December 15, 2025
Description
Tired of paying outrageous prices for original H&K .308 mags when Magpul ones can be had for a few bucks? You have come to the right place. I designed an adapter that can be used to convert a Magpul LR/SR GEN M3 .308 magazine to fit an HK417 / MR308 / MR762.
The design was inspired by this model by hsiltala on Thingiverse. However I wasn't happy with the fitment in my MR308. The mags were difficult to insert and they were not drop free. So I designed my adapter from scratch in Fusion360 (design file is also attached, if you need to make further modifications to fit your rifle).
The print profile is already configured with custom support and the ideal print orientation. You basically just need to print it. There are two different versions of the adapter included. Plate 1 has a regular version and plate 2 has a version with tighter internal dimensions. Depending on your material and printer calibration, one may work better than the other.
I have printed mine in eSUN PA-CF (PA6/66 with 20% CF), which is extremely tough. Unfortunately it also creeps a bit over time. So I had to glue mine onto the mags. Depending on what material you use and how rigid it is, you may be able to get away without the glue. Feel free to experiment and post your results in the comments for others to learn from :)
The printed adapters are pushed onto the magazine from the top. The cutout for the mag catch goes to the left and the shorter side with the small protrusion in the middle goes towards the back (see picture 2 and 3). I found it easiest to push them onto the mag in the back first and then angle them down until they snap over the mag body in the front. From there, apply even pressure until they are flush with the feed lips. To fully seat them in place, use a rubber hammer and a punch and alternate hitting the adapter in the front and back until it is flush with the magazine body in the rear (see picture 5).
If you notice that the adapter does not stay in place after some use and wanders up on the mag, add a few drops of super glue between the mag body and the adapter when sliding it on. That will fix it in place and ensure reliable feeding.
Unfortunately, this alone will not let you use the magazines in the HK417 / MR308 / MR762. The front of the magazine body (from the front edge of the magazine lips all the way to the front of the body) is too tall and will contact the inside of the receiver, before the magazine is fully seated. In order to fix this, you need to remove some material from the walls of the mag in the front. Use the lowest point on the front of the mag body as a reference and continue that height back towards the feed lips (see picture 6-8). You do not need to modify the feed lips!
The easiest way I found to do this was to use a side cutter to take of the material on each side, followed by some filing to smooth everything out. This does not have to be precision work. The walls are not critical to the stability of mag, especially with the beefy adapter added on the outside. They are also not involved in the feeding (unlike the feed lips), so although this modification is permanent, it does not prevent the mag from being used in standard .308 AR (unless the adapter is glued into place).
hsiltala also suggested filing down the original overtravel stop for the mag catch. But I have used my mags with my adapters without this modification without any issues. This might allow for a more positive lockup, but it was not necessary for me. If you have issues with too little engagement on the mag catch, feel free to try this. However, this is a modification that will prevent the mag from being used in a standard .308 AR.
As with all firearms related parts, make sure that you act in a safe and responsible manner. You are dealing with 3D printed and hand modified magazines here. So maybe don't trust your life on them. But for having fun at the range, they are perfectly sufficient. I have been using four of them on 10rd PMAGs in my MR308 for some time now. And except for the walking up of the adapters (which was fixed with some super glue), I have had zero issues with them.
The grip extensions for the 10rd mags in the pictures are designed by @martin.hardi and can be found here on MW.
License:
MakerWorld Exclusive License
TheBigOne
