Greetings all.

I just initiated the purchase of my first machine gun using one of the PAWS Olympic M16 lowers that Ruben had for sale.

I have a couple of Sig MCXs, one of which was a "hybrid pistol" which is an MCX upper mated to an AR lower (Sig M400) using the adapter block needed to work with the MCX recoil assembly.

So I thought "I wonder if this would work on an M16 lower" and set off to search, and found this thread at the top.

https://www.ar15.com/forums/Armory/SIG-MCX-upper-on-M-16-lower/23-483170/

That sealed the deal for me.

I already purchased the SSF trigger and mcx trip lever that will be needed, but I will need the rest of the parts to complete the lower when the stamp arrives.

Do you have any recommendations for any sources of these parts? It looks like they're widely available, and mostly manufactured by one or two companies.

Thanks!

Justin

Don't know anything about the MPX but congrats on the purchase.

Thank you!

As to my parts question, it looks like it's all standard stuff.

If you bought one of the stripped PAWs lowers from Ruben you will need the following parts.

1. M16 lower parts kit.    

I would highly advise buying a Colt M16 lower parts kit from ken Elmore at Specialized Armament Warehouse.   The "Colt" kits on gunbroker are junk and the rest are mainly a hodge-podge of non-Colt and or used parts.  Ken is slow to process orders and ship but he is the only place I know where you will get new/verified Colt M16 fire control parts.

http://www.specializedarmament.com/%E2%80%94-parts-kits/3-lower-receiver-parts-kit-auto/

2.   You will also need a buffer tube.  I personally like the POF enhanced buffer tubes as they protect the lower receiver threads.  Otherwise I would probably pick up a Colt tube from Ken.  

https://www.rainierarms.com/pof-buffer-tube-anti-tilt-6-position

3.  You will need a buffer (probably an H2 or H3) a buffer spring, castle nut, and receiver extension back plate.  Personally I prefer Colt for buffers and springs but castle nuts and back plates it doesnt really matter.   (although I like the VLTOR SASE units)

4.  Obviously you will need some sort of pistol grip.

You will also need the SIG MCX specific parts.

1. The auto sear trip (which you already have)

2. The MCX buffer tube adapter.

https://www.sigsauer.com/store/mcx-stock-adapter.html

http://soldiersystems.net/2015/02/09/ar-lower-receiver-extension-adapter-for-sig-mcx-upper/

The MCR upper requires this adapter on a standard AR lower as it has the buffer in it and makes the back of the AR receiver solid to work with the MCX carrier/recoil spring assembly.

My understanding is that while Sig has promised to offer these adapters for sale, they have never actually produced and shipped any other than some prototypes that have made it out to select individuals.   Maybe this has changed recently but this part may require you to find somebody to make one as a one off.    

I would also recommend if you are going to build out the lower yourself to purchase the correct tooling to install the trigger guard and mag catch pin if you don't have them already.  Nothing sucks like damaging a lower installing these two parts and on a 20K M16 receiving you don't want to just use a punch and hammer.

I personally like the LCGW and units.

https://littlecrowgunworks.com/product/bct-bolt-catch-assembly-tool/?v=7516fd43adaa

https://littlecrowgunworks.com/product/rpp/?v=7516fd43adaa

The only other piece of advice I can offer is to make sure the lower is in-spec before you start building it out.  The forged Oly M16s are good receivers in my opinion (I have two) but both had challenges fitting mil-spec uppers or accept standard PMAGs.    Both of these issues can be remedied by sending them out to M60Joe and/or Victor to get blueprinted and reanodized.   I suspect that a Sig MCX upper may not drop onto the Oly lower and you probably don't want to have to file on the Sig upper to make it fit.


Congrats on the new M16 and Happy building!

Thank you for the info! I have a friend that has an M16 and he also recommended the same source for parts.

I actually have the MCX stock adapter because I recently purchased an MCX "hybrid pistol" which is an MCX upper on an AR lower..., with the stock adapter installed. That's ultimately what made me ask if this combination would work.

Thanks for the tips on tools!

Justin

@jbntex
As always, a wealth of information for anyone.  I do have a few questions:

1) Why go with 5.56mm POF RE versus .308 with A5 buffer system?  Cost difference is minimal if buying individual components, and the A5 system has shown improved reliability.
2) Have you used an end plate with a QD socket?  It seems like that would be handier for swapping uppers and moving slings.  Granted, if your SASE has HK hook and that attachment point, then it's not an issue.
3) Regarding the tools, what about the Wheeler Engineering Delta Series AR-15 Armorer's Bench Block?  I have one, and it supports the lower very securely with a non-marring surface.

1) Why go with 5.56mm POF RE versus .308 with A5 buffer system?  Cost difference is minimal if buying individual components, and the A5 system has shown improved reliability.

I am always hesitant to recommend the A5 buffer system to folks on the internet.   I personally have never really seen the need for it from a functional standpoint as you can get really heavy carbine buffers nowadays that almost mimic the weight of an A5, which at that point all you get it the slightly different spring compression given the longer buffer tube that I think is somewhat dubious in terms of really moving the needle in terms of reliability.  Thats just me as many folks have great experience with the A5 setup.

However, my primary concern in making an A5 buffer recommendation is that there are many folks who might not realize that the A5 system can *only* be run with an A5 buffer.  If you accidentally put a carbine style buffer in an A5 tube the carrier gas key will smack the top of the receiver buffer ring as the stack up tolerance is now wrong.  It doesn't take many rounds with the carrier gas tube smacking into the buffer ring to quickly do significant and permanent damage to the receiver.

For a buffer tube setup I just prefer to tell folks to stick with the standard length carbine tube and buffers to keep from ever accidentally cross using the wrong buffer and damaging their receiver.

2) Have you used an end plate with a QD socket?  It seems like that would be handier for swapping uppers and moving slings.  Granted, if your SASE has HK hook and that attachment point, then it's not an issue.

I have never used one with a QD socket.   For carbine competition I have always used a Blackhawk CQD sling with an HK  clip for probably 10 years.  With the HK clip I can cross use it on my MP5/HK53 with an A3 stock or an M4 style carbine with the VLTOR SASE HK backplate loop.  For some reason I just never really trust those quick disconnect slings, especially if the rifle on the end of the sling is a transferable HK or M16 machinegun.  Call me old school as I just prefer a solid HK "carabiner" style clip on my slings.

3) Regarding the tools, what about the Wheeler Engineering Delta Series AR-15 Armorer's Bench Block?  I have one, and it supports the lower very securely with a non-marring surface.

I have the wheeler upper receiver block and it works well for rebarreling uppers, but have never used the Wheeler lower block but it looks like it is a pretty solid piece of equipment. I may have to order one if you recommend it works well

jbntex,

Thanks for taking the time to respond.  The A5 buffer makes sense, and I see how the HK clips would work great.

Regarding the lower block, I absolutely love mine.  I bought it on a whim a few Black Fridays ago, and it's been great.  It's been used for around 25 lowers between my friends and I (mainly Anderson and the like) but we have had NO issues with it.  I generally do the bolt catch first, since it needs installed before the pivot pin.  From there, I do the trigger guard, then work my way from front to back.  What I really like is the support without marring anything.  My first lower has bite marks from where I used Vise Grips to get in those two roll pins, and with this I haven't had a single issue.  It is as simple as toothpick in bolt catch, roll pin punch, done.  The trigger guard is even easier, as the front will click in place and the opposite ear is fully supported.  It also has a hole all the way through so if you want to swap out trigger guards, the pin will fall straight down when knocked out.  Pretty handy.  I haven't used all the features on it, but the ones I have used have impressed me with the thought put into the design.