İs there Penetration difference between 18.3, 18.4, 18.5, 18.6 etc. Bore diameter shotguns.
Penetration depends on pellet speed, size and, to some extent, hardness, To the extend that bore diameter affects pellet speed, bore diameter will affect penetration, all else remaining the same.
There are two schools of thought on whether backboring or overboring affects velocity. If you listen to the manufacturers, you will hear that overboring lowers friction of the shotload and wad in the barrel and this increases velocity. They also argue that recoil is reduced with larger bores, but if Newton has anything to say about it, when velocity goes up, so does calculated recoil.
Obviously, if you overbore too much, the shot wad won’t seal properly and everything goes to pot. On a cold day, I tested an .800″ bore Baker barrel which did just that with a certain shell.
The other camp holds that a larger bore diameter lowers velocity because it increases volume and this lowers pressure and hence lowers velocity. Lower velocity does lower recoil.
The only way to know for sure whether a change in bore diameter will change velocity is to measure the velocity of a certain gun with a certain shell. Then backbore the gun to a new barrel diameter and remeasure with the same shell. I’ve not personally done that, so I have no facts to back up my opinion.
I do know from personal experimentation that tightening a choke will increase velocity. It works the same way a garden hose works. The pressure in the water pipe doesn’t change, but when the nozzle of the hose is full open to a wide spray, the water doesn’t go very far. Then the nozzle is tightened down, the water comes out faster (though still with the same basic water pressure) and the water squirts further. Works the same way with a shotshell. I’ve seen differences as high as 50 ft/sec going from Cylinder bore to Full choke.
But that’s an aside and not what you asked. I don’t absolutely know the answer to your question, but I lean towards larger bores lowering velocity due to lowering pressure, not increasing it due to lowered friction. But take your pick.
Perhaps one of our knowledgeable readers will clear up my waivering with some hard facts.
Shotgun Report’s Technoid