Being a Virtus owner (5.56) I have grown to love the platform. I had an opportunity on a great deal. I picked up a Legacy Hybrid MCX pistol/ This one....
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Some things I would like to do out of the gate (I envision using this as a SHTF truck gun, shooting at night and for general purpose fun)

I plan on changing the brace (I hate the current one)
I plan on upgrading the optic
I plan on upgrading the trigger

The questions I have are as follows-

Any little nuances I need to know about the .300 MCX?
Will a SB retractable brace work on this receiver?
Are the Magpul .300 optimized mags worth the money or should I not bother?
What kind of accuracy are .300 MCX owners getting with SS FMJ?
What is the best bang for the buck replacement trigger? SIG Duo? Hiperfire? Gissele?

Thanks in advance!

That upper is pure sex.

Agreed-Until you work your way to the brace.....

A friend of mine uses his mcx exclusively in 300 blk. Has an 11 inch 556 barrel for it, but I haven't seen him use that in a good while.

I haven't put it on paper, but its plenty accurate with sellier and bellot 200 grain ammo. Accurate enough to lessen the ground hog population with ease.

Ammo feeds reliably with gi mags, pmags, lancers, just fine!

His has a geissele 2 stage, feels good.

You'll be happy with it, if not, PM me

The telescoping PDW/MP5 style brace won't fit the hybrid lower.

If it performs anything like my Virtus, you won’t be getting a PM from me.

What rail is that?

I believe it is a midwest industries rail. His srd762 just barely fits inside it.

I like the rail on it, but it would be nice to have a longer rail for suppressor use. I will check it out. Thanks!

I'll get a definitive answer and get back to you, but I'm pretty sure it's an MI rail

ETA: 10 inch midwest industries rail

Sunday bump for info...

I think the MCX/MPX brace would work. You've already got the adapter to use a 1913 brace on there, so I don't see why the MCX one would not work.

Congrats on the new MCX! It looks pretty sweet (_other_ than that brace, lol). Thought I'd give you a few answers to some of your questions (from my perspective - anyway).

*NOTE: I do not own a legacy MCX, I have the Virtus tho!

Nuances:
Meh, prolly not much. I was having issues at first when I was shooting my 6.75" 300BLK un-suppressed... Failure to eject. I remedied this by adding a POF roller cam pin and a stout O-ring to the extractor -- now -- no issues whatever. Subs, supers -- it's all good! (Although, I can't cycle subs un-suppressed -- didn't wish to though as that doesn't seem very useful to me.)

Magazines:
IMO if you are shooting 30rd subs - yes - absolutely get 300BLK specific mags. I too, use the Magpul's and they work great for me!

Accuracy:
Out of a 6.75" barrel shooting sub-sonic I'm right about 1 - 1.5 MOA @100y Which is well within my limits of what a battle rifle _should_ be able to perform.

Trigger:
Mine came with the Sig DUO and TBH it was fine. I swapped it recently for the Geissele MCX SSA -- but I'm not thinking that that was a necessary upgrade after using it now... Oh well. hahahaha! I like the flat blade better! Be careful what trigger you throw in your MCX, they are known be very hard on them... only a handful of manufacturers make triggers that'll work with the MCX.

Good luck and have fun man!

p.s. Have you looked into Gear Head Works Tailhook yet???

That is the one that was and is on my radar. I will do some more investigating.

Thanks for the input! The Tailhook looks good, I was originally eyeballing the PDW style braces but do like the side folder. Let me know if you want rid of that DUO

I got a 11.5” mcx sbr but still want a 300bo 9”. I could have bought the gun 3 times over already but got two ar15 shorties in 300bo but like to have the sbr in 9” 300bo. Can’t decide to just get barrel conversion or jump all in get another rifle. I use to think get another rifle lol.

Excuse the semi-necropost, but I wanted to make sure you (or anyone else) doesn't burn money. I have a few MCXs (including a hydrid which is now a standard lower on a rifle build) and have the collapsing brace, collapsing stock, a couple of Kate Moss folding stocks, folding knuckle with V7 storage tube, minimalist plus, and the Sig folding brace.  Anything folding will fit the hybrid, but none of the collapsible stocks will (the rail ones, not the folder/collapsing); the buffer tower of a standard lower (which the hybrid is with an adaptor) will block the rails from continuing into the upper receiver slots.

Thanks for the heads up. As a stop gap I pulled the Sig brace and trimmed it by 2 inches. It will work for now until I settle on something else.

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