There are a few circumstances that you will encounter while learning to pick standard pin tumbler locks that are worth noting.
i. Small keyways
Common on padlocks, some pin tumbler locks have very small keyways. This can make picking difficult, so it is recommended that you dedicate some time to picking a variety of padlocks with smaller keyways. Try and stay away from the exceptionally inexpensive padlocks, as they offer very little in the way of quality and pick resistance. There are a wide variety of padlocks made by Master that range from fairly simple – in that the only thing that makes them tricky is the smaller keyway – to quite challengind.
You may find it beneficial to make your own custom low-profile picks for picking locks with small keyways, as even the smallest of commercial picks available can be awkward to navigate in a small keyway. Illus. (blah) shows a home made low profile pick made by the author from a piece of street sweeper bristle.
One more common element associated with a smaller keyway is smaller pins. This can frustrate picking by making it more difficult to lift individual pins without unsettling other pins. Again, practice is your greatest ally here.
ii. Restrictive keyways
The more wards and the deeper the wards are on a lock, the more difficult it is to navigate a pick within the keyway. Locks with this characteristic are referred to as having a restrictive keyway (not to be confused with a restricted keyway, which refers to a manufacturers regulations with regard to distribution of a particular key blank). In some extreme cases, the wards on either side of the keyway actually converge at, or cross the imaginary centerline of the keyway. This is called a paracentric keyway. These can be especially challenging to pick because it is difficult or impossible to move the pick vertically within the keyway. In some extreme cases, such as the Medeco Keymark locks, a custom pick can be fashioned from piano wire. This allows for an exceptionally low profile pick. Paracentric keyways are more common in pin tumbler locks found in Europe than they are in North America.
iii. Spring-biased plugs
Most commonly found on padlocks, sometimes the plug itself is actually spring-biased, causing the plug to be rotated back to its natural resting position by a large spring inside the lock. Though this does not produce any added difficulty to picking, it does change the feel somewhat and require an increase in tension to overcome the spring bias.