How to Adjust Feed Lips on Plastic AI Pattern Mags?

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Maple57

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So the Cadex chassis finally arrived for this rifle and I put it together today.

Its a Defiance Deviant Tactical action with a 1:7 Bartlien barrel chambered for 223 in Cadex barrel contour.

It's throated to jam the lands at 2.560" with 75 grain Hornady ELDs, which is as long as the MDT and AI plastic mags will allow without modification.

The AI mags feed perfectly with 2.560" long rounds but the MDT and Ruger 223 RPR mags do not.

I measured the distance between the feed lips and the AI mags are considerably wider at the front than the MDT and Ruger versions, which looks to be the reason AI mags work and the others do not.
(FYI the Ruger and MDT mags feed fine in my 223 RPR, just not in this new Rem 700 foot print rifle... And the AI mag does not feed in the RPR.)

I have a tool to adjust metal feed lips and have my arms around that for the 308 but the plastic mags put me in unchartered waters.

So how do I adjust the feed lips on plastic mags?

File them?

Any ideas?

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You can fiddle with/file them but if the AI mags run, you're probably better off buying a couple more of those and keeping your Ruger and MDT mags for your RPR.
 
rugbydave said:
You can fiddle with/file them but if the AI mags run, you're probably better off buying a couple more of those and keeping your Ruger and MDT mags for your RPR.
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Houston we have a problem... Real AI mags are no longer being produced for 223.

Cadex offers Accurate mags for 223 but they are made for shorter OAL... Something like 2.35" or so and I don't want to run them that short.

I do think however that Accurate 223 mags might be the easiest of all to modify because they have plastic liners inside the steel 308 mag. They can be disassembled and the plastic inserts can be separated, so it would be fairly easy to adjust, I suppose.

But I'm still stuck with a bunch of plastic mags that need an attitude adjustment.
 
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Maple57 said:
Houston we have a problem... Real AI mags are no longer being produced for 223.

Cadex offers Accurate mags for 223 but they are made for shorter OAL... Something like 2.35" or so and I don't want to run them that short.

I do think however that Accurate 223 mags might be the easiest of all to modify because they have plastic liners inside the steel 308 mag. They can be disassembled and the plastic inserts can be separated, so it would be fairly easy to adjust, I suppose.

But I'm still stuck with a bunch of plastic mags that need an attitude adjustment.
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The real AI polymer mags come up for sale on the EE from time to time, maybe a WTB ad will find you some. I've got 6 of them (not for sale) I've picked up over the last couple years, they're out there.
 
A heat gun will not work, the magazines are a fiber reinforced polymer and you will damage them before any modification can be down. In order to properly modify the MDT or AM, you're gonna have to remove material with a file, dremel, belt sander, etc.

Like Dave said, you're better off shopping around for the AI mags and stockpiling them if they work (however, the tend to break down and split at the centre seam over time) or modify the others. May even be worth a call or email to MDT to see if they can help you out?
 
I have a really small die grind that can use one inch sanding disks. I'm going to give that a try on one of the MDTs and see how it turns out.

Looking from the side with a round in the chamber I can visually see the AI mags align the cartridge with more of a nose up attitude.

The plan will be to put the mag in a vise with a dial indicator on the front of a case in the mag and measure how much it rises as I grind away material.

If I use find sanding paper (like 240 or 320) and take my time I think I can get it done. It's a pain to have to do this though.
 
Maple57 said:
I have a really small die grind that can use one inch sanding disks. I'm going to give that a try on one of the MDTs and see how it turns out.

Looking from the side with a round in the chamber I can visually see the AI mags align the cartridge with more of a nose up attitude.

The plan will be to put the mag in a vise with a dial indicator on the front of a case in the mag and measure how much it rises as I grind away material.

If I use find sanding paper (like 240 or 320) and take my time I think I can get it done. It's a pain to have to do this though.
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I wouldn't suggest a die grinder especially if it's not much you need to remove. I suggest a file if you have one as you can keep fairly even pressure across the whole feed lip if you need the round to sit up higher. You should be able to file the forward portion of the feed lips to get it to nose up a bit. AI magazines also has a higher feed lip and if you are running a control round feed action it seems 223 Remington like the rounds to sit a bit higher.
 
buji88 said:
I wouldn't suggest a die grinder especially if it's not much you need to remove. I suggest a file if you have one as you can keep fairly even pressure across the whole feed lip if you need the round to sit up higher. You should be able to file the forward portion of the feed lips to get it to nose up a bit. AI magazines also has a higher feed lip and if you are running a control round feed action it seems 223 Remington like the rounds to sit a bit higher.
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I already tried the file and found it difficult to control.

Next up, I'm going to run a 1/4 inch rod through a fired 223 case and solder it on. Then slop it up with lapping compound, put it in the mag, chuck the rod in a drill and spin it with upward pressure. Reverse the drill back and fourth along the way. I think this will be the easiest to control. Confidence is high.
 
Maple57 said:
I already tried the file and found it difficult to control.

Next up, I'm going to run a 1/4 inch rod through a fired 223 case and solder it on. Then slop it up with lapping compound, put it in the mag, chuck the rod in a drill and spin it with upward pressure. Reverse the drill back and fourth along the way. I think this will be the easiest to control. Confidence is high.
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Fair enough, curious is the back portion of the magazine hitting the action at all? How much up and down movement is there in the magazine to the action?
 
The MDT mags fit perfectly. A little too snug to drop out. Cycling the bolt doesn't hit the mag or anything.

It's clear that the problem MDT mag geometry is designed for SAMMI spec loads of like 2.25 OAL.

Since my loads are so long, they need more upage at the nose.

The RPR mags sit too low and I cant strip one off the mag, so I wont bother with them. I'll keep them for the RPR.
 
Maple57 said:
The MDT mags fit perfectly. A little too snug to drop out. Cycling the bolt doesn't hit the mag or anything.

It's clear that the problem MDT mag geometry is designed for SAMMI spec loads of like 2.25 OAL.

Since my loads are so long, they need more upage at the nose.

The RPR mags sit too low and I cant strip one off the mag, so I wont bother with them. I'll keep them for the RPR.
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The polmyer 223 magazine is meant for up to 2.550" in 223 Remington, you should have plenty of room. My 2.390" 75 GR Eldm's run perfect!
 
buji88 said:
The polmyer 223 magazine is meant for up to 2.550" in 223 Remington, you should have plenty of room. My 2.390" 75 GR Eldm's run perfect!
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Yes mine will feed fine if the round is short enough... But I want to run them as long as possible.

Try actually feeding a round or two that are actually seated as long as the mag will allow.... Then you'll probably share the problem.

I've been examining the problem and think I need to add a 6 O'clock scallop at the mouth of the chamber... similar to how they do it on an M14.

The sharp edge catches less than half of the meplat, so it doesn't need much help... Probably 0.030" chamfer would get me there for a full 2.600"
 
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