The
following is a generally agreed upon list of shotgun TRUE
BORE DIAMETERS all gauges (NON-BACKBORED BARRELS) and a Iist
of generally accepted choke constrictions (IN THOUSANDS OF
AN INCH) for the various named choke constrictions.
(EXAMPLE: You have a 12 gauge shotgun with a true bore diameter
of .729 and have .019 choke constriction--therefore you have
a MODIFIED choke with an inside diameter of the the choke
to be .710.)
MISCELLANEOUS
1. Non-American shotguns (12 GA.) normaIly have a true bore
diameter of approximately .005 less than US barrels or generally
have a true bore diameter of .725.
2. Back bored barrels (12 GA.) usually have a true bore diameter
of .740 to .745. |
| Gauge |
10 |
12 |
16 |
20 |
28 |
410 |
| True Bore Dia. |
.779 |
.729 |
.667 |
.617 |
.550 |
.410 |
| Cylinder Bore |
.000 - .779 |
.000 - .729 |
.000 - .667 |
.000 - .617 |
.000 - .550 |
.000 - .410 |
| Skeet I |
.005 - .774 |
.005 - .724 |
.004 - .663 |
.004 - .613 |
.003 - .547 |
.002 - .408 |
| Improved Cyl. |
.010 - .769 |
.009 - .720 |
.007 - .660 |
.006 - .611 |
.005 - .545 |
.004 - .406 |
| Skeet II |
.015 - .764 |
0.12 - .717 |
.010 - .657 |
.009 - .608 |
.007 - .543 |
.006 - .404 |
| Modified |
.020 - .759 |
0.19 - .710 |
.015 - .652 |
.014 - .603 |
.012 - .538 |
.008 - .402 |
| Improved Mod. |
.025 - .754 |
.025 - .704 |
.020 - .647 |
.019 - .598 |
.016 - .534 |
.011 - .399 |
| Full Choke |
.035 - .744 |
.035 - .694 |
.028 - .639 |
.025 - .592 |
.022 - .528 |
.015 - .395 |
| Extra Full |
.040 - .739 |
.040 - .689 |
.035 - .632 |
.027 - .590 |
.024 - .526 |
.021 - .389 |
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