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MPX stock adapter mounting...

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Just mentioned this in another post, but figured I’d post it on its own in case any other MPX shooters came across similar weird issues I was having...


When I first got my MPX put together I was experiencing some pretty weird issues with the gun just not sitting/feeling right, and with optic height, them seemingly being too low at a normal AR absolute co-witness height, and then with a riser added being maybe too much “heads up” with an odd chin/cheek weld; this is with a TTI stock adapter mounted as it looked natural with the adapter covering all the available pic slots on the butt of the receiver...


Then I read somewhere that the MPX receiver is taller (like Sig’s MCX) than a traditional AR-type receiver, and noticed that TTI was mounting their adapter 1 pic notch lower than I had it mounted on their more recent MPX’s... 


So, turns out, if you put the barrel and buffer tube against a straightedge, the adapter mounted 1 pic notch down is closer to keeping the stock in-line with the bore, otherwise mounted higher, most of the buffer tube and the stock is actually higher than the barrel.


Anyways, an AR absolute co-witness optic height = a lower 1/3 height on an MPX, and with the stock adapter mounted 1 pic notch down, feels exactly as it would on an AR.
Conversely, with the the stock adapter mounted higher, an AR-height absolute co-witness optic will be lower and feel like it’s on a low mount...


YMMV, some guys might actually like the buffer tube up high, but for me remounting the stock lower cleared up a bunch of what was feeling awkward about the MPX for a couple months, so maybe this helps someone out there...?




Edited by ck1
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  • 1 month later...
27 minutes ago, Nikolai412 said:

Hi, this is right up my alley at the moment. I bought the KAK mpx stock adapter and it won't even try to fit on there. It seems now that the TTI adapter is literally my only other option, so thanks for the alignment info. Could you tell me how it attaches with no pic bolts? Is it pretty snug?

There’s a 5.5mm screw (iirc) that keeps it in place which is accessible before mounting the buffer tube. You have to tighten this 5.5mm screw first plus the tiny set screw on top and then you can install your  buffer tube and stock. It’s a solid setup. I have this setup on both my MPXs and no problems whatsoever.

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I like the KAK buffer tube adapter better. With the TTI adapter you have to remove the buffer tube to access the set screw if you wish to remove your stock at all.  With the KAK adapter you simply remove the 2 or 3 thru-bolts and you can remove the whole stock assembly as one complete unit.  It cost a couple dollars more than the TTI but it is a better design IMO.  I'm not bashing TTI, as I love their other products, just not this one. YMMV

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Which adapter is better, TTI vs KAK I cannot comment on; because I only have experience with the TTI one (which is solid). The KAK one looks solid too.


I was more trying to point out that how you mount it up (the stock adapter), height-wise, matters a bunch more than maybe first glance as far as feel and optic height are concerned.

The MPX receiver is different enough from how a “normal” AR-9/15 sits that optic height and feel is definitely different depending on how one chooses to mount their optic/stock; it’s not apples to apples. The MPX’s are a little wonky in that regard, and are actually closer to a CZ Scorpion than an AR variant in most ways... for optic height and cheek-weld. The MPX can be a bit tricky coming from a more traditional AR-9 platform, but it’s not as simple as “just get a low mount” or whatever, because how high you mount the tube on the back of the receiver changes things.


I can say this after fumbling around with it and trying and buying different stuff: IMHO, mounting the stock adapter 1 pic notch down seems best, it’s naturally more in-line with the bore, but it means using a 510C specifically (which was my favorite) is out unless you prefer a lower 1/3rd height setup, because that’s as low as that sight can be mounted. Adapter mounted 1 pic notch down with an MRO on Trijicon’s stock low mount equals an absolute cowitness height, and feels exactly as an absolute cowitness on an AR platform. An H1/T1/Holosun 20mm sight on a factory low mount will be too low (shotgun height), and so an ADM low mount or maybe Midwest (which is a bit higher and about equal to Trijicon’s MRO low mount) is needed to bring it up to an absolute cowitness height... this is all after seeking a traditional “absolute cowitness” feel and cheek-weld on the MPX.


Phew, I know, confusing AF LOL 😝 

Edited by ck1
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