Tony,,, I have a disco in .177 and .22 and both of them wear 6.5 inch TKO's. The TKO's did a great job on both rifles and I didn't find that the TKO did better on either caliber,,, it just worked fantastic on both :)
As for chopping the barrel and threading it...... I've never done either so I'll let other, more knowledgeable members give you advice on that....
Jeff