I consider a 'sidearm' a pistol or a compact SMG that is stored in a holster, anything else is a secondary. They are almost the same thing, especially in airsoft where most people use their secondary'(s) as a sidearm.
Well they can be completely worthless, or they can be your best friend. I say completely because some people learn to reload alot faster(with AEG's) among other things.
You could basically toss that aside since were talking about sniping however I thought it should be known.
As far as personal experience goes my pistol is my CQB/urban gun of choice and my trustful backup during ambushes. I could probably bore you guys to death by writing paragraphs on why I do this, but I will save you the time and dumb it down alot........
The pistol when used correctly is the most maneuverable weapon in airsoft, therefor it is a prime weapon for smaller areas(ie. ambushes and CQC)........ Done.....
When you purchase a pistol go name brand. Some really good brands are KSC, KWA, Tokyo Marui, Western Arms(WA not WE), and KJW. When you purchase with one of these brands get the brands mags, Ie TM hi-cappa gets TM mags Not WE hi-cappa mags. This will ensure better consistency and quality. Also I made the mistake of buying a gun with $40 mags a pop, make sure you really like the gun, then research some other stuff about it such as mag cost and reliability. I have found that there have been very few situations where I have needed another mag, in the past my extra GBB mag money has gone towards more games
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Thats pretty much all you need to know about side arms. Secondaries have a totally different thought process. I'll let someone else fill you in on that stuff.
sticks