Introduction: How to Convert a 30-Round Magpul PMAG M3 AR-15 Magazine for Legal California Use

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About: I am a law abiding and responsible gun owner, and I try to make people aware of the 2nd Amendment. Since I live in California, it is often very difficult to comply with gun-related laws because they are so re…

WARNING! DO NOT ATTEMPT THIS MODIFICATION WITHIN THE STATE OF CALIFORNIA! It is illegal to import, assemble, or receive large capacity magazines. You must perform this modification outside of the state of California before bringing the magazine into the state!

This Instructable will show you how to permanently modify a new 30 round Magpul PMAG M3 magazine (which are readily available in most states near California such as Nevada, Arizona) to accept no more than 10 rounds of ammunition. This permanent modification makes the resulting magazine 100% legal to import into California, and possibly into other states that have limits on magazine capacity.

This is the latest version of the Magpul PMAG. They made previous versions, and the one before this one was called the M2, and there are a few others I have seen. Please note that this modification will only work on the latest generation magazines, as far as I know. You can identify a PMAG M3 because it has a dot matrix LCD sort of grid array on both sides, which are intended to be used for marking with a light-colored paint pen. The older PMAG magazines did not have this matrix.

These magazines are commonly used in AR-15 rifles, which are semi-automatic rifles. This means that when you pull and hold the trigger, one bullet is fired. To fire another bullet, you must let go of the trigger, and pull it again. Please note that AR-15 rifles are not "assault weapons". That term generally applies to fully automatic rifles and machine guns, which an AR-15 is not. However, these magazines are also used in full automatic rifles such as the military M16 and M4.

The method I will demonstrate in this Instructable involves installing a piece of sturdy plastic tubing in the base of the follower, which limits how far towards the floor plate the follower can travel. This limits the number of rounds that can be placed into the magazine before the plastic tube contacts the floor plate and prevents any additional rounds from being inserted.

In order to make the modification permanent, the plastic tube is permanently epoxied with special plastic epoxy into the follower.

Finally, to assure additional permanence, the floor plate is installed on the magazine body and riveted to the internal spring base plate to prevent the floor plate from ever being removed, and a drop of plastic epoxy is used to permanently seal the rivet to the floor plate.

First, several notes on legalities:

I live in California, so it would be a felony for me to perform this modification within the state because importing (bringing them home with me), manufacturing (assembling the parts), or receiving (ordering through the mail) the stock 30 round magazines (because they have a capacity of more than 10 rounds) would be a felony. Curiously, it is legal to possess magazines that have a capacity larger than 10 rounds, but if you are caught with them, you will likely be arrested, and the prosecutor will try to trace back to see how you acquired them which will probably trigger one of the above restrictions (importing, manufacturing, or receiving).

I did this conversion in Arizona, so I was legally able to import the modified 30-round magazines into California because they can only ever accept 10 rounds or less. Reversing the modifications would render the magazine unusable.

The California Department of Justice has testified in court that in order for a magazine to be considered modified "permanently", you must use a rivet and epoxy in the modification of the magazine to accept 10 rounds or less. You can find very deep discussions of this on the web site www.calguns.net. Because of the California DOJ testimony, and because the 2014 law prohibiting the importation of "conversion kits", it is possibly illegal to import the loose parts that could be used to assemble a complete magazine, the modifications I demonstrate in this Instructable should be able to stand up in a court of law as being a permanent modification. However, there are no definitions or court cases yet, so we have to error on the safe side. We also know that a rivet and epoxy could be removed, and any modification could be reversed, so there really is no such thing as a "permanent" modification. Reversing this modification, however, would constitute a felony because you would be converting a 10 round magazine into a 30 round magazine.

I am personally comfortable using magazines modified in this fashion so they can not accept more than 10 rounds of ammunition. To the best of my knowledge as of the date of publishing this Instructable (Jan 1, 2016), I believe that they are 100% legal to import into California, and are 100% legal to use, sell, and otherwise treat as if they were factory 10-round magazines because they cannot be modified to hold more than 10 rounds without destroying the magazine parts. However, I am not an attorney, and I have not had this modification reviewed by any legal expert or court of law. Please use your own judgement on using this or any other method to modify any magazines to accept 10 rounds or less.

OK, now on with the show!

Step 1: What You Will Need

Here's what you will need in order to perform this modification:

  • Magpul PMAG M3 30 round magazine (I used the windowed version, $18)
  • Ball point pen (I highly recommend the Paper Mate 1.0 M black pen, 10 pack for $1)
  • Measuring tool (I used a Harbor Freight digital caliper which costs about $20)
  • Multi-tool or blade and pliers for cutting, scraping, grasping
  • Plastic epoxy (regular epoxy will not adhere permanently to the plastic magazine)
  • Pad of Post-It notes for mixing the epoxy on
  • Toothpicks or wooden matches for mixing the epoxy
  • 11 rounds of .223 ammunition for testing
  • Pop rivet (1 per magazine, 3/16" diameter, any length over 1/2")
  • Washer (stainless steel M5 for backing up the pop rivet)
  • 3/16" drill bit
  • Drill motor
  • Pop rivet tool
  • Butane lighter for heating the pen body to bend it slightly

Once you have all these items and materials together, we can move to the next step.

Step 2: Disassemble Your PMAG

Take the PMAG completely apart, including taking the spring off of the follower and off of the spring base plate.

To do this, you just press the little button on the bottom in, then slide the floor plate off. Watch that you hold the spring so it doesn't go flying!

Step 3: Prep the Pen

The pen barrel is used as the sturdy plastic tube to limit the travel of the follower.

To prep the pen, you need to cut it to length, shave down the thin part a bit, and fit it into the follower.

  1. Remove the cap from the pen and discard it.
  2. Use the pliers to grasp the pen tip and twists and pull to remove it, then discard it.
  3. Shave off just a bit of one side of the narrow part of the pen barrel. This is to allow it to fit into the follower.
  4. Do a dry fit into the follower. It should fit in only one way because the flat spot you shaved will go towards where the tip of the bullet goes on the follower. You should be able to press it in all the way and seat it. If it is too hard to press in, shave off a little more.
  5. You can also scrape off the logo and lettering at this point if you want to, although nobody will every see it again unless you have to go to court and the prosecutor disassembles your magazine to inspect the modifications you have done. I scraped the logo off of mine.

OK, now on to the next step!

Step 4: Epoxy the Pen Into the Follower

This step attaches the plastic tube permanently to the follower.

  1. Mix up about a pea-sized bit of epoxy on the Post-It nite and mix it thoroughly. I used 5 minute epoxy, so it really does start to set up fast. You need to work fast!
  2. Pull the pen barrel out of the follower and put epoxy on the narrow end of the pen barrel, then fit it back in to the follower. Make sure the flat shaved part locks in to the follower correctly, and make sure to push the pen barrel all the way in to the follower.
  3. Apply epoxy to fill the gap all the way around the pen barrel and the follower. Do not get any epoxy on the sliding surfaces of the follower. If you do, wipe it off before it cures.
  4. Set the pen barrel and follower assembly aside until the epoxy is fully cured and no longer sticky.

OK, next step!

Step 5: Bend the Pen Barrel With Slight Heat

The pen barrel is straight, but the magazine body is curved. We will now heat up the pen barrel and bend it slightly so it doesn't bump into the spring when the follower is depressed.

  1. Heat up the pen barrel with the butane lighter carefully to avoid melting it, and just warm it enough so you can put a gentle bend in it.
  2. Adjust the curve to match the curvature of the magazine.

OK, next step now!

Step 6: Cut the Pen Barrel to Length, and Check Round Capacity

Now you need to carefully cut the pen barrel to the exact length to allow 10 rounds to fit into the magazine, but not 11.

  1. Measure exactly 3.5" from the receiver and mark the pen barrel with the blade. In my tests, 3.5" was the perfect length for 10 rounds, but your results might be a little different.
  2. Cut the pen barrel exactly on the mark you made.

Now reassemble the mag completely, and see how many rounds you can get in. You should be able to fit 10 rounds in easily, but not 11.

If you can fit 11 rounds in, then you will need to remove all the rounds, disassemble the magazine and add a short extension to the end of the pen barrel. You can use a piece of the original pen tip that you removed in a previous step. Just cut a piece of it about 3/8" long and slide it into the pen barrel. You will need to epoxy this so it doesn't slip. I had to do this on one of my magazines, and you can see the modification I made in the last image.

If you can fit 9 rounds but not quite the 10th round, or the 10th round fits very tightly, remove all the rounds and disassemble the magazine, then trim off about 1/8" from the end of the pen barrel. Then reassemble the magazine and check again.

If you can get 10 rounds in and not 11, then you did great! This is the check that a law enforcement officer or range master is likely to perform if anyone questions your magazine that looks like a 30 round magazine. If you state that the magazine has been modified, they will probably still want to verify it for themselves.

We are ready to close the magazine up permanently now. Let's move on!

Step 7: Drill the Floor Plate, Install Washer, Assemble, and Rivet It

Note: Do NOT pop the pop rivet until the magazine is fully assembled! The last photo shows the pop rivet in its final position just for clarification, but it will not be popped until the magazine is assembled.

Now we are ready for the final set of steps. We need to permanently install the floor plate so that it is impossible to open the magazine and remove the pen barrel extension.

  1. Make sure you have checked the assembled magazine can only hold 10 rounds and not 11 rounds
  2. With the floor plate and spring base plate removed, seat the spring base plate into the floor plate the same way it goes when the magazine is assembled.
  3. Use a 3/16" drill to drill a hole through the floor plate that is centered in the existing hole in the spring base plate.
  4. If the hole isn't perfectly centered, you can hog it out a little bit to be sure it is centered.

Now we will seat the washer into the spring base plate, since we will not be able to access the area where the washer goes once the magazine is assembled.

  1. Press the M5 washer very firmly over the hole. The ribs on the sides will hold it in as long as you press it in very firmly.
  2. Turn the spring base plate over and tap it on the table to be sure the washer is in very tightly. You don't want it to fall out after you reassemble the magazine!

Finally, we will reassemble the magazine and install the rivet.

  1. Reassemble the magazine. Do not pop rivet anything until after the mag is assembled!
  2. Visually confirm that the washer is still in place.
  3. Carefully insert the pop rivet into the hole on the floor plate, through the spring base plate and through the washer.
  4. Use the pop rivet tool to squeeze the rivet fully and "pop" it.

At this stage, unfortunately you can no longer open the magazine for cleaning, lubrication, etc., but we must do this modification with the rivet and the epoxy in the next step in order to be in full compliance with California law.

OK, one more step and we are done!

Step 8: Seal the Rivet With Epoxy

This last step is important in order to be in full compliance with the definition of "permanent modification" that is found in the California DOJ testimony. A rivet alone is apparently not enough to be considered "permanent".

  1. Mix up a pea-sized blob of plastic epoxy.
  2. Place a drop of epoxy on the head of the rivet, and run the epoxy around to seal the edges of the rivet.
  3. Allow the epoxy to fully cure until it is no longer tacky or sticky.

Note that I used clear plastic epoxy. I considered tinting it black (you can do that by putting a few drops of wet spray paint of the color you want to match into the epoxy during mixing) but I decided to leave it clear, because I think it would be easier to show to a law enforcement officer or range master to verify that it is indeed riveted and epoxied closed and can not be opened for modifications.

That's it! Your magazine is now what a reasonable person would consider permanently modified to accept no more than 10 rounds of ammunition.