doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Backboring a 20ga

Chuck,

I have a Fox 16 that I chamber sleeved down to a 20 bore. The chamber were ruined and I wanted to save the barrels without major expenses. As I had several 16 with 30" barrels I decided toi try to make a set of 20/16 barrels. It is choked full and full and gives full and full choked patterns with 20 shells. My chamber sleeves are longer than the chamber area as I turned them on my lathe to enter the bores by 1 3/4". They have a slow taper outward to the 16 bore. I know that I could have made them longer to perhaps get a better seal and burn out of the 20 shells but the short chamber sleeved replacements work well. Crony reading are almost exactly the same as my 20 Fox with 28" barrels. Maybe 30-50 fps difference.

I had to drill out both chambers because they were so pitted that it was not safe anymore. Storing two paper hulls for 50+ years, with high humidity, caused the shells to crumble and they in turn caused the chambers to pit. I ended up taking almost .035 of the chamber wall thickness out in one and .030 in the second. Then made a chamber casting of both chambers and first two inches of the bores to figure out how to make the chamber inserts. Wasted a ton of metal on the lathe and milling machine before I got two that fit as I wanted them. It taught me a lot about machine operation and just how difficult what seemed like a simple plan could end up as another 100+ hour education.

So in effect I have a over bored 20. I did pattern it and found that it patterned one ounce loads slightly better than it did the factory AA's 7/8 I used for a base line. If I had to do it all over again I would just go back as a 16 or even find another barrel. I have the correct chamber reamer available and figure that my custom barrel chamber making days are not a real good use of my time. Then again if I came across a 20 with bad chambers I might try to make a set of 28 chamber sleeves with about a 10-12" barrel insert. Be a fun gun to shoot skeet with and maybe a few released quail.

Jon,
Thanks for relating your experience. I just measured the walls and got between .045-.049" . These barrels are HEAVY. I wouldn't feel goosey about taking .010" a side off.
That might make enough weight difference to make the effort worth it to me.

In searching for "the" gun, considered; ability to use modern pressure ammo, cost, single trigger, ejectors, quality of construction, durability, weight. The BSS doesn't have the weight I was hoping for but has the other requirements covered. With a BSS being a relative bargain, I can put a little money into it and not feel too bad. Besides, nothing I found that was near what I was looking for was anywhere near whatever I could put into a BSS.

If I had a set that had wall thickness of .045 the loss of .010 or even .015 would be no big deal and has to help them swing better. Either do that or start a sterods program so you can swing that gun better. That gun sounds like it is almost a 12 gauge by barrel weight. Does it balance near the hinge pin?

It balances 'bout 1/2"-3/4" forward of the hinge. Barrels are 26" and weigh 2 lbs 15 oz. O.D. is 'round .715 average or little less.

Well when you consider back boring a 30" 12 barrel .010 reduces weight by 2-3 ounces the amount of metal removed from a small bore will be less by about 1/3. So if you reduce both of the barrels by 3-4 ounces total combined and then remove 3-4 ounces of wood from the stock to re-balance does this get you to the weight you are looking for? I can not see much more than 6 or maybe 7 ounces total weight reduction. Some guns just can not be made real light.

Jon,
The weight in these barrels is pretty significant. Also, the forend iron has a strap like a piece of railroad track. It could take some lightening pockets on the underside and not even show and drop an once or two easy. The bar of the action stout and could use some lightening pockets ala Parker or CSM 21 style. That kind of whittling is cake on a mill.

But, I just saw that new S&W 20g again in the pro-shop at the range. Nice part for a Turk gun...very nice. Very well sized reciever and nice fit & finish for a $2.3K gun.

Woodsman, spare that tree. Chopperlump

Shot a few boxes of 20g shells using some Gauge Mate chamber inserts in a 16g LC Smith. Summer weather. Not a hint of a bloop. Shells included AA and STS - they are easier to remove from the Gauge Mates. Federal shells worked OK, too, but are harder to push out of these inserts. Shot a few 16g shells, too, and could not feel or hear any difference between them and the 20g shells in the inserts. Did no pattern testing.

Tempting to think of overbore as similar to using a larger gauge. The wads of the larger gauge would contain the shot within their petals a bit better than the wad of a small gauge going through an overbore. The shot column would start out shorter in the bigger gauge for a given load. Overbore could allow use of a bit lower pressure to get the same speed as the gas seal area would be a bit larger. The bigger gauge would be able operate lower pressure and equal the speed of smaller gauge for the same load.