lip spacing a little toward the front. It's touchy that way however, so go slowly.
If you only load one round, does it still do that? If not, it might indicate too much mag spring pressure. On all my 450b mags, I remove some of the spring windings from the bottom since the rounds are not stagger stacked like 5.56's are.

How about a side view image of the mag with just 1 and a second image with a full load of cartridges in it.

BTW, Welcome Aboard!

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Re: Bolt Catch Depressing Cartridge

Postby Pathfinder » Fri Oct 26, 2018 7:00 am

Thanks for the feedback Hoot,
I definitely see what you mean about leveling out the rounds.
Colt on the left, Stoner on the right in each pic. Stoner mag is definitely the worst. I primarily use the Colt.
1st pic is one round, 2nd is 5 rounds. I don't see much difference. It's happening no matter how many are in the mag.
I didn't realize there was so much variance of round orientation between these two magazines. I'll experiment with the Stoner first.
I haven't had other feeding issues other than loading with an open bolt. Proper extraction has been a challenge, but I think that's solved.
Worst case, I'll just adjust my loading habits with this rifle and make sure to "bump" the top round forward if loading open.

This build was a total whim. I was buying some reloading supplies from Midway and just happened to see their 450BM Stoner barrel for not a lot of $$. Fishing this year had been crap, and college football was months away, so I had time on my hands for another project. I had no idea how much fun these would be to shoot !
As it turned out, that particular barrel was a problem child (not just for me it seems). I eventually got another barrel from Bear Creek which appeared absolutely identical. They assured me it wasn't, so I swapped things out. It still took some tweaking. Bear Creek was nice enough to send me one of their extractors to try and So Far, So Good.

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Re: Bolt Catch Depressing Cartridge

Postby ScottyPotty » Sat Oct 27, 2018 4:03 am

Put the upper on a different lower and see if it improves.....

I had that issue, then it progressed to the bolt not locking back 100% on an empty mag. I looked at everything and couldn't really see anything wrong, so I ended up putting the upper on a different lower and everything worked fine. I decided to change out the bolt catch spring and that was it, the bolt catch worked and I never had anymore issues with the tilted case or locking back on empty mag.
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Re: Bolt Catch Depressing Cartridge

Postby Hoot » Sat Oct 27, 2018 5:43 am

Pathfinder wrote:...snip...
I get the same result when switching lowers as well...snip...
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Re: Bolt Catch Depressing Cartridge

Postby Bmt85 » Sat Oct 27, 2018 8:19 am

Yeah, the round at the top of the fully loaded mag seems to be tilting up too much. Push the sides of the mag together where the front of the feed lips are (about middle of the mag). That should level them out a bit, but I wouldn't go for completely level. My 5rd Bushmaster mags needed this. I also tweak the very front edges of the feed lips up a bit. My thoughts are that it releases the cartridge a little sooner and smooths the feeding a little (so it's not an abrupt release from the mag). If you still have problems, you should be able to file the catch down a touch, and at a slight angle. That should keep it from catching the rim, but still function.
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Re: Bolt Catch Depressing Cartridge

Postby Pathfinder » Sat Oct 27, 2018 4:52 pm

Thanks for all the feedback. It's appreciated.
I'll work on those magazines.
I've got five lowers to work with, but I'm pretty sure all my bolt catch components have come from Anderson kits.
I'll try some different parts on my next lower build and see if that helps as well.

Thanks again !
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