Spring and buffer for a 5.5 inch buffer tube

T

Tool

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What are some good spring and buffer choices for a 5.5 inch buffer tube? May I use mil-spec spring kit? I think they are designed for 7 inch tubes.

I'm building a lower that will be used for two uppers: DDM4 PDW and DD MK18.

Thanks.
 
I wouldn't do that, i forsee lots of technical difficulties in your future, good luck.

if i did do something like that, it would be with the understanding that i would have to use the entire special proprietary system from DD as probably nothing else will work with the non-standard setup. Most likely it will be a specific, proprietary buffer and spring and you will not be able to use anything else.

And that buffer system may not be compatible with a mk 18 upper. Id call DD if i were you.

Generally all ar stuff is interchangeable and tunable as long as you stay mil-spec-ish. As soon as you start using specialty proprietary parts, all bets are off.
 
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With short buffer tube, first thing to make sure is the proper function of the action.

It should lock back on empty mag before the buffer bottoms out in the tube. You can test that without firing. If it doesn't, you will need a shorter buffer. A flat spring may be needed as a conventional spring can also limit the travel of the buffer in the tube.

From lock-back position, the action should pick up round from the mag reliably. If not, you need a strong (stiffer) spring.

All good, then you need to siphon off proper amount of gas to power your machine. I'm a believer of adjustable gas block. It doesn't have to be continuously adjustable by turn a screw. Any method to vary the flow of gas suffices. It isn't that much of a mystery really. Turn the spigot down if the flow is too high. I have milsurp rifles older than Stoner's design and they have adjustable gas designed in. No problem.

Don't want to have adjustable gas? Buffer / spring weight is the option left. Don't overlook the urethane disks inside the buffer. They are there to absorb excessive energy, just like the shock in a car's suspension.

With enough tinkering and thinking, it may work.

-TL

Sent from my SM-N960U using Tapatalk
 
Thanks for the advice.

I just spoke with the DD technical support. According to them:
(a) Both their MK18 and DDM4 PDW are mil spec and use the same BCG, so it will be possible to share one lower.
(b) For the shorter tube, DD uses the Maxim spring and buffer kit, and recommends me to purchase them directly from Maxim.
 
Midway sells a"Strike" brand short tube,buffer,and spring. Its a flatwire spring and a short,propriatorybuffer. No military buffer will substitute.

I don't know the DD components.

You know,there are only so many suppliers of this stuff. Example? Aero makes a lot of uppers and lowers that other people put labels on.

The capacity to forge and finish 7075 receivers is a bit capital intensive.

I can't say,I don't know, but the source Maxim and Strike may be selling the same parts.

You may need an adj gas block.
 
KAK also sells a variety of 5.25" buffer tube setups..

https://kakindustry.com/ar-15-parts/stocks-buffer-tubes

And "NO" you can not use a typical 3.25" buffer or typical AR15 carbine recoil spring. The compressed typical Carbine spring will still be to long for the amount of space in the 5.5"ish buffer tube.

BTW... the KAK kit's "stock" is IMHO kinda lame.. You might consider one of these as a replacement..
https://magpul.com/moe-sl-m-carbine-stock-mil-spec.html?mp_global_color=118

And if you are trying to "shrink" the overall profile of your firearm... also not my look .. but it does "shrink" down the profile.
DO NOT order these direct from Fab Defense.. my CC was hacked from them.
https://www.opticsplanet.com/fab-defense-ergonomic-folding-black-pistol-grip-for-m16-m4-ar15.html
 
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Many thanks for sharing the information. It's interesting to know there are foldable AR-15 grips.
 
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