Max fpe for shooting indoors at a farm

If mixing calibers, the easiest and most relevant factor is FPS. At the same fpe, the .177 will be more apt to penetrate than .22. They are more similar in penetration at the same fps.

Head type is also important. Flat nose (wadcutters are less likely to punch through than domes.

Lead less so than alloy. 

Fortunately you don’t need much to take down birds cleanly. Inside of 20-30 yards, a wadcutter at 500-600fps is plenty. The trick isn’t finding an accurate wadcutter at those distances...inside a dime, for example. In my experience, the RWS Meisterkugelns and often times even the Basics fit that description. 
 
I'd go with 12 fpe in .22 for indoor shots. Make sure of your back drop! Even at 12 fpe the pellets will pass through the small birds.
The key with permissions is not to take chances with possible damage to their property. (Even if they say they don't mind)
Don't do it!
Surgical extraction is the best! Improves your skills, and makes the owner more likely to give you a good recommendation to others who may be on the fence about allowing shooting on their property.
Even on outdoor shots.

Always remember that what goes up will come down! Be sure of your pellet flight path!

Ricochet, or deflection of the pellet should always be a consideration when taking a shot.

Shoot safe!
And Have FUN!
 
Bob's right on anything much more then 500 is going to be a problem. I tried a Benjamin pump and that didn't work. It just wasn't accurate enough for me. I'm using a HW urban pro but I shot all the pigeons with the Benjamin ( I missed a bunch but hit them eventually). My urban pro is plenty accurate enough but it's likely to be a little hard hitting. I took a coil out of the main spring and I plan to chronograph it. I may take another. I ordered a tune up kit with a new spring so if I go too far I've got a back up. Lots depends on materials in the barn. Obviously aluminum is going to be very easy to punch thru. It fact you probably don't want to even chance shooting at anything with aluminum in the back ground. Even at 450 to 500 FPS if you miss and hit the metal it will crack pretty resoundingly and probably shoot thru aluminum .



ALSO watch for bounces and ricochets at those lower velocities.



Keep us informed as to how you make out. I don't know it there is much difference between a 177 and a 22 in this application. Seems like the 177 might penetrate more but I don't know if its significant or not.