Thermold 20 Round Mag Modded to CA Legal 10 Rounds - How To

[Jubz]

I found some great PMAG alternatives; great quality, inexpensive and in stock. These mags are great for target shooting, but they're not as robust (polymer is not as thick) as the PMAGs. I like the size of the 20 round mags and not having to worry about anti-tilt issues since the mags are straight. Being in CA, these will need to be modified to 10 rounds. I had put this info together for some friends; thought I'd share this easy mod.

Thermold 20 Round Mag - 10 Round Modification
Make sure you get the higher quality US made version (Zytel) and avoid the Canadian Version (Maple Leaf stamped) with cheaper brittle plastic. No need for anti-tilt followers since the 20 round mag is straight. Here is how to easily modify this mag to CA legal 10 rounds using a Magblock. Basically you cut down the Magblock and insert into the spring, then put back the spring plate and mag’s base plate. CA legal requires you make this mod permanent (pinned and glued) and that is your responsibility. The below shows a version which you can return it back to 20 rounds for use outside of CA.

• MAGBLOCK UNIVERSAL MAGAZINE BLOCK 5.56/.223 - $5.95 each (this exact model is what I used for below)

Thermold 20 Round Magazine
US made versions will have stamps of “MASTER MOLDER WILSON N.C. U.S.A.” and “LAW & GOV’T USE ONLY” on the sides, and “PAT. NO. x,xxx,xxx WILSON N.C. U.S.A.” on the bottom plate. Avoid the Canadian version stamped with a Maple leaf.


Disassembled


Magblock Modification to 10 Rounds for a Thermold 20 Round Mag

• Template - cut a business card down to 1 5/8 inches height
• Place the template on the Magblock and slide down until it stops on the bottom ridge/platform
• Cut/saw/Dremel the Magblock above the template, this will give you a snug 10 round fit
• Block sand the cut top and bevel the corners, this will result in 10 rounds with a little play but not able to fully insert an 11th round; insert/check/sand as needed to get that little play to avoid excess pressure on the feed lip

Insert Modified Magblock

• Insert the Magblock into the spring and push spring down onto the plate; no need to put the spring into the Magblock’s spring clip
• Note the Magblock’s bottom plate extension/tang accomodates the magazine’s internal guide/ridge
• Also note the placement of the magazine’s spring plate over the bottom of the Magblock; that little round bump for the spring attachment will be forward of the Magblock’s ridge

Complete Assembly

• Slide on the magazine’s floorplate while depressing the spring plate and it will lock into the middle hole
• Additionally, the magazine’s bottom outside corners (left corners below) are sharp; hit this lightly with sandpaper to take a little off those edges