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Modern 16ga shells are 2 3/4". Older shells were usually 2 1/2" or maybe 2 9/16". You can buy 2 1/2" 16ga shells from Polywad and also RST I believe.
Period number = no. of electron shells. Therefore neon has two shells.
if an element has 1 shell in its atom it is in period 1and so on...
2 shells=period 2
3 shells=period 3
4 shells=period 4
5 shells=period 5
6 shells=period 6
7 shells=period 7
2 3/4" or 3" shells
Yes, you can.