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Modernizing a Winchester 37
by Roy Seifert

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Disclaimer:  This article is for entertainment only and is not to be used in lieu of a qualified gunsmith.  Please defer all firearms work to a qualified gunsmith.  Any loads mentioned in this article are my loads for my guns and have been carefully worked up using established guidelines and special tools.  The author assumes no responsibility or liability for use of these loads, or use or misuse of this article.  Please note that I am not a professional gunsmith, just a shooting enthusiast and hobbyist, as well as a tinkerer.  This article explains work that I performed to my guns without the assistance of a qualified gunsmith.  Some procedures described in this article require special tools and cannot/should not be performed without them.

Warning:  Disassembling and tinkering with your firearm may void the warranty.  I claim no responsibility for use or misuse of this article.  Again, this article is for entertainment purposes only!

Tools and firearms are the trademark/service mark or registered trademark of their respective manufacturers.

About 20+ years ago a coworker approached me and wanted to know if I was interested in purchasing an old single-shot shotgun.  I asked him if he had any details about the gun and he told me that it was owned by his father which he had inherited when his father passed away.  I told him I would be interested so he brought it into work (this was when we could still do those things!) and we went out to the parking lot so I could take a look. 


It turned out to be a single-shot, break-open 12 gauge Winchester model 37 Steelbilt with a 30” full-choke barrel and an external hammer.  The Winchester 37 was manufactured from 1936 until 1963 and most did not have a serial number.  Because it was manufactured before 1968 it is legal to own without a serial number.  This gun is a classic piece of Winchester workmanship.  Who knows how much game this gun has harvested over the years?  It had a lot of surface rust on the barrel from being stored under my coworker’s bed for years, but some 000 steel wool and gun oil took care of that.  The receiver was starting to take on a plum color, but other than those two issues, the gun was in excellent condition.  The bore was clean and bright with no pitting.

I used this gun for trap shooting at one time, but I hadn’t used it for years.  Since I have been working on shotguns lately, (refer to my article Modifying a Winchester 97 for Competition) I decided I wanted to modernize this old gun.  My modernizing plan was as follows:

  • Measure the chamber to make sure it would take modern 2 3/4“ shells.  If not, then
  • Lengthen the chamber to take modern 2 3/4“ shells
  • Lengthen the forcing cone
  • Ream and tap the barrel for interchangeable choke tubes
  • Replace the hammer
  • Replace the poorly-fitted recoil pad
  • Install a fiber-optic bead
  • Refinish the wood

Measure the Chamber


Even though the chamber was stamped for 2 3/4” shells, the chamber gauge #080-546-012 I purchased from Brownells would not fit up to the 2 3/4“ mark.  This meant the chamber was probably designed for 2 3/4“ roll-crimp paper shells, not star-crimp plastic shells.  The forcing cone was also very abrupt so I wanted to lengthen both the chamber and forcing cone.

Lengthen the Chamber and Forcing Cone

A long, gradual forcing cone can enhance the accuracy of a shotgun because it doesn’t deform the shot so much.  Since I also wanted to lengthen the chamber to shoot 2 3/4“ plastic star-crimped shells; I could perform both processes at the same time with a long forcing cone reamer Brownells #080-661-012.


I removed the barrel from the receiver, then removed the extractor, extractor guide, extractor sear and extractor spring from the barrel.  I installed the barrel in a padded vise with the chamber up.  I lubricated the long forcing cone reamer with cutting oil and used a large tap handle to ream the chamber and extend the forcing cone.  Because the reamer had straight flutes, when the flutes became full with chips, the reamer became difficult to turn.  I removed it from the barrel always turning it clockwise to prevent breaking the teeth, and cleaned off the chips.  Each time I removed the reamer I also cleaned the chips out of the barrel with brake parts cleaner and checked the chamber with the chamber gauge. 


I stopped reaming the chamber and forcing cone when the chamber gauge fit up to the 2 3/4“ mark.  I could have continued lengthening the chamber to accommodate 3” shells, but this gun was never designed for them so I stopped at the 2 3/4“ length.


To finish the job I polished the chamber and new forcing cone with a 12-gauge chamber Flex-Hone® I purchased from Brownells #080-608-512.  This tool is a wire brush with abrasive balls on the ends of the bristles and comes with a nine-inch shaft.  It should only be used with Brownells Flex-Hone® Oil #080-008-609 to ensure the polishing is performed correctly.  I attached the hone to my drill and applied the oil to the hone.  I polished the chamber and forcing cone by running the drill at a slow speed (<750 RPM) and moving it in and out.  I flushed the barrel with brake parts cleaner, then oiled it with CLP.  Now the chamber and forcing cone are clean and polished smooth.

Ream and Tap the Barrel for Interchangeable Choke Tubes
In order to ream the barrel for interchangeable choke tubes the barrel had to be at least 0.845” thick.  The barrel measured 0.850” thick 1 1/2“ back from the muzzle.  Cutting the barrel at this point also removed the fixed choke, but wouldn’t make all that much difference to the performance of the gun.  Before cutting the barrel I marked it 3 1/2-inches behind the original bead with a center punch so I would know where to drill and tap the barrel to move the bead.  This location would be behind the choke tube where the barrel was nice and thick.

I have done this process before on other shotguns; refer to my articles Threading a Shotgun Barrel for Choke Tubes and Modifying a Winchester 97 for Competition for details.  After cutting the barrel I set it vertically in my bench vise with the muzzle up and began reaming the barrel.  To facilitate the reaming process I put about 50-pounds of bullets on top of the tap handle.  I reamed the barrel until the shoulder of the reamer squared off the front of the muzzle.  I used a rubber polishing wheel to round off the sharp edge on the outside of the muzzle left from cutting the barrel, then cold-blued the exposed metal.  I then tapped the barrel using the choke tube tap.  I cleaned and lubricated the new threads and installed a Winchoke® full choke tube.  Before installing the choke tube I lubricated the threads with choke tube lube.  This prevents the choke tube from becoming stuck as the barrel heats up from firing.

I purchased a 3-56 shotgun sight convenience kit from Brownells #078-021-256 that included a #45 drill bit, and a 3-56 bottoming tap.  I used the #45 drill bit to drill a hole in the barrel where I had marked it previously.  To start the tap perfectly straight in the new hole, I left the chuck of my drill press centered over the hole, put the 3-56 tap into the hole, then tightened the chuck just enough so the tap would turn freely.  I used a set of vise-grips to start the tap about 2 turns; I reversed the direction of the tap after every 1/4 turn to break the chips.  After the tap was started I used a small tap handle to finish tapping the hole.  I cleaned the hole and the threads of the original silver bead with acetone, then applied a small bit of blue thread locker and installed the bead.

Replace the Hammer
This shotgun had a very small, curved spur on the hammer (Winchester calls it a cocking lever) and the knurling was worn almost smooth.  It took so much thumb pressure to cock the shotgun my thumb would be very tired and sore after shooting a couple rounds of trap.


I found a replacement hammer on ebay, of all places, for $15.00 with free shipping.  That was cheaper than from Numrich.  If you notice in the above photo my hammer has a thin curved spur, whereas the new hammer has a wide straight spur.  Although I can’t find any manufacturing records for this gun, I’m guessing my gun was an early production model.  Customers probably complained about how difficult it was to cock the gun so Winchester redesigned the hammer with a straight spur.  As a side note; a friend of mine also has a Winchester 37 and his hammer has a wider spur, but it is still curved, not straight.  It seems that this hammer went through a number of production changes.

After I removed the butt stock from the receiver I could see that two pins held the hammer in place.  The top pin connected the firing pin to the hammer, and the bottom pin was the hammer pivot pin.  I removed the two pins and withdrew the hammer from the receiver. 


The pivot hole in the new hammer was too small for the original pivot pin so I tried to ream it with a drill bit, but because the hammer was hardened I broke the bit.  I had to set up my hobby CNC milling machine to mill the proper diameter hole of 0.080”.  I used a 1/16” square end bit and milled 0.0005” deep each pass and kept it well oiled.


I installed the hew hammer into the receiver and found that the trigger sear did not fit into the ‘V’ notch in the front of the hammer.  I used my high speed rotary tool and a cutoff wheel to expand the ‘V’ notch, then polished it with a fine stone.  The straight spur now makes the gun very easy to cock and the trigger sear fits into the ‘V’ notch. 

Replace the Poorly Fitted Recoil Pad

(Photo courtesy of Hogue)

I purchased a brown Hogue EZG grind to fit recoil pad size medium #00711 from the eCop Police Supply web store on eBay.  I purchased brown because I thought it would nicely set off the refinished stock. 

Since the pad measured one-inch thick I cut one-inch off of the butt stock.  I put two layers of masking tape around the stock and cut it to the same angle of pitch.  I don’t remember where I read this, but somewhere I read that you need to use a sharp saw blade to prevent splintering the wood.  I purchased a new blade for my table saw that cost $60, and it not only cut the walnut like it was butter, but left no splintering.


After cutting the butt stock, I marked and drilled new holes for the recoil pad.  I mounted the pad onto the stock and scribed a line around the stock.  I have trouble seeing the scribed line, so I filled it with white paint.  I painted the scribe line, then wiped off the excess paint with an alcohol-soaked cloth.  This made the scribe line stand out so I could see it better.  This worked so well that even my errors and overruns showed up.


I then sanded the pad using my modified jig and sanding table.  For details refer to my article Installing a Recoil Pad.  Every time I do this I get better at it, and the white line really helped me from sanding too far.

Installing a Fiber Optic Bead


My Winchester 1300 Defender came with a slip-on fiber-optic front sight.  In reality it was a Hiviz MPB sight.  It slips on over the barrel, and a V-notch in front sits against the bead which prevents the sight from moving under recoil. 


Since I use an electronic holographic sight for my Defender, I took the slip-on bead off of the Defender and put in onto the 37 barrel.  As you can see from the above photo it sets back a ways from the muzzle, but that green dot is big and bright!

Refinishing the Wood
The butt stock and forearm had a few dings and scratches in them along with 70+ years of age.  I wanted to steam out the dents and generally refinish the wood.  I removed the butt stock from the receiver and the forearm from the barrel.  The forearm had one screw holding the mounting hardware in place so I removed the screw and mounting hardware.


I used Klean-Strip® KS-3 premium stripper I purchased from my local home improvement store to strip the old finish from the wood.  This stripper is a thick paste which sticks to the wood and doesn’t drip off.  I applied the stripper with a disposable brush and let it set for 15-minutes.  I could see the stripper start to work almost immediately.  Then I used paper towels to wipe off the stripper and the old finish came with it.  The above photo shows the forearm before and after stripping.  I performed the stripping process three times to each piece of wood, then cleaned the exposed wood with odorless mineral spirits as described in the instructions.  I was careful not to apply any stripper to the inside of the forearm; I didn’t want to have to refinish that area.


Once the wood was stripped I applied a wet towel to the wood and pressed a hot iron to the wet towel.  This process steamed out many of the dents, but also raised the grain.  I could not steam out some of the deeper dents and scratches, but as my shooting friends say, that gives the wood “character”.


After the wood dried I sanded the raised grain “feathers” with 400-grit sand paper until the wood was nice and smooth.  I applied some fast-drying polyurethane to the inside of the hand guard and butt stock to help prevent moisture gathering and causing rust.


I decided to leave the walnut its natural color.  I followed the finishing video found on the Boyds’® gunstocks web site using Birchwood Casey® Tru-Oil®.  This process takes some time, but the results are outstanding.  I applied six coats of Tru-Oil and sanded with 1,000-grit sand paper after the third coat.  I have always liked the smooth, semi-glossy finish Tru-Oil imparts to the wood as you can see from the above photo.

Repairing the Auto Eject
At the time I lengthened the chamber and forcing cone I removed the auto ejector parts so they wouldn’t become damaged.  The four parts involved in the auto eject mechanism were 1) ejector, 2) ejector sear, 3) ejector spring, and 4) ejector guide. 


When the barrel is closed the ejector (1) rubs against the breech face causing it to be pushed inward until the notch in the ejector clears the ejector sear (2) hook.  The ejector spring (3) being under tension causes the ejector sear (2) to rotate so the hook falls into the notch.  The hook at the top of the ejector (arrow) is positioned under the rim of the shell, and the shell is now fully seated into the chamber.


When the barrel is opened and starts to swing forward the ejector is still locked in place by the ejector sear.  As the barrel continues to swing forward the lobe on the front of the sear (arrow) contacts the bottom of the notch cut into the barrel hinge pin causing the sear to rotate so the hook drops out of the notch.  The ejector spring causes the ejector to spring forward until it is stopped by the ejector guide (4) causing the shell to eject out of the chamber.


The barrel hinge pin is either pressed or silver-soldered into the receiver so it cannot rotate.  A notch is milled into the center of the pin to accommodate the lobe on the front of the ejector sear.  The pin is installed so the notch is at an angle to trip the ejector sear when the barrel is opened.

On my gun as the barrel was swung open, because the ejector was under spring tension, the ejector was constantly rubbing against the breech face and would open slowly.  This allowed me to pull the empty hull out of the chamber manually, but the ejector would not snap open. 

I installed each part individually to make sure it moved freely and found that the ejector sear was binding and would not pivot.  I polished the edges with 400-grit wet/dry sand paper, and polished the ejector sear pin so the ejector would rotate freely.  I then installed the ejector, ejector sear, and ejector guide without the spring to see how they all worked together.  After polishing the ejector sear and pivot pin everything worked as it should.  I lubricated and reassembled the parts with the spring and now everything works as it should.

Lightening the Trigger Pull
Although the trigger was crisp, it was very heavy.  I decided to lighten the trigger pull by replacing the trigger return spring. 

This spring has a specific diameter to fit into a recess in the frame to prevent it from slipping, and a specific length to ensure the trigger spring guide rod stays in contact with the trigger.  This constant contact keeps the hammer back so the firing pin does not protrude from the face of the receiver to prevent loading or ejecting a shell. 

New Spring (bottom)

I didn’t want to cut the original trigger return spring because I couldn’t find a factory replacement spring or guide rod in the Numrich catalog or exploded diagram, so I decided to replace it.  Again I believe this was an early production model because none of the diagrams I found had the long trigger return spring and guide rod.  I never throw away a spring; you never know when you might need one to fix another gun.  I cut a piece of 0.231” OD spring from Brownells spring kit No. 71 #025-071-000 three coils beyond the length of the guide rod.  This lightened up the trigger pull to 5 1/4 pounds.  You can see that the new spring is made of a narrower gauge wire and is one coil shorter which makes it lighter than the original.



This was a fairly easy project.  Most of the modifications I performed with hand tools.  Although I used a drill press to drill and tap the bead hole, I could have accomplished this with a hand drill.  I now have a 1930’s era shotgun modernized for the 21st century.  Just for fun, I may remove the fiber-optic bead and install a flush-mount choke tube and use this gun for cowboy action shooting.



   © Copyright 2014 Roy Seifert.