THe barbs are designed to go into tubing. So a 1/2" barb fits into 1/2"ID tubing making its OD 1/2". The barb's walls have some thickness, so that means that the ID of the barb is less (yes a stock 1/2" hosebarb is close to 3/8" ID). That's why many of us either:
- Use 5/8" hosebarbs (their inside diameter is right at 1/2" and you can still get tygon and clearflex or silicone over them nice and snugly)
Use copper pipe fittings instead of barbs for much nicer IDs
Use a drill press and open up our 1/2" hose barbs more for higher flow.
To FURTHER add to the confusion, Eheim pumps are european, so the barbs aren't exactly 1/2" and the threads aren't NPT. I have found that a 3/8"NPT x 5/8" barb will go in ok on the 1250 outlet, but use silicone "Goop" and DON'T tighten it all the way in. You will crack the housing if you do this, especially with a brass barb but even with nylon ones. I had to JB Weld a friend of mine's pump back together the other day who had done this.