Chris:
This testing is for 12 bore. But empirical results appear to go for small bores too:
"Long Shells in Short Chambers”, Sherman Bell with technical assistance from Tom Armbrust in “Finding Out for Myself” Part V, Double Gun Journal, Winter 2001
The test barrel was a ‘heavy’ Krieger barrel with piezo-electric transducer at 1” from the breech and an Oehler velocity gauge 2 5/16” from the breech.
Tests were performed:
1. 2 1/2” chamber with a 7/16” forcing cone – “British chamber” (BC)
2. 2 1/2” chamber with a 1 1/4” forcing cone – “Modified British chamber” (MBC)
3. 2 3/4” chamber with a 1” forcing cone – “American Chamber” (AC)
Each load was tested in each chamber at least 5 times.
No significant change in velocity was noted between the 3 test chambers
It was not unusual to have 300-600 psi spread between shots
LOADS
1. 7/8 oz. in Federal plastic hull at 1185 fps in the 2 3/4” chamber
2. 7/8 oz. in Estate plastic hull at 1206 fps
3. 1 oz. in Federal paper hull at 1137 fps
4. 1 1/8 oz. in Federal paper hull at 1120 fps
5. 1 oz. in Remington plastic hull at 1205 fps
6. 1 1/8 oz. in Winchester plastic hull at 1186 fps
7. 1 1/4 oz. in Federal plastic hull at 1091 fps
8. 1 1/4 oz. in Winchester plastic hull at 1136 fps
9. 1 1/8 oz. with GOEX FFFg in Federal paper at 1184 fps
10. 1 1/8 oz. Bismuth in Federal plastic at 1091 fps
I only include the standard target or game loads that were tested
……..BC……...MBC…....AC…..Difference BCvsAC
1…..5725…….5905…..4834……891 psi
2…..6225…….6065…..5009……1216 psi
3…..5865…….5325…..5263……602 psi…..“long cones do reduce pressure”
4…..5845…….5665…..5058……787 psi
5…..7805…….7845…..7577……228 psi
6…..11,125….11,045…10,787….338 psi
7…..6892…….6805…..5953……939 psi
8…..8765…….8545…..7870……895 psi
9…..5405…….5265…..4603……802 psi
10…6765…….6365…..6380……385 psi
Interpretation and caution:
1. Results in light British 12b or American small bore barrels may be different.
2. Results in YOUR barrel would require drilling the chamber for the transducer thereby destroying your gun.
3. Paper hulls with star crimps showed little difference compared to plastic hulls. NO roll crimp paper hulls were tested.
4. The increase in pressure comparing 1 oz., 1 1/8 oz., and 1 1/4 oz. loads was similar.
5. Longer cones MAY reduce pressure. This was NOT a comparison of standard 1” with 2 1/2” cones however.
6. “Shooting 2 3/4” shells in 2 1/2” chambers does make them produce more pressure-but in most cases it is less than a 1000 psi increase. I see no reason, related to safety, to modify an original 2 1/2” chambered gun to shoot 2 3/4” shells, if the 2 3/4” load you intend to use would develop pressure that is safe in that gun, when fired in a standard chamber!”
7. “We found that lengthening the forcing cone in a 2 1/2” chamber usually helps mitigate the pressure increase that comes from shooting 2 3/4” or 3” shells in the short chamber.”
8. “Shooting heavy 2 3/4” loads in a light gun is a dumb thing to do. Shooting 3” magnum duck loads is even dumber.”
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