ar15 A2 rifle barrel length
 

What is the standard length A2 style barrel? I just got my firearm and I want to change the hand guards to ones with rails. Do I just take my existing ones off and measure the length or measure the barrel length?

Measure the handguards, should be 12" for rifle length.

those definitely look like rifle length in the picture, looks like you have a 20" barrel.

there's 3 standard size lengths that I'm aware of. carbine and mid-length, both typically have a 16" barrel unless LE only or if it was sold as a pistol, and then the full length or rifle length hand guards, which are typically on the 20" barrels.

I don't know how far the barrel threads into the upper, but it looks like it might be a 20" barrel. I was thinking about changing the barrel out at some point to a carbine 16" but for now the hand guards with rails will suffice. I am changing the A2 stock to an adjustable. I believe I will need:
stock, buffer tube, buffer, spring and end plate all for the adjustable stock...and of course it's going to be MILSPEC.

Barrel length and gas system length are 2 different animals. A standard A2 style AR would have a 20" barrel with a rifle length, 12" gas system but that doesn't mean you have that.

Gas system are generally described as rifle length 12", mid length 9" and carbine length 7". Standard handguards that require the delta ring and hand guard cap will need to fit the gas system length...free float hand guards with a low profile gas block do not, that is you can mount a 13" handguard on a carbine gas system if the tube fits over the gas block.

If you just got it what did you buy? The shipping info with the rifle should describe both the barrel length and the gas system length. 20" a barrels are almost exclusively 12" gas systems but not gonna guarantee that...all types of hybrids on the market today.