1) K and L Frames
Assuming the guns have barrel lengths the same or shorter than the holster, a K will fit in an L holster. An L Frame will not fit in a K frame.
The K will be loose in an L frame holster. Do not do this. The holster should hold the gun securely. Do not be the idiot who drops his gun on the floor in public. One guy did it last week (early Jan 2014) and sent a bullet zinging through a restaurant.
http://www.digtriad.com/news/local/...-Gun-Fired-After-Man-Dropped-It-In-Restaurant
Your carry gear is not the place to skimp. I have tried the $10 Uncle Mike's holster and decided it was crap after it chaffed badly and came out with the gun on the draw too frequently.
2) Covering the sights
"This is certainly not a problem with the ramp sight on my 686 and with my 386 (which does have a tritium front sight) the barrel does not exend past the body of the holster so its not an issue for me. It doesn't look like the design could cause problems with a tritium of FO sight but without a gun to experiment with, I can't comment for sure."
The non-body side of the holster protects the sight. You will learn after you break a lamp or two, or ding the ramp. I did this several times and Trijicon fronts are not cheap. Door jams and chairs love to reach out and smack front sights. I guess some people have to learn by doing.
If you want to carry a gun whose muzzle is not covered, that is your call.
Another good feature of a holster is one that protects the rear sight. I have smacked them into things and broken lamps or moved the sight blade. Holsters that offer such protect are less common. DeSantis and El Paso Saddlery place a small tab of leather on the retention strap of their pancake holsters. I find this sufficient.
3) Hoffners Ultrux
I make my own kydex holsters. I spent quite a bit of time trying to get it to work. Either way, the attributes I mentioned make it a poor design.
4) Safety
I forgot the most important reason why a holster must hold the gun securely: safety. A loose gun can work its way out. A gun whose muzzle is exposed can be pushed upward due to pressure from body movements and objects. Other weird things can happen. Maybe two years ago, a man shot himself while sitting in his car. His gun was in a well used Yaqui Slide type holster. The pressure of the seat caused the gun to move up slightly. The worn leather was soft and worked its way into the trigger guard. The gun discharged.
http://www.itstactical.com/warcom/f...her-holsters-can-cause-accidental-discharges/
Remember, your revolver has nothing preventing that trigger from being pulled. While more pressure will be required (and the cylinder will have to rotate), never assume it will not happen. The gun could be loose enough that you feel it and you try to pish it back in without checking. In a class, I saw a guy discharge a SIG in DA by pushing rather than backing off. One of those round plastic coat stays worked its way into the trigger guard and he "pulled" the trigger by holstering.
Be careful with carry because you really can shoot yourself in the butt!