Given the choice I would go with the easy to screw in nipples.
They are less likely to wear out the soft steel of the breech where the nipple screws in.
When I mentioned that a similar sized thread to the ones that easily screw in that takes a bit of wrenching I was thinking you had a nipple with similar threads but with a #11 cap nipple on it.
If you do have this combination I was just suggesting that it could be used if light wrenching is all it takes to install it.
If you do decide to go with the tighter fitting nipple, put a drop of Anti-Seize on the threads to ease installation and to prevent galling.
You can buy a tiny tube of this Anti-Seize for a couple of bucks at any auto supply store.
They are less likely to wear out the soft steel of the breech where the nipple screws in.
When I mentioned that a similar sized thread to the ones that easily screw in that takes a bit of wrenching I was thinking you had a nipple with similar threads but with a #11 cap nipple on it.
If you do have this combination I was just suggesting that it could be used if light wrenching is all it takes to install it.
If you do decide to go with the tighter fitting nipple, put a drop of Anti-Seize on the threads to ease installation and to prevent galling.
You can buy a tiny tube of this Anti-Seize for a couple of bucks at any auto supply store.