Why is my bolt binding as it moves to the rear - How to Fit VMAC9 / MAC-11 9mm Uppers

As I mentioned in the first post, a fit of nostalgia over the MACs and then seeing the 3D printed firearms coming out using MAC uppers started me digging. What really cemented the deal is that you can build a VMAC9 very affordably. I’d argue it’s one of the best deals out there and at the end of the day, you have a modular firearm you can change over time as your needs change.

There are a number of companies that make complete uppers and I am only going to list the ones I heard good things about. Just remember that any person or business can produce a defect. I look for general trends, comments about customer service and how much money is being spent. The reason for this is that under a certain price level, I will gamble. Conversely, once somethings is over a certain level, I will not.

So, here’s the list if you are looking for MAC-type uppers and I’d recommend you do some research to here what folks are currently saying about a given company:

JC Arms – https://jc-arms.com
Lage Manufacturing – https://www.max-11.com/index_2020.php
Masterpiece Arms (MPA) – https://masterpiecearms.com/cat/defender-series/
Practical Solutions – https://macmachineguns.com/
Velocity Arms – https://www.velocityfirearms.com/product-category/vmac9/vmac9-upper-kits/

Velocity Firearms Uppers

Being new to building a MAC, I decided to one stop shop and buy the lower and upper parts kits from Velocity Firearms. I went with side cocking uppers because I wanted a Picatinny rail on top to mount optics. I bought their 5.5″, 11.25″ and 16.7″ complete uppers – they had their barrels installed and came with complete bolt and spring assemblies.

Up to this point, I had three assembled lower receivers. Once married with their respective uppers, I created two pistols and one rifle subject to the applicable regulations of the ATF and laws of Michigan. The two pistols had to be registered as such. Note, I am not a lawyer and this is not legal advice. I will tell you one thing – know what laws and regulations apply to whatever you build.

This is the 5.5″ pistol upper on a lower during test fitting. The magazine is a refinished Israeli 20 round Sten unit. You can get a barrel with either just the original 3/4″x10 rear threads or with the 1/2″x28 front threads also.

This is the 11″.25″ upper. They used an adapter on the barrel’s 3/4×10 threads to mount an AR handguard. What you see there is a free-floated handgurd where a ring is turned backwards to extend and lock the handguard in place and a set screw is tightened to secure it.

This is the 16.7″ upper and the cylindrical handguard you see mounts directly on the barrel’s 3/14×10 rear threads.

Fitting Will Likely Be Required

The front of the upper receiver has a trunnion that holds the front in position. The rear of the upper is held up by a spacer. I’ve seen it called the “Flat Rear Spring”, “Sear Spring Holder” and “Rear Spacer”. Regardless of the name, it’s height matters a great deal.

Velocity Firearms calls this rear spacer the “VMAC9 Flat Rear Spring”. It sets the rear height which is critical to smooth operation. This must be sitting flat at the rear and may need to be bent either closed or open a tad to set the height.

See those two holes on the right? The top is where the end of the recoil rod sits and the bottom is for the ejector rod. The upper receiver is sitting on top of the flat spring. The top hole is for the recoil rod and the bottom is the ejector rod.

This is the bolt but it is sitting on it’s side – the left side is face up and the bottom is in the shade. The important thing is to look at the rods – the recoil rod with the spring is at the top and would be at the top in the firearm. The bare steel is the ejector rod.

So here’s the problem – the bolt is moving front and back in the upper tube. It’s vertical travel is limited. The rods are vertically aligned at the rear by sitting in the holes in the receiver. If the upper, bolt and end of the rods aren’t sufficiently aligned then the bolt will bind as it moves towards the rear.

Why was my bolt binding?

Well, I hinted at the problem above. My flat springs were sitting on the bushing of my Practical Solutions 1913 rail adapters. When I racked the slide back, the alignment issue was so bad that the bolt would jam open at the rear plus the upper did not want to go back into the raised portion of the lower.

I write these posts after I build firearms and may seem like I have things figured out but let me assure you – at the time, I was having a WTF? moment. I somehow wound up talking to Sam Schneider, who owns Practical Solutions, who is a class 2 manufacturer specializing in MACs and Class III firearms. He has a wealth of knowledge about the MACs and explained the importance of getting the vertical spacing right. My “flat spring” sitting on the bushing of his adapter was throwing everything off when the bolt travelled backwards.

Ahhhh … That made sense. The bolt is at the mercy of the upper’s channel and it also rides the recoil and ejector rods that are spaceed by going into the back of the receiver. Of course it would make things bind up if they were off.

So, after talking to Sam, I knew I needed to fix this. I would cut a slot so the “flat spring” could sit flat plus it would cradle the oblong bushing so it could be tightened.

I used my calipers to find the center and then mark the offset on both sides so I cut out the notch with a Dremel. I deburred it when I was done.

I noticed some welds in the corners that I ground down out of the way.

Once the notch was done, and the welds were gone the spacer could sit flat.

At this point, it cycled better but I could still feel it binding just a bit. The last step was to crush the flat spring just a bit in my vise to lower the upper a bit further.

This took me three or four tries – I’d squish the flat spring just a bit and then test fit and cycle the VMAC. By the last time, it felt really good so I called it even. The trick is to not rush and just bend it a little bit followed by testing. I bought a couple of spare flat springs just in case.

Summary

There are a lot of MAC/VMAC upper options out there. Make sure your flat spring is seated on the bottom of the lower and is at the right height. If you feel binding, odds are it is too high and you can see this because the upper is hitting the top rear of the lower and not wanting to seat fully. Just take your time and bend the flat spring and test over and over until it fits.

I hope this helps you out.

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