The original standard was set here. These choke tubes are made for a number of older and current Winchester shotguns. They are flush fit for a good look -- and tapers are optimized for the tube length to create consistent, uniform patterns. The bore diameter for guns using the Standard Invector system is considered to be the standard size and they are not back-bored in either 12 or 20 gauge.  Standard Invector Choke Tubes are not interchangeable with Invector Plus tubes and are mostly for use on earlier Model 1300s, Speed Pumps and some other shotguns provided with rifled choke tubes. Check you current choke tube or documentation or on the barrel inscription to confirm the type of choke your shotgun requires.  Average Pattern Percentages Measured using the number of pellets in a 30" circle at 30 yards. Varies with shot size and/or lead vs. steel shot. Steel patterns generally exhibit a higher percentage of pellets inside the circle. IMPORTANT: The following values are approximate. They vary between gauges and the type, length and design of the choke tube.  75-85% – Extra Full Trap55-85% – Full70-75% – Light Full45-75% – Improved Modified35-65% – Modified30-60% – Light Modified25-55% – Improved Cylinder20-45% – Skeet15-45% – CylinderVery open – Spreader