I haven't done any work with the 709, but as an "over the top" cast bullet shooter, I have made chamber casts of my 738B, PT111 Pro, and PT138 Pro and rendered the dimensions to very detailed drawings. You will see that my bore measurements are the same leading me to believe that any 9mm or 380 Auto is barreled from the same stock.
I have done this work with all the Taurus semi autos except the 40 S&W and the 25 ACP, 32 ACP, and 45 ACP are over bore as well. Some manufacturers use this, over boring, to help reduce pressure at the shot but it is only speculation as to what is on Taurus' mind. I see a lot of comments about factory ammo not producing the velocity specs, even with like length barrels used in the figures and this is why, the barrels are dumping the pressure behind the shot.
Here are the cast bullet sizes that I recommend for the various barrels
- 25 ACP - .256"
- 32 ACP - .314"
- 380 Auto - .358
- 9mm Luger - .358
- 45 ACP - .457"
This is what the barrel of a 25PLY looks like with a Speer .251 Gold Dot pressed into it. It could be any of the barrels I've checked but it is easier to get the bullet in and out of a 25 caliber barrel.
If you want to tighten up the performance of your Taurus semi auto, both accuracy and velocity, shoot a properly sized cast bullet.
As a note, the barrel stock that Taurus uses on the 357 Mag, 41 Mag, 44 Mag, and 45 Colt is appropriate for the caliber and matches the stock used in the Rossi rifles. Of course, they have the cylinder gap to relieve pressure in the revolver applications.