I have a Twister with a Tower 40 (OS 40 FP ) engine with a #843 muffler on it. I want to replace the existing muffler with a tongue muffler with a pressure fitting on it to reduce weight on the nose. Which ones work best on this engine? I have seen different ones with one set of holes and others with more rows of holes. My LHS doesn’t have them so where to buy? Do not mind a used one either. Thanks, Tim

How much weight are you trying to save?

It's true that the OS muffler is quite heavy but if you do not need to reduce the weight to the full extent of the tongue muffler, I suggest the Randy Smith tube muffler is worth a try.

I have an ARF Cardinal that has an FP 40 that is a gem. It has a 6.75mm ID OS venturi, Bob Zambelli hemi head and the  Randy Smith muffler with no other modifications.

It draws positive comments for the engin runs whenever I take the model out.

Will do. Gill
And Perry said my Twister is a little nose heavy, but flies great, so I’m just trying to keep my pressure tank and lighten the nose with a tongue muffler. Tim

You could first use tail weights to see where you want the cg to end up.  When you find that spot, remove the tail weights and muffler and see how that compares.
You might find out that moving the engine back is a better solution. Or a lighter engine.
Or the tongue muffler and a little tail weight.
Lots of options for you.
A slightly nose heavy first plane isn't the worst thing.