Fair. I'm not going to bother giving and exact measurement as finding one of my dozen tape measures is a bit of a pain in the as s, but the term "standard length" is vague, opaque.
I should have used the term Original Equipment Manufacturer (OEM), or factory stock. A term that would have lead the reader to understand that whatever length of magazine tube the gun originally came with is designed to hold no more than 4 shells of whatever the gun is chambered for. 3" chamber in the case of the Beretta 1301. So 4+1=5
From factory, there will, or ought to be an installed plug, a long plastic rod that restricts the magazine to no more than 2 shells of whatever the gun is chambered for. This is done to comply with migratory bird hunting regulations that themselves are in place to mitigate the injuring of birds while hunting. This "plug" I refer to as a "hunting plug".
From factory, there will or ought to also be a magazine spring retainer. This prevents the spring from resting on the magazine cap, permitting the removal of said cap without concern of the spring pressure launching the cap into someone's face, across the field, or lodged into drywall. The spring retainer also assists with being able to thread the cap back on without having to fight with, struggle against spring pressure.
Personally, I take these retainers out and store them with my magazine plugs (as well as caps if/when I put extensions on the magazine). In my mind it allows a little relaxation in the spring which equates to longevity and freedom of movement in the magazine.
If you can't put 4, 3" shells in the OEM magazine with the hunting plug removed, try to carefully remove the spring retainer. Shotgun shell hull length is measured from expanded or opened, not crimped, and there are variances in crimping. These variances can result in a really tight fit when loading the magazine to it's designed maximum. The tolerances in the magazine can be less forgiving than the tolerances in the hull crimps. The 1/4" to 1/2" gained by removing the spring retainer can be the difference needed.
Now, if you want to forgo all that bullshi t, get a Canadian legal +1 extension and use it to replace the OEM magazine cap. The extension allows you to remain compliant while allowing an additional 2.75" shell.
I hope that helps...officer.
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