doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com What is considered safe min wall thickness?

What are the pitfalls of using a Skeets bore gauge for the inside measurement and a micrometer for the external measurement and just doing the math for the wall thickness where depth of pits are not needed?

I am sure others will chime in, but one issue is that the bore may not be concentric with the outside of the barrel.

Not just not concentric, but the inner bore, and outer wall will not be precision circles in cross section. We are talking mostly hand made guns here, the barrels hand filed and fitted by a barrel maker, who may or may not be a master of his art. Then factor in older guns, which may have been subjected to barrel repairs, bore honing etc. that may or may not have been carried to by a qualified, and competent smith. There are a myriad of conditions that will lead to localized wall thickness variation.
The current tools used to measure wall thickness aren't perfect, but are very affordable, and with patience and skill can give a good indication of wall thickness. But it's always prudent to have a competent SXS specialist gunsmith confirm your findings!

Larry;
Either I totally goofed in what I said or you misread it badly. Certainly after the max peak is reached the pressure begins to fall & does so continuously till the load exits the muzzle. The lower & higher were meant to be in comparison to the other load. If two loads give the same ballistics at different pressures the one which began with the lowest max chamber pressure will have the highest barrel pressure. Exact curves will of course vary with different loads, but about 4" from the breech is the general vicinity where the two curves will cross.
Thus to use the same two examples I used before if the MWT is out around 18"-24" from the breech the load which had 8K max chamber pressure will have higher pressure out at the MWT area than will the load which started with 10K max chamber pressure. Certainly in both cases the pressure Out there is going to be much lower than the max chamber pressure, it don't increase anywhere after th peak unless it hits an obstruction.
The point is though assuming a sound gun with ample wall thickness in the breech area with a thin area Down the barrel a ways you are not doing it any favor by lowering the chamber pressure if you do so without reducing overall ballistics of the load.
Lets say for example again that the proof of the gun is good for a load giving that 10k PSI max pressure. Say you have a load which pushes 1oz shot out a 1200 FPS at that pressure. That load will give the lowest pressure "Down There at the Thin Spot" than any load pushing that same 1oz of shot out @ 1200 FPS but with less chamber pressure than 10K PSI.

OK. Got you now. Thought you were talking about a pressure increase somewhere along the curve. Comparison between two loads of different peak pressures: we're on the same sheet of music.

This question comes up so often, and this thread will again if someone uses the 'Search' function, so thought I'd post this template I use when measuring barrels. The format changed a little from the Word doc but it might help those with gauges

Date_____________________________________

Maker___________________________________

SN______________________________________

...................................LEFT...........................RIGHT

Chamber_______________________________ ____________________________________

Bore __________________________________ ____________________________________

Choke _________________________________ ____________________________________

_________________________________ ____________________________________

Choke Length___________________________ ____________________________________



WALL THICKNESS

End of chamber ___________________________ ____________________________________
(.105�)
Forcing cone ___________________________ ____________________________________
(.100�)
9� from breech____________________________ ____________________________________
(.040�)
9� from muzzle____________________________ ____________________________________
(.025�)
MWT_________________@_________________ _____________________@_____________

Comments:

Unfortunately, when I first started measuring for guys, I gave them the sheet and didn't keep a summary but am doing so now. Here are previously posted results:

http://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=224090&page=all

Fox A grade 16ga, 26" barrels, 2-7/16" chamber, 0.085" and 0.086" minimum wall thickness at the end of the chambers, .734" and .734" diameter at the end of the chambers, 5/8" long forcing cones, #4 barrels, gun weighs 5lbs 9oz, serial 302XXX.

Parker VH O Frame 16ga, 28" steel barrels, serial 134XXX, 2-9/16" chamber, .082" and 0.085" minimum wall thickness at the end of the chambers, 0.730" and 0.732" end chamber diameters, 3lbs 2oz unstruck barrel weight, gun weighs 5lbs 14oz.

Fox XE 16ga, 30" barrels, serial 301xxx, .072" and .080" minimum wall at chamber ends, 2-7/16" chambers, .733" and .733" end chamber diameters, gun weighs 6lbs 3oz

Parker DHE 16ga, 32" steel barrels, serial 212XXX, 0.090" and 0.092" minimum wall thickness at chamber ends, 2-9/16" chambers, .735" and .736" diameter at chamber ends, barrel unstruck barrel weight 3lbs 11oz, gun weighs 7lbs 4oz.

Lefever H grade 16 gauge, 28" twist barrels, .090" and .092" minimum wall thickness at chamber ends, 2-5/8" chambers, .729" and .728" diameter at chamber ends. Gun weighs 6lbs 4oz

Ithaca Flues Grade 4E 16 gauge, 28" Krupp fluid steel barrels, 0.091" and 0.092" minimum wall thickness at the end of 2-5/8" chambers, daimeters of .728" and .730" at the end of the chambers, gun weighs 6lbs 6ounces.

Parker DH 12 bore, 28" Damascus barrels, 1 frame, .086" and .090" minimum wall thickness at end of the 2-5/8" chambers, gun weighs 6lbs 14oz. Serial #84XXX, My Skeets gauges are in capable of measuring the diameters at the end of the chamber.

Bissel Birmingham boxlock, 20 gauge, 25" steel barrels, made in the 1920s, .084" and .092" minimum wall thickness at the chamber ends, 2-1/2" chambers.

AYA #1 20 bore sidelock, factory 2-3/4" (70mm) chambers, 30" barrels, built and proofed in 2006, .092" and .091" minimum wall thickness at the end of the chambers.


Minimum chamber wall thickness
http://parkerguns.org/forums/showthread....ents&page=4

English:
James Purdey (1898) 12b., 2.5" chambers: 0.100.
James Woodward (1909) 12b., 2.5" chambers: 0.098
James Woodward (1909) 12b., 2.5" chambers: 0.103
Boss (1897-8) 12b., 2.5" chambers: 0.090
Westley Richards 12b., 2.5" chambers: 0.090
James MacNaughton (1895) 12b., 2.5" chambers: 0.095
Wm. Pape (1898) 12b., 2.75" chambers: 0.100
EM Reilly (1887-1904) 12b., 2.5" chambers: 0.091
Henri Egg (1870) 12b., 2.5" chambers: 0.108
WH Monks (1875-87) 12b., 2.5" chambers: 0.116
WC Scott (1905-6) 16b., 2.75" chambers: 0.105
Francotte (1938) 12b., 2 5/8" chambers: 0.098
Westley Richards (1905) 20b., 2.5" chambers: 0.098
WW Greener (1922) 20b., 2.75" chambers: 0.098

Belgian:
Francotte (1894-5) 12b., 2.5" chambers: 0.085
Francotte (1896) 12b., 2.5" chambers: 0.085
Francotte (1930) 20b., 2.5" chambers: 0.095

French:
Verney Carron (1950s) 12b., 2 5/8" chambers: 0.100

German:
Wilhelm Brenneke (1902) 12b., 2.5" chambers: 0.110

Some of the 12g for which I have numbers

12g 1913 Sterlingworth
Chamber & forcing cone lengthened and honed by Briley
End of chamber L .107 / R .109
End forcing cone L .097 / R .102
9� from breech L .040 / R .045
9� from muzzle L .030 / R .033

12g 1903 Smith 00
End of chamber L .110 / R .107
End forcing cone L . / R .
9� from breech L .048 / R .049
9� from muzzle L .035 / R .035
MWT L .031 3� from muzzle / R .027� 7� to 5� from muzzle

12g 1906 Smith 00
Bore .730 and possibly honed. Chamber and FC had been lengthened.
End of chamber L .112 / R .118
End forcing cone L .111 / R .118
9� from breech L .044 / R .053
9� from muzzle L .027 / R .034
MWT L .025 10� from muzzle / R .034

12g 1929 Smith Field
End of chamber L .095 / R .101
End forcing cone L . / R .
9� from breech L .043 / R .045
9� from muzzle L .038 / R .039
MWT as above 9� from muzzle

12g 1905 Remington 1894 A Damascus
End of chamber L . / R .
End forcing cone L .112 / R .107
9� from breech L .060 / R .058
9� from muzzle L .037 / R .035
MWT L .037 9" to 5� from muzzle / .035 9� to 3� from muzzle

very useful data. sounds like old ed's formula of .030 and .090 is a good guideline for what is safe and what is not.

Mark Beasland may chime in if he's not hunting or working, but I recall him telling me about wall thickness at the end of the chamber on the Verney-Carron 20ga he had made a couple years ago. I was surprised it was so thin. But that's modern metallurgy, which likely makes some difference.