doublegunshop.com - home Forums General Discussion DIY Gunsmithing @ doublegunshop.com MWT at forcing cone - 16ga and 20ga

For a quality fluid steel 16ga or 20ga barrel, what is the MWT advisable at the forcing cones?
Thank you.

Bill, I’m just back from a road trip. If you don’t get an answer when I get caught up I’ll find my copy of CIP specs.

Thank you, Mark. I appreciate whatever data you can share.

That’s a good question, and it’s smart to think about wall thickness before touching the cones. For most quality fluid steel 16 or 20 gauge barrels, many folks like to see a minimum wall thickness around .025"–.030" in the forcing cone area, with a bit more giving extra peace of mind. That said, every barrel is different—age, maker, and condition all matter. A proper measurement with a wall thickness gauge and, when in doubt, input from a competent smith is always worth it.

I think in front of the forcing cone area, the minimum wall thickness would be much higher than Mr. Griffin quotes. Those numbers are ok for minimum wall thickenss down the barrel.

That’s a good question—and one where you’ll get slightly different answers depending on who you ask.

For quality fluid steel 16ga and 20ga barrels, a commonly cited practical minimum in the forcing cone area itself is around .025"–.030", assuming the barrels are in good condition and free of significant pitting.

That said, many experienced smiths prefer to see more thickness than that right at the chamber/forcing cone junction, since it’s the highest-pressure area. General guidance suggests something closer to ~.080"–.090" at the end of the chamber/entry to the cone for a comfortable safety margin.

Bottom line:

.025"–.030" = often quoted minimum workable in the cone
More is better near the chamber end
Always measure carefully and consider barrel condition before any work

When in doubt, a competent double-gun smith’s opinion is worth it.

Bill I texted you a page with CIP minimums. Numbers are metric unless hand written notes.

I sincerely hope the .025" to .030" MWT given for the forcing cone are gets corrected, or better yet, deleted entirely so nobody finds this Thread and gets the wrong idea.

For good quality fluid steel barrels, .090" is a safe conservative minimum for 12 gauge. 16 and 20 gauge guns and loads often operate at slightly higher pressures, so something like .095" minimum for the beginning of the forcing cones would be better.

That .025"to .030" MWT figure given for the forcing cone area is flat out wrong! .030" is about twice the thickness of a paper matchbook cover... and it's crazy to think that is a safe MWT for the forcing cone area. .025" would be a good minimum for the thinnest part of the barrels at roughly 9" behind the muzzles. Pressures are much lower at that point in the barrels. When you get down in the .020" or thinner range at the end of the barrels, they become much more susceptible to dents, or bulges and splits from very minor obstructions such as snow.

This is not to say you won't find guns with barrels a bit thinner than .090" at the beginning of the forcing cones. But many of those guns were proofed at pressures much lower than modern SAAMI or CIP standards. You may get away with shooting a gun that has a wall thickness of .075"to .080" or thereabouts, but you aren't leaving much of a margin of safety. Especially if using standard pressure loads.

It seems to me that if Mark can do Free Tagline Advertising in every post he makes here, he could have also posted his page of CIP minimums here for the benefit of everyone. He is another of the Free Tagline Advertisers who refuses to say whether he is paying Dave the $12.00 fee for each and every sale resulting from that Free Advertising of a For-Profit Business venture. I'd think any advertiser who is playing by the rules would be happy to say if they are paying the fee. I suppose it would be different if one had something to hide or be ashamed of.

It would seem to me that each individual on the forum can decide what they would like to post, who they share information with and what they put in their tag line.

I did not know that Karen had been made an administrator and now had say over other posters content.

Or were you just imagining that Princess?

By the way, your mailbox is full

SKB, Keith has a good point on the subject of what wall thickness is safe in front of the forcing cone. Mr. Griffin keeps saying a minimum should be .025" to .030" to be a safe minimum. No one should read and believe that. I wonder if Mr. Griffin is thinking of the choke cone area. Wall minimum thickness at the forcing cone of .025" to .030" is, of course, dead wrong, in more ways than one. It would be sad and dangerous to have someone follow Mr. Griffin's thoughts.