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si gen 3 upper/alternate lower.


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Bought a complete 308 upper with an si upper installed. Not a big fan of the lines honestly, but do to the lack of required kac wrench, im pretty much set with it.

I REALLY dont care for the lower. I have no need for grip grooves.

So,long story short, im hoping someone has experience pairing an si upper with another brand of lower.

Jd machine, a local outfit, looks to be the ticket, but they're by appointment only now.

Any help would be appreciated.

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  I have a SI Defense Gen. II Billet Upper & its mated to a very early CMMG Billet Lower . The combo worked with any Steel Mag. I put in it , but will not work with Magpuls Pmags reliably . They are not attached to a Barrel right now , but will be shortly . Not sure which is the problem , but I believe its the CMMG lower , I can put the SI Defense upper on a DPMS Lower & Pmags will work , but no Upper I put on the CMMG lower will work reliably with them . 

  All about Tolerance stacking , this is a CMMG Lower that was made before the LR PMag was made , if fact , many years before , so I don't blame CMMG for the Stacking problem. 

  Mixing different manufactures Receivers is a crap shoot . <dontknow>

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Thank you boss.


I too had an early cmmg lower.  Got lucky and swapped it for the newer release.


Glad to hear the dpms model will, or should work. Big fan of the simplicity of the Bushy lowers, which are essentially the same thing.


Im gonna do my best to attempt to find a few more options before committing.

Edited by geistacwm
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My PWS MK216 uses SI-D upper and lowers. PWS switched over to their own forged lowers after using SI-D for the MK2 MOD 0 and initial year of MOD 1s.

I've put my upper on some other LR308 pattern lowers, but it also didn't work on some due to the front pivot pin ear being too wide.

Never had any issues with G2 or G3 PMAGs...

Edited by FaRKle!
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