http://www.mcace.com/shotguninserts.htm
Quote from above link...
"Our insert barrels have a standard length of 10 inches and are $65.00 each. This is an excellent length for many calibers, particularly the 22LR, 22 Mag, 32ACP, 38 Special, 45 ACP and 45 Long Colt. Pistol barrel lengths in these calibers are often less than 10", yet they still achieve excellent results and great hitting power. Six inch shotgun inserts shoot better than pistols of the same length.
Longer insert barrels are available in all calibers. While more costly, they have several advantages. The longer barrels develop more velocity and hit harder, while reducing the noise of the shot. The 32ACP, 38 Special, 45 ACP and 45 Long Colt are surprisingly quiet in the longer barrels. The extra length is particularly useful in calibers such as the 45-70. The use of the 45-70 inserts makes an effective double rifle or combination gun at a very reasonable price."
This is an interesting idea for quiet loads without a suppressor. It would make sense that using a 10" pistol caliber insert in a 26" 12 gauge barrel would allow the area forward of the insert to act as an expansion cahmber and cool some of the gases which would ordinarily be heard as muzzle blast from the pistol round. Sort of like a 6"x.72" suppressor with out baffles or an endcap.