Modification of the Magpul PRS-1 stock to fit
a H&K Enhanced Buffer Tube
By Tim
Recently I purchased a Magpul PRS-1 stock for my PTR-91. I had earlier purchased the enhanced buffer tube from HKparts.net since it was a good product and at very reasonable price, a rarity in today's economy.
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Unfortunately this buffer tube will not fit into the stock without modification, so I headed to the internet to find more information about it. Turns out the only thing out there is outdated stuff that's poorly described with no pictures, so today I decided to save the next sucker the headache and go over what I did to make the buffer tube fit in the stock. Read all the way through first and feel free to ask questions.
When I first researched how to fit the buffer tube into the stock I read a lot about using a dremel. I felt like this was a poor way of doing it that could easily go wrong. Well my brother, who is a machinist, suggested drilling out the stock with a large drill bit. That seemed to be a better way of doing it, so with the help of my machinist brother, we got to work on it.
This is the stuff you'll need. Your stock of course. A 13/16 size drill bit, about two to three inches longer than the buffer tube, metal cutting preferably and a drill. You don't need a strong drill at all and in fact you might injure yourself with a strong drill if the drill catches. A moderate power one will work just fine. You might need a chuck adapter if your drill's chuck isn't big enough to hold the drill bit. You'll also need a vise with padding to hold the stock and of course safety equipment. You might need someone else to hold the vise if it's not secure and to watch to see when the drill bit goes.
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The first step is to drill out the piece that fits into the back plate. I don't have a before shot of my piece but there's a chunk of plastic in the back that holds the screw for the short buffer tube. You'll have to drill this out.
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Set up your stuff like this. Do not have the rest of the stock attached! You'll insert the drill bit into the hole in the stock from the top and use the hole as a guide as you drill down. I experienced no chatter at all and the sides of the hole were barely scuffed up. Run the drill at full speed to get better cutting action and to avoid the drill bit catching. Be very careful! DO NOT LET THE DRILL DROP OR PUSH IT! Guide it down slowly. Expect it to take about a minute. Do not rush it! You don't want to push the drill because it might catch, possibly damaging the stock and hurting yourself. If you have someone helping you, they can see when the drill goes through the stock.
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Once that's done, strip down the second major part of the stock, ensuring you know where what screw goes where and how. Especially note the position of the push buttons that fits in the indents of the adjustment wheels. Now, attach the second part of the stock to the first part using the two screws and diamond plates.
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Set it up like this. You'll continue using the first part of the stock as a drill bit guide. Again, insert the drill bit into the hole of the stock using it as a drill bit guide. Again no chatter was experienced when I did this. Run the drill at full speed to get better cutting action and to avoid the drill bit catching. Be very careful! DO NOT LET THE DRILL DROP OR PUSH IT! Guide it down slowly. Expect it to take about a minute. Do not rush it! You don't want to push the drill because it might catch, possibly damaging the stock and hurting yourself. It is especially possible for the drill bit to catch due to the random voids in the second part of the stock.
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Stop drilling often and insert the buffer tube into the hole to see how far you have to go. Remember! There needs to be a generous gap for the back plate.
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Now that you are done with the hard part, you might want to consider putting epoxy or some kind of harding material in the voids of the second part of the stock to increase rigidity. You shouldn't need to but it's up to you.
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Of course, you might have a drill press. If you already have a drill press I'm sure you'll be able to adapt this to using a drill press instead of a drill, but keep in mind there's no wiggle room with a drill press. If the drill press catches on the stock, there's a good chance it'll break it. It's up to you. Since you have a drill press, chances are you have more experience than me.
Also, I can not stress this enough. Some of ya'll are 20 year machinists and some of you don't know what chattering or catching is. Do not try this if you do not understand it. Get someone who knows what they're doing to help you. You can break your $224 stock or even injure yourself.
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