Scott, per the Auto Ordnance web site, The Thompson T1-14 models with a barrel length of just 16.5”, it is 1.5” shorter than the existing T1 models with the 18” barrels. The BATFE recently approved the permanently affixed Cutts compensator as a part of the 16.5” barrel. The pinned Cutts compensator offered on the 18” barrel models are not considered part of the barrel.
Details here >>> auto-ordnance.com/thompson-1927a-1/

Hi Scott and welcome. Both models are the same length. The 16.5" Finned barrel is (18" with compensator). The 18 inch listed model also has a 16.5 inch barrel and is 18 inches with the compensator. The devil is in the details of how they were listed. One lists the 16.5 inch without the compensator (18 inches overall with) and one lists the barrel as 18 inches with the compensator. Both are 18 inch overall barrels. Hope this makes sense. Cheers and stay safe!

Hi Scott, the pinned Cutts compensator on the 16.5" model is considered part of the barrel., which lengthens the finned barrel length to 18”, and puts the overall length for the carbine at 41” .
The pinned Cutts compensator offered on the 18” barrel models are not considered part of the barrel. If you put them side by side, the 18" model would be 1.5" longer because the compensator is not added to the total length.
Stay safe & have a great day!

The Thompson T1-14 models with a barrel length of just 16.5”, it is 1.5” shorter than the existing T1 models with the 18” barrels. The BATFE recently approved the permanently affixed Cutts compensator as a part of the 16.5” barrel. The pinned Cutts compensator offered on the 18” barrel models are not considered part of the barrel.