I bought M5 bolts with barrel nut a year ago. Then I bought new M5 bolts a few days ago to replace the old ones. The new bolts do not seem to enter the barrel nuts. Upon closer inspection it seems that the thread on the two is not matching.
When I look for bolts I get size like M5x70mm. But it never says anything about the thread size in the description. Are all M5 bolts supposed to have the same thread size?
Metric, like SAE, describe bolts and nuts in form of base diameter x thread pitch.
For M5 nuts/bolts, there are 2 main pitches, .5 and .8 mm. With the .8 being most common by far. The pitch in metric is the actual space between threads, while with SAE, it is expressed as number of threads in 1 inch.
With all that being said, almost every hardware store will have a sizer board where you can get the exact sizes of your bolt or nut.
While metric fine threads exist, you won't find them in furniture. Even in industrial quantities they're far more expensive and reserved for special purposes.
It's much more likely than one of your screws isn't M5. 10-32 UNC is very close, and more common in consumer goods than metric fine, even in metric countries because of legacy designs and international markets. M6 could be confused with M5 if you're not familiar with them, and is very common on furniture fixings. M6 is also close to 1/4-20 UNC.
