Assembling a VMAC9's lower receiver's internals - applies to many semi-auto Mac-11-type pistols

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Need to start this post with a reminder to be legal and safe.

By this point I had welded my receivers, parkerized them and also prepped some old Israeli Sten magazines. It was time to assemble the Velocity Firearms MAC 9 firearms. Two would be registered as pistols and one as a rifle. They all use the same internals in the lower receivers so I bought complete internal lower parts kits from Velocity Firearms plus complete uppers from them as well. Note, the grip assembly with the mag catch is a separate part. I just bought it from them pre-assembled.

Parts Identification

These are the lower parts for a Velocity Mac 9 (VMAC9):

Lower receiver / welded frame
Back grip assembly – includes the Plastic back grip housing, mag catch, mag catch spring and mag catch pin
Optional: This is a Practical Solutions rear 1913 adapter.
Wire retainer for the selector lever and trigger pin
Hammer pin retainer e-clip – it’s still in its bag so I wouldn’t lose it
Takedown pin assembly – tubular body and a hex socket button head screw.
Safety selector
Trigger bar – note bevel goes face up
Sear plate – This is positioned the right way – bevels go down, front slots to cradle the trigger bard are down.
Sear spring
Sear spring holder / spacer – bevel is at the top. This holds the upper in position
Trigger
Trigger pin – note the slot is set further back for the retainer wire
Hammer pin – note slot is out at the end for the retainer e-clip
This is both the hammer and the hammer spring – the spring is oriented properly. The hammer is resting striking face down.

The Practical Solutions 1913 Rear Rail Adapter

I knew I wanted a 1913 Picatiny rear rail for mounting braces or stocks to. I looked at a number and the Practical Solutions model received good reviews. I’ll tell you it is solidly made for sure.

The Practical Solutions unit is well made and easy to install. To create a solid mount that doesn’t flex, I would recommend bedding it with epoxy so there is no “wiggle” room between the rear of the receiver and the adapter.

For the most part I like it because it’s easy to install. You will probably want to bed the adapter in the rear or else it will likely move. I mixed up some epoxy, black dye and 1/32″ glass fibers to make a putty that I then applied between the adapter and the rear of the receiver. Both parts had release agent on them just in case. I did this later because I didn’t realize how easily it could move with a brace or stock acting like a lever arm.

There is a lot of adjustment room to accomodate a wide variety of builds. This is a pro and a con.

Also, the bushing will push the spacer up just a bit so you might want to grind/sand/cut a radius so the spacer can seat properly. Alternatively, just bend the spacer. I just bent the spacer until the upper would fit.

Either notch your spacer to avoid that bushing or bend the spacer to reduce the height enough.

Ok, I found out about this after I fully assembled the VMACs – the rear of the VMAC and the Practrical Solutions adapters do not sit perfectly flat against one another so the adapter could flex left and right. Honestly, not a big deal. I just mixed up some black glass fiber reinforced epoxy and created a filler/spacer between the adapter and the rear of the weapon. Problem solved. By the way, my intent was never to “glue” them. I applied release agent on both surfaces so they can be removed. If you don’t have a release agent like Mann Ease Release 200, use PAM cooking spray or a bit of shoe wax to the surfaces before you apply the epoxy. If you want to bond them directly, go right ahead.

Let’s step through the assembly

Install the plastic grip assembly taking care the mag catch’s half circle end sits outside of the mag well.
Take the sear plate, tip the rear up and lower the front forks down so they slide under the bullet guide’s arms. The beveled front and rear rails are face down. Another way of thinking of the orientation is that the trigger bar slides under the front of the forks and sits in the machined opening. Make sure the sear plate can slide all the way forward to the center support and all the way to the rear. If there are any welds in the way, grind them so everything slides smoothly.
At this point, I start lubricating as I go. If it slides, grease it. If it rotates, oil it. I use SuperLube grease applied with an acid brush to all surfaces that need greasing. Oiling was done with SuperLube oil. I like the two as they are synthetic and don’t get gummy plus they have PTFE (the generic acronym for Teflon) added to improve lubricity. The coarse parkerized surfaces will want to slow everything down while they wear in.
Slide the sear plate forward, install the spacer to the rear with the beveled face up. Try test installing an upper at this point. You may need to squish the spacer a tad in a vice to allow the upper to seat easily.
Slide the sear plate spring onto the sear and move the sear to the rear so it is all inside the sear spring holder / spacer.
Insert the trigger bar with the beveled face up. I used needle nose pliers to do this and slid it under the front of the sear plate.
Move the whole trigger bar and sear plate rearward so you can drop the trigger into the hole in the receiver and also such that the trigger is in front of the trigger bar.
Insert the trigger pin from left to right. This requires a little bit of manipulation to get the pin through both sides.
Install the hammer spring it it hasn’t been already. It should be oriented the way you see above.
Install the hammer in the rear. It sits like an AR hammer with the flat face forward. The spring legs rest between the sear plate’s legs.
Install the hammer pin
Put a daub of grease on the receiver around the hammer pin. This is an old trick and will hold the e-clip in position while you slide it on. I’d recommend using something non-marring to push the e-clip on and you’ll feel the cip snap into position. Another trick is to lower the pin such that the slot is even with the receiver so you don’t have to try and get the clip at the right height while fighting everything else.
Install the retaining wire by sliding the forward portion under the front reinforcement. The hooked portion sits in the groove on the right “ear” of the bullet guide. I’ll show photos below.
Technically, you are supposed to lift the retaining wire up and insert the selector under the wire. This is what adds the tension to lock the selector and trigger pins in place. Uhm .. there was no way I could do that. If you can get it together/ go for it. I was getting pissed off and needed to think of a different approach. So, if you were able to lift the wire up and install the safety selector lever then you are done with the lower. Speaking for myself, I had to add the following:
For the life of me, I could not get the selector lever in due to an elevated area on the inside and I couldn’t get the wire any higher. Instead, I backed off the trigger pin and inserted the selector because the tension was gone. The issue then became how to get the trigger pin in. What I did was to align the pin on the hole and used a grooved screwdriver (spanner bit) to forcefully push the pin and retaining wire down facing the hole while pushing from the other side so the pin would pop into the hole. That worked. One reason it worked was that I could see what I was doing and how to move things whereas with the selector lever and wire, I couldn’t see what was going on.
Note, you will need to help the sear plate and hammer to go into the cocked position before you install the upper.
I did a quick function test while controlling the hammer so it wouldn’t smack into things. Pulling the hammer back, the sear plate would grab it. Pulling the trigger back, the sear plate would release the hammer – again, I caught it vs. letting it fly free. I squeeze the trigger back, moved the hammer back, let go of the hammer while the trigger was squeezed and the sear plate grabbed it. I pulled the trigger and the hammer “fired”. I then made sure the safety worked. Everything was good.

Now For Photos

I find the sear plate fascinating. Yeah, I’m wierd. It’s almost like Ingram was designing a miniature bullpup. He had to create a trigger linkage that could bridge the distance from the trigger located in front of the mag well / grip to the rear where the hammer was. He came up with this design. Folks, this is pretty damned ingenious. First off, I have it flipped upside dowhn and the left would be to the front of the weapon and the top right would be facing the rear. A sear is what holds the bolt, hammer or striker back until enough pressure is applied that said device is released. In the case of the MAC, the sear plate is a forged piece of steel and let’s cover some of the unique design elements (remember it is inverted for this picture): 1) The hammer bar sits in the notches in the arms of the plate. 2) The plate has bevelled skates/runners/thingies that are bevelled to accomodate the radius of of the receiver bend and also reduce the surface area of the plate so there is less friction against the receiver plus they will probably push a fair amount of debris out of the way. 3) the plate is split into a fork to pass the left and right of the mag well. 4) the rear triangle is a lightening cut 5) the sear spring sits on/around the protruding “tail” at the rear of the plate.

Unless you like the fun of “where did that part go?” I’d recommend putting the parts in a magnetic parts tray.

The grip is installed on the steel mag well by keeping the bottom of the mag catch (the half moon shape) on the outside of the mag well so just the square “shelf” of the catch is inside. That half moon sits on the bopttom of the slot in this photo – which would be the top of the slot when the VMAC is upright. The grip is held on by one screw. If that screw doesn’t want to start or you do get it together and then can’t insert the mag, odds are that whole mag catch is accidentally inside the grip.

I did it the wrong way – see how the whole mag catch “arm” is in the mag well? I could install the grip but I could not insert mags. This photo is out of sequence so you can see the mag catch.

See how the squared off “shelf” of the mag catch is the only thing sticking into the mag well? That’s what we want – that is correct. I did not need to do any alterations for surplus or polymer mags. Both mag types lock up nice front to back but are loose left to right.

I welded the heck out of the sides. After looking at the plastic grips a tad bit more closely, I could have done a small weld on that rear tab and it would have cleared the grip. Look at the photo above – the top inside edge of the grip is hollow and would have cleared it. Live and learn I guess. That mag well will not budge given the weld I did on the sides and front corners but I would have done that rear tab had I known.

The intent of the photo is to show you the orientation of the trigger bar, sear plate, sear spring and the spring holder / spacer.

I don’t have a photo but make sure the sear place and trigger bar can slide unobstructed. If there are welds or anything in the way, grind them out, then blow out the receiver before you coat everything in grease.

Insert the sear plate with the rear tipped up at an angle. This will give you enough room to slide the front legs under the bullet guide. Yeah, I grease everything that slides.

Slide the sear platforward and insert the spacer. If your spacer has a bevel, it goes face up as shown in the photo. Since I installed the Practical Solutions adapter already you can see it is riding high. The solution is to compress/crush/bend the spacer and trial fit the top. Bend a bit and test over and over until you can full seat the upper.

Slide the sear spring onto the sear plate and side it backwards.

I use needlenose pliers to lower the trigger bar in place with the beveled edge to the front and face up.

Push the trigger bar, sear plate and what have you rearwards and insert the trigger forward of the trigger bar as shown.

To be clear, the top pin is the trigger pin. You can tell because it’s groove is further away from the end and that’s where the retaining wire will sit. The lower pin is for the hammer. The groove is right near the end because the retaining e-clip actually sits outside of the receiver. When looking down at the open receiver with it facing forward, the pins are inserted from the left to the right. In other words, the heads will be on the side opposite the ejection port.

Compared to an AR or AK, the hammer assembly goes in without a fight because the hammer can rest without spring tension during installation. The hammer pin can easily be pushed through.

Put a daub of grease on the pin and surrounding area. This will help hold the e-clip and keep it from flying into another dimension only to be found by the next person who owns your shop. I like to lower the pin so the groove and e-clip line up and then I use a non-marring surface to push the e-clip into position. You will feel it click into position.

Here is the pre-bent retaining wire. It was designed by someone who literally wanted to remove all the joy that is left in the world.

So the hook on the retaining wire rests on the bullet guide. You install it by sliding the far end first. There’s no tension yet. That comes when you install the safety selector. That’s also when the swearing starts. You can’t see or get ahold of that wire thanks to that tab that is there, in part, to make this a semi-auto receiver so I wasn’t about to cut it off or bend it.

You are supposed to lift the front of the spring up so this opening is more clear and slide the selector lever in.

Okay, so imagine you are sliding this into the hole. The spring hits that first shoulder to the right of the flat area in the shaft. I could not get the damn wire, which is now under tension, to clear that shoulder and go into the groove. In hindsight, bevelling that surface would probably have solved my problem. At the time, I was pretty pissed off after trying too convince it to go in, pulling up on the wire, etc.

My workaround was to back off the trigger pin and release the tension from the wire. I then installed the safety selector switch. I then used a spanner screw driver bit to push the trigger into the hole. Pushing it down also tensioned the wire so it took some force. The trick was controlling it.

The receiver is sitting with teh bottom cradled in my big woodworking vise. Note the safety selector is installed with the retaining wire in its groove. The trigger pin is lined up with the hole. What I did was push down with the spanner screw driver while pushing the pin to the right with my left hand. I had to use the vise because it took two hands but it worked. I did those for all three lowers and it works just fine.

Well, the retaining spring is installed.

You do need to manuall bring the hammer back and catch it because there is no upper to stop forward travel or bring it back. You absolutely can install an upper with the hammer all the way down and wonder why it’s not cocking. Look in the trigger well – if you see the hammer laying flat it’s because you forgot to cock it before installing the upper. When you see tips like that, it means I did it 🙂

Be sure to do the function testing outlined above. All weapons should be cautiously test fired to ensure safety.

Summary

I was actually impressed by the design other than my extreme dislike for the retaining wire. The trigger is better than I expected. Not great but not horrible either.

In the next post, I’ll take about the first upper and how I configured the firearm.

I hope this helps you out.

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