Hunting- if I buy an AR15 rifle then what are pluses of 16 vs 20 inch barrel?

More maneuverability and lighter.... that's pretty much it.

20 Inch Ar15 Barrel

You typically get a little more velocity from a 20" barrel. It is thought by many that the longer barrels are more accurate. My son has a 16" Bushmaster and built a 20" heavy barreled upper. He does get touching bullet holes at 100 yds with our hand loads with the 20" barrel. I think most of the accuracy is due to the heavy barrel and the quality of it. The 16" has iron sights so I am not sure how accurate it is. I am getting ready to build an AR-15 upper. It will be a heavy barrel. I was thinking about a 20" barrel, but they really make an AR muzzle heavy. I then decided to get an 18" but I am seriously considering a heavy 16" barrel. For hunting, I don't know that the length makes much difference, but if you want to shoot longer distances, or quite a few rounds in a short time, I would get a heavy barrel. Also a free float hand guard will help a lot with accuracy.

As has been mentioned above, a 20'' barrel's rifle length gas system lessens the harshness of the recoil and improves the extraction reliability of spent cases.

A 20'' barrel also allows more complete powder burn, increasing bullet velocity. Military 5.56 rounds rely on velocity for fragmentation, lower velocities greatly decrease the effectiveness of the bullet on a live target. Higher bullet velocity also means less bullet drop over range.

You will get significantly less muzzle blast from a 20'' barrel and a longer handguard that allows for more hand positioning options.

A 16'' barrel is more handy, and some may prefer the balance of the rifle with the shorter barrel.

20 Inch Ar 15

Shorter barrels are generally lighter and easier to "swing"

Longer barrels tend to have a bit higher of a muzzle velocity. Will a paper target at 100 yards notice the difference? no but if you are going strictly for a long range gun then get the 20. If you want a plinker IMHO get the 16"

The shorter barrel might be good if you are hunting in heavy cover where the longer barrel might get tangled up in brush. You will also save a few ounces of weight with the shorter barrel. Those are really the only pluses.

the 16 inch barrel will be lighter and have better manoeuvrability due to the reduced length. the 20 inch barrel will have more power and accuracy due to the increased velocity from the bullet being in the barrel longer. so, choose the 16 inch barrel if you're planning on hunting in brushy country and the 20 incher if you're planning on hunting in wide open areas.

that is much less confusing to purchase an entire larger receiver assembly. yet definite, a pragmatic gunsmith ought to eliminate the previous barrel and setting up a sparkling barrel alongside with the different proper areas to pass with it.

Actually there's not a difference besides the weight and that's not much of a difference. In the 16" you can shoot a target just as accurate as the 20" barrel. The 20" let's you shoot at long range but like I said you can get the same results with the 16"

Maneuverability, accuracy according to some people, my dad went with a 20 inch barrel for hunting but thats just him, varies in people