If the barrel is perfectly concentric to the bore especially the muzzle and and the threads are machined perfect you may use the exact specific caliber. It actually does have a sound difference and you can either remove the barrel and put it on a lathe to measure actual concentricity or if it's real bad you can notice it just by eyeballing it.
Here's a simple trick if you don't have a lathe.
Put 3 to 4 criss crossed layers of masking tape on the exit part at end of the LDC be sure it's taped fully on the face of that LDC and wrapped at the end tube portion to keep it from blowing off. Color the round edge around the circumference with a Sharpe. Measure the exact center diameter of that tape that's covering that hole from the outer diameter divide by 2 Mark it. Poke a hole the size of the pellet right there. Follow with tip of Sharpe to blacken that hole. Take a shot and see where there's still black and if it's even all around that hole then you are close. If it's off centered then that's life.
The outer Sharpe diameter circle trace is for measuring reference to the hole in the middle after carefully removing the tape in one piece.
It should give you a general non scientific idea whether or not you can go with a same caliber LDC
If it's really close it's not recommended to keep screwing and unscrewing the LDC on that specific gun and better to just EPOXY it on.
Thanks for the interesting ideas.
Re screwing on and off, I am finding myself doing that on the Marauder pistol all the time because even with the cone adapter, my DonnyFL Koi is not getting enough clearance. It's a pain unscrewing the moderator(and watching out I don't lose the o-ring) every time I fill my tank. I have had some wild flyers puzzle me, and I wonder if that's the reason.
Anyway thanks guys. My .25 moderator sounds useable at least on a .177 and on my present .22.