Tutorial: How to install a PRS stock on a KJW/TK M4 GBB

How to install a PRS stock on a KJW/TK M4 GBB

Sometime ago I looked at my KJW/TK M4, and thought that I don’t really like the stock it has, and that I would like to replace it with something different than everyone is running around with. After searching for some time, I have chosen the Magpul PRS stock.

So I ordered one online with the required buffer tube, the package arrived two days later, and that’s where the problems started… It appears that the KJW design is a lot different than the Magna system, and the original firearm, so I needed to do a couple of modifications to use my new stock (I was really determined at this point 🙂 )

Warning: You will need power tools and a lathe to follow my tutorial.

KJW M4 has two buffer tubes… well one that is a real buffer tube, and a fake one covering it. So the real one is screwed in directly in to the lower receiver, and has a wide and thick nut at the end. And the next one is made out of some pot metal, and is just covering it (basically You just slide that pot metal tube over the real one, and secure it with a tiny – I mean tiny – screw underneath) Just throw it away, we won’t be needing it ever again 🙂

So the first problem is, that the KJW tube does not want to fit in the PRS buffer tube, due to bigger outer diameter, and especially that big and thick nut at the end.

buffer tubes


So we have to get rid of it, but unfortunately it holds the buffer return spring, and stops the buffer itself. The only solution here is to decrease it in diameter to fit in the PRS tube. We also have to remove about half of the buffer tubes thread, because it’s diameter is the same as the tubes.

stock lathe


So the next problem is the thread on the end of the PRS buffer tube. KJW/TK lower receiver doesn’t have a thread other than the one for the stock tube. So we will have to remove the thread form the PRS one.

PRS tube


Now we created another problem, and that is, how do we secure the PRS tube, without the thread that we removed in the previous step? The answer is quite simple – we will make a new thread for a screw, in that big nut at the end of the KJW tube.

Stock nut:

stock nut

Drilled and threaded:

nut thread


And that’s it! In this quick and easy way we have modified our parts so, that now the KJW buffer tube, slides in the PRS tube.

Assembly (I’m sorry for the poor quality of those photos, was making them late in the evening):

1. Screw in the modified stock tube.



2. Slide the PRS tube on the modified KJW one.

prs mod


prs mod 2


3. Put on the stock





4. secure everything with a screw that goes through the stock, the PRS tube, and in to the nut at the end of the KJW buffer tube.



5. Admire your awesome “new” rifle 🙂


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