Here are my findings concerning the Century Tantal keyholing issue.
When the Century Arms International Tantals were originally introduced (in early 2007) they were using U.S. made .223 barrels with 1x9 twist and had "TTL' serial numbers built on D.C. Industries receivers. Over the next 18 to 24 months, Century started experimenting with various twists (i.e. 1x9, 1x7) in "TTL" and a few very early "Y" serial numbered weapons using D.C. Industries and NoDak receivers and still had problems due to the .223 bore of the barrels. In approximately May/June 2009, Century started using the correct 5.45 barrels with 1x8 twist barrels all using NoDak receivers with "Y0020xx" serial numbers.
My Tantal is a "Y0020xx" serial number NoDak receiver built in Aug 2009 with a correct U.S. mfg. 5.45 barrel w/o chrome lining (I believe it measures .219 or .220) with 1x8 twist and has not had any keyhole problems using surplus corrosive or modern ammo.
From all indications and reviews, the new "TTL-C" serial numbered weapons appear to be using the correct 5.45 barrel with 1x8 twist. My simple rule of thumb...the "C" in the serial number stands for "correct".
As a side note, I purchased an early D.C. Industries Tantal complete receiver (TTL006xx) from an individual that was going to change-out the original .223 barrel, but lost interest. I used that receiver to finish building an older original "matching numbers" Tantal parts kit with factory chrome-lined barrel. She is most definitely an accurate shooter and loves any ammo I feed her & no keyhole issues.