2013
03.03

Last fall, my friend Max helped his mom pick out a shotgun for home defense.  Being a fairly small lady, she took a liking to a youth model 20ga. Mossberg 500 and had a great time with it at the range.  However, the 22″ barrel that came with it was a little long for a home defense gun, and I was asked if I might be able to chop it down to the legal limit of 18″ (in the US, a shotgun with a barrel of less than 18″ is considered a short barrel shotgun, and is subject to NFA restrictions).  The small amount of gunsmithing that I’ve done has always been for myself, but I was happy to take a stab at the project (worst case, I’d just have to buy a new barrel to replace the screwed up one).  Additionally, I needed to attach a rail-mounted Streamlight flashlight to the shotgun (that has no rails).  Sounded like a fun challenge, so I ordered some odds-and-ends from Brownell’s in preparation for the task.

The first step would be to start hacking away on the barrel.  Since the barrel has a vented sight rib, you can’t just use a pipe cutter to cut the end off (besides, I never liked the thought of that method anyhow – it would leave a nasty burr on the inside, and is rather a crude approach when you have access to machine tools).  Also, 18″ is right through the front edge of a rib, and erring on the side of caution is highly recommended – I would have to trim the rail back to the rib and leave a bit of barrel sticking out underneath that to make sure I’m on the legal side of 18 inches.

I clamped the barrel in a padded vise and proceeded to use a number of wraps of electrical tape through the cut area – protecting the existing finish is paramount.  I’m a garage gunsmith, but I don’t want my work to look like it was done by a garage gunsmith – I’d like the result to be something I can personally be proud of, so I took extra caution to prevent errant nicks and dings.  I should also apologize for the horrendous pictures here – I left the protective plastic sheet in place on my phone, hoping it would help protect the lens (which it does, but when the LED ‘flash’ turns on, it illuminates the sheet and ruins the photo).

Measure twice, cut once.  Or in this case, measure at least half a dozen times – anything under 18″ isn’t an ‘oops’, it’s a federal crime (if you don’t have an approved Form 1 for the shotgun).

I cut through the rib with a cutoff disc in a Dremel and stopped before I hit the barrel.

Then I lopped off the end of the barrel with a hacksaw.

Next was to clean up the front of the sight rail and end rib.  I taped the heck out of the barrel stub end and then used a flat file to smooth out the end of the rail and give the corners just a little radius so they wouldn’t catch on anything.

I used some vinyl drawer liner material with double-sided tape to line the lathe chuck jaws.

With the barrel inside a plastic bag to help keep it protected, I took light facing cuts across the muzzle until all evidence of hacksaw-ery was eliminated.  Then I used a small piece of sandpaper to knock down the sharp inside and outside edges.

Since the barrel shortening had removed the front bead sight, I needed to drill and tap further back on the sight rail to remount the bead.  I clamped the barrel in the mill vise and eyeballed it to make the sight rail level.  I then used an edge finder to indicate in the sight rail so I could be sure of drilling right on its center.

I drilled just behind the front rib and carefully tapped it out with a 5-40 tap.  That was all for machining on the barrel itself – the bare steel simply needed bluing.  Unfortunately, I don’t have any photos of this, as I was trying to simply do the job correctly rather than documenting it.  However, my general approach was to first thoroughly degrease the surfaces to be blued (the end of the barrel and the front of the sight rail and rib).  I wiped them down with rubbing alcohol and started a pot of water boiling on the stove.  I would dip the end of the barrel into the boiling water for a little while to heat it up, then pull out the barrel, shake off any water droplets, and quickly apply Brownell’s Oxpho-Blue Creme to the bare steel areas with a cotton swab.  After letting the solution blue the metal for 30 seconds or so, I wiped it off and dunked the barrel end back in the boiling water to clean off the solution and re-heat the metal.  I repeated this perhaps 8-10 times.  When everything was done, I washed off the barrel end once more, wiped it off, and applied some Remington gun oil to the newly-blued areas to keep them protected.  I think the end result was perhaps a touch lighter than the factory finish, but it’s hard to tell, and might just be due to the machined/filed surfaces rather than being factory polished.  At any rate, it looked good.

Now, how to mount that pesky flashlight?  Originally Max and I were thinking of drilling and tapping the magazine tube (the tube under the barrel where the shotshells reside) for Picatinny rails.  While this might work, I’d have to figure out how to align the magazine tube properly in the mill vise, and I’d have to contend with what would likely be a thin wall on the magazine tube (and interfering with the feeding action would be a very bad idea).  I then figured that attaching a rail via scope rings would be the best method – not only would I not have to drill through the magazine tube, but the user could adjust the flashlight position though a full 180 degree arc.  I purchased a sight rail with scope rings, as I had a crazy idea – mount the rings to the magazine tube and flip the rail inside-out…  The rail needed a bit of machining – I had to add extra slots (for the cross-pin on the flashlight to lock into), and I needed to machine the rail’s underside on each end so that the scope rings could clamp onto it from underneath.  With all of that complete (forgot to take photos, sorry), I had to adapt the 1″ scope rings to the 7/8″ magazine tube.

I used some scrap 1″ OD tubing that had a 3/4″ ID, and bored out the interior to 7/8″.  I then sliced off two rings with a parting tool.

I clamped each ring in the mill vise and lowered the quill so that the slitting saw sat on the top of the ring.  Then I raised the knee up by half the outside diameter of the ring plus half the thickness of the slitting saw’s kerf.  Slotting each ring allows them to collapse slightly and grip the magazine tube when each scope ring is tightened.

The mounting system worked perfectly, and the flashlight was easily attached.

The resulting platform turned out great, and she’s very happy with it – that’s all I could ask for!

6 comments so far

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  1. Very interesting, thanks for the post.

  2. Very cool.

    I’ve just gotten access to a lathe and milling machine and I’m am giving serious thought to building a muzzle brake for a Mosin-Nagant. I haven’t found a lot of information so far, so I thought I would see if you have any suggestions.

    What I have so far:
    Make sure the brake is concentric with the rifle bore
    .040 over the diameter of the bore seems to be a good size
    Angling the vents back 30-45 degrees is probably the most effective

    I’m probably going to make the prototypes out of aluminum. If one lasts a few hundred rounds, I’ll consider it a success.

    Any words of wisdom?

    • I wish I had words of wisdom to offer, but my knowledge of muzzle brakes is rather limited. However, it sounds like you’re definitely on the right track regarding the design and implementation – the rifle has a solid kick to it, and a brake would certainly be appreciated by many shooters!

      I assume you’re looking to do a clamp-on type of design rather than threading the muzzle for attachment?

  3. Mike,

    Max’s mom here. Thank you for all your great work! You and Max gave me just what I needed for my home defense needs or the zombie apocalypse (whichever need arises first). I’ll be heading to the gun range soon….weather permitting. I am more than happy with it, I’m absolutely thrilled with it!

    • Super – so happy to know you like it!

  4. Mom is very happy. Just trimming down the barrel makes the Mossberg “feel” much more handy. With a little luck (and forethought) the trimming of my Benelli Supernova barrel will produce better pictures…