In case any new shotgunners are out there reading this ...thinking some guns are strong enough to do this ....even once in a while.......( its not ok, to do it at all !! ). Noone, with any sense, I hope -- would want to intentionally fire a gun -- shotgun, handgun or rifle -- with a partically blocked chamber, forcing cone or barrel ....and think its ok ...!
Shooting a 3" shell in a gun chambered for 2 3/4" shells...you end up shooting a shell in a gun with a partically blocked chamber, forcing cone and/or barrel...
The petals on a shell, depending on the brand of shell, are about 3/8" long....so that is why a shell called a "3" shell" ...unfired is only about 2 5/8" long unfired...( 3" is the fired length ). So that is why an Unfired 3" shell ......can be unsafely loaded into a shotgun with a 2 3/4" chamber...
I have a buddy that owns a used gun shop and sells a lot of used shotguns....and this issue of chamber length comes up all the time - he says at least once a day ( espeically on older field guns ) / and I am always shocked when a buyer implies that checking the length of the chamber in a gun is best done by just dropping an unfired shell in there...and thinking that is how its measured --- its not !!
( and a little off topic..of 12ga shell length ) ...its also possible to drop a 20ga shell into the chamber of a 12ga...and the 20ga shell will drop all the way down and stick in the opening of the barrel...and it goes down far enough ...to allow an unfired 12ga shell to be loaded in behind it ---- in effect creating a bomb ..if that 12ga shell is fired with a fully blocked barrel with that 20ga shell. That is why in the field...or even on sporting clays fields...I would never allow anyone to carry both 12ga and 20ga shells in their vest ( maybe a new shooter is shooting the 20ga / so the other shooter carrying a 12ga...says they will carry all the shells ( 12ga and 20ga )...its a recipe for disaster ! Because in the heat of the moment ...its easy to forget ...and drop the wrong kind of shell ( a 20ga ) into a 12ga.
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While I understand some guys might think firing 3" shells in a gun chambered for 2 3/4" might be ok once in a while....like others have said....its a ridiculous risk to take !
The same risk applies on all gagues of shotguns...( 12ga, 16ga, 20ga, 28ga and .410 ) ....you need to match the shell to the chamber length of the gun. Some guns, especially field guns may have 3" chambers...or even 3 1/2" now...but there are a lot of "target" guns out there with 2 3/4" in 12ga thru 28ga / or 2 1/2" chambers in .410..../ read the markings on the barrel...and if the barrel is not marked..talk to a gunsmith or an experienced shooter that understands how to measure the chamber length --- please don't guess !!