doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com How thin is to thin?

Frank,
Can you provide any information on what the inside of the choke profile looks like in this gun? (where the taper starts, how much choke etc.)

Had a Greener once that did the same thing--long before I had a wall thickness gauge. It is since long gone.

Bores .724/.725", chokes ~ 4-1/2" long. Both as normal for early Fox 12 ga guns. Both chokes were opened from .039/.040" to .025" with a parallel section at the muzzles; choke work was done before this happened. Parallel sections were probably about 1" long which is about where the bulges became apparent but they may have started farther back from the muzzles. Walls were measured with a good Galazan vertical wall gauge. All the preceding from my records.

This did not result from stuck wads in one or both barrels.

Sorry, I can't supply more info because the ice is too thick on my neighbor's pond and the water is too cold to go diving for the barrel that's been there since I tossed it. RIP. Lesson learned on avoiding thin walls at the muzzles, on choked guns you really want to shoot more than a few shots per year..

That's scary as fuk!

I know of genuine original H&H barrels struck down to 0.019" to achieve weight balance as new.
But this gun was intended for use in the UK with normal UK nitro 2 1/2" loads. Nowadays, guns could be subjected to all sorts of pressures/loads in different countries: from high brass tooth-rattlers to gentle low pressure and no doubt H&H's recommendations reflect this.
I have successfully re-proofed Damascus down to 0.014" and, although I would not recommend this, it was safe to shoot afterwards.
When considering the location of thin parts of a barrel, it should be noted that although a thin patch can be anywhere, in a post 1880 English gun it is most likely to be found at around the half way mark.
Barrels made after the early hammergun era and beginning of choke era, were generally struck up with the 'Eiffel Tower' profile: ie. wide at the breech (to contain pressure), thinning toward the half-way mark (to save weight and aid 'balance') and then thickening (albeit rather less) towards the muzzle (to protect from dents).
A muzzle area as thin as 0.018" would be most unusual in my experience but I agree with JDW that pressure there is so light as to be unlikely to be the lone cause of that ring bulge.

Some new scary stuff here Jay
http://www.picturetrail.com/sfx/album/view/17546456

Don't confuse Max pressure with muzzle pressure. What the maximum chamber pressure is has little to do with remaining pressure at the muzzle. In fact if you push the same shot load to the same MV with two diffwrent loads the one having the lowest max pressure will almost certainly have the highest muzzle pressure.
"In My Mind" these bulges in the choke confirm that some component of the load, be it the shot charge itself or merely the wad, but some component is "Checked" just enough in its velocity to produce a pressure spike. This presents a situation just like an obstruction, though the load itself is the obstruction. This would occur irregardless of the wall thickness, though it doesn't become a problem until the bbl wall gets excesssivly thin.
I cannot say this is universely followed but I have checked a good number of guns on which the bbls were sort of "Swamped", ie the od was slightly larger starting a bit behind the chokes giving an increased thickness going into the chokes.
I would personally prefer to have that .020" thickmess at around to 18"-22" point than either near the muzzle or much closer to the breech unless the gun is a cylinder bore in which case it could extend all the way to the muzzle.

My theory is that it was mechanical constriction of the payload (wad-shot) that bulged Frank's barrel, not gas pressure.

Just a swag, but it would take about 3200 psi to yield a .700 bore/.018 wall 4130 normalized tube. I don't think that's likely. The tube is more likely than not thinner just behind the max constriction and therefore more likely to yield there from gas pressure, rather than at the maximum constriction/thickness area. Also, if the payload suddenly slowed at the choke, the payload itself is something like 3/4"-1" long and that would put the base of the wad behind the maximum constriction/thickness area where it would exert any spike in pressure (and therefore, bulge) from a theorized gas dynamic pulse.

As usual, Toby posted pretty much exactly the points that needed making, so I'll endorse them rather than post them again. Saved me a job!

Not necessarily. Even if we consider a leisurly MV of 1100 FPS & a total load length of 1.5" (probably both faster & shorter in actuality) the the load is moving at 13,200 inches per second. At this rate it would take .000114 secs for the total charge to pass a given point. Tests conducted with intentionally obstructed bores have definitely proved the bulge or burst usually occurs at a point farther along the bbl than the obstruction. I do not believe that gas hammer should automatically be ruled out here.