Newbie here and this is my first build. I have Aero upper and  lower, RRA LPK 2stage match trigger RRA Operator stock . I am looking at the Green mountain 18" and 20" fluted stainless barrel. Rifle is to be used for varmint and Coyote. Which barrel would I get the best bang for the buck?

Either one would be fine I'm sure. The rifle length has system does work just about perfect from a mechanical/engineering standpoint in a 20 inch barrel.

I would choose the 18" due to shaving off a little bit of weight on that heavy set up.  Plus it would be a little easier to maneuver.  The velocity loss is very very minimal between the 2, and I am wanting to build up an 18" with a Faxon Gunner barrel myself.  The difference between the 18 and 20 is less then 100fps and probably closer to 70 or so fps.  If it was the difference in a 10.5 vs 12.5 barrel that is a world of difference due to a lot larger velocity increase.  But there just isn't very much difference with this setup.  So 18 rifle length gets my vote.  Good choice on Green Mountain!

Right....18" to save a little weight unless you really want that 600+ yard varmint.

Either will work fine.  I kinda like the 20's right now.

muzzle lift with the 20 is almost non existent. if that is important then go that route. reaquirred 2nd shots are smooth. here in ny we don't get to use a muzzle brake. I have a 20" shilen match grade barrel and it is very nice.

My 18" ARP is spot-on out to 700 yards.

Every GM barrel I have tried has been excellent...my go-to rifle has the 16" GM-M8 barrel and I can go 600 yards with it, so I think you will be fine with either one.

Faced this same dilemma recently and I opted for the 18". Figure by the time I add a muzzle device I'll be at 20", don't wanna push it any further. Plus, for the type of shooting in do, another 2" isn't going to make or break me.

Thanks for the input guys. I currently hunt with a RRA ATH carbine which has an 18"heavy barrel 43oz. according to their web site, and is a tack driver. The GM 18" & 20" fluted barrels weigh 33.6oz & 35.2oz .so they are both lighter than what I have now. I am leaning towards the 20"for a little more bullet stability / accuracy. I don't plan to shoot 600-700 yards, Hell my old eyes cant see that far, but I will be attempting 3-400 on prairie dogs. I think the extra 2" might help.

My gun? 20" and no fluting. In fact I have 3 uppers that are exactly that. I also add about 7" worth of muffler on the end of mine .

However, this is your gun, so build it to YOUR liking. In all fairness, though, you didn't mention caliber (or I missed it). In typical loads with the typical military cartridges, 20 doesn't really gain much over 18. If you're shooting something hotter, like "match" ammo, then you'll start seeing the benefits of the added length.

My build is .223. I don't reload yet but want to with this build.

I concur on the Green Mountain barrels. I currently have two and they are superb performers. My 20" GM barrel is also spot on at 500+ yards
V

I would recommend going with a 18" barrel, BUT if you already have a 18" barreled rifle then maybe go with the 20".  I think you'd be happier having two different size barrels then having the same length you already have (just lighter).    

went shooting yesterday. let my wife shoot my 20" shilen ar. 300 yds hitting the gong,  second time she shot a ar 1st time she shot this one. me I had to final site my new scope in, at 100 yds sub moa. to the point going through the same hole.

shilen barrels are a heavy barrel but not a bull barrel. a lot light than some out there. was so impressed ordered a 24" for my 308.

At 300-400 yards that extra 2" isn't going to be noticed.  I am not going to stop you from getting the 20" or say you are making a mistake, because both barrels are nice barrels and will serve you well.  But I imagine it would probably be hard to even notice much of a difference with a 16" over 20" at that range.  The 18"s just don't loose that much velocity from the 20" and you wouldn't notice any more difference between the 2 then you would switching to a different brand/load of ammo.  Also the 20" is not more accurate then an 18" - that is a myth that most people believe.  A shorter barrel is not less accurate then a longer barrel - and often the shorter barrel is more accurate due to stability/rigidness.  The less a barrel can flex, the more accurate it can be and you will often hear the term barrel whip.  So a long skinny barrel is going to be less accurate then a short thick barrel on average.  With having 2 barrels of the same heavy weight profile that are identical just different lengths it is doubtful you would ever be able to tell the difference even in a group size of 10 barrels from each.  But if there was a difference, the nod should go to the shorter barrels.  

The thing that makes a longer barrel nice for shooting longer ranges isn't the accuracy it is the velocity.  The more velocity you have, the less bullet drop and wind drift you have to worry about.  And at very long ranges the extra velocity will keep your bullet supersonic for a longer distance meaning it will stay stabilized for a little bit longer.  How much depends on the barrel length in question and what ammo will be used.  In this case the difference in a 18 and 20 is almost nonexistent especially at 300-400 yards.  At 500-600 you may notice a slight difference but even then it would be hard to notice.  With the lightweight bullets you gain about 75ish fps and if I had to guess with the heavier 75-77 grn bullets that would likely be used you would probably see a lot less difference maybe 50fps or even less.

as has been said already, unless you're going to be stretching 5.56 to it's limits for long range shooting then get the 18".  

whichever you think looks better

I built a 20" 556 using a Faxon barrel a while ago, and a good friend of mine purchased it from me.



I've also done more 18" guns and will be finishing another 18" rifle soon.

18" barrel gets my vote.