If one were to build a carbine in .260, at what barrel length would the long range ballistics start to really take a dump?  I'd like a rifle that could easily be called on to hunt deer at 200yd max ranges (more realistically here in MO, <100yds), and also still have good long-ish range ballistics for target shooting.  I know a lot of guys shoot 16"-18" .308 guns at long range, and that barrel quality is an important factor.  I like the flatter trajectory and better BC of the .260 projectiles.  What do you guys think?

For a short barreled hunting rifle most people go .308 or larger because of terminal bullet performance. Bullet performance/ expansion tends to be a real problem when you  below 7mm diameter.
Look at it this way, in WW2 there were very few countries who used a cartridge in their main battle rifle smaller than 7mm, and most of them were .308 and larger. This is simply because larger diameter bullets kill better and in expanding type bullets this is especially true because larger bullets expand more reliably.

While it is true that larger calibers over 7mm tend to expand easier when using expanding projectiles, the .260 Rem. has plenty of gusto to get the job done. The 18" bbl. Should be
just fine, you will only loose about 100-150 fps vs. the 20-24" version...

Just remember, accuracy beats horsepower (within reason) and lower recoil aides in accuracy...I took a spike buck @ 110 yards w/ my 223 based 6.5mm PCC round w/
complete penetration and expansion of the 120gr. Speer HCSP.

Good Luck

What are you talking about?

US troops were very likely to face 6.5s in World War II either in the form of
Italian 6.5x50mm Carcanos or 6.5mm Arisakas......Quite a few countries fielded
6.5mm weapons in both World War I and World War II.

And what is this about bullet performance/expansion tends to be a real problem when you
go below 7mm? This isn't the 1930s....modern high quality hunting projectiles perform
very well.

Roger that.  If you are building a carbine, might as well do a Grendel if you're looking for a shorter barrel, lightweight carry gun for hunts.  As far as good expansion goes, check this out:

Posted by one of the guys on the Grendel Forum.  120gr Nosler Ballistic Tip, 197yds, shot through rear of deer, ended up at the base of the neck.  Final weight: 89.1 grains:

http://www.65grendel.com/forum/showthread.php?1531-Nosler-120-BT-30.0-grns-748...-Deer-at-197-Yds-74.25-retained-Bullet-weight

You might note that those using 6.5mm weapons were the loosers and they were also a great minority of the weapons fielded, though I realize you're just being argumentive.
The top battle rifles of WW2 were 1903 Springfield 30-06, M1 Garand 30-06, 1898 Mauser 8mm, and Brittish Enfield 303 not necessarily in that order, don't forget the 7.7 Jap too.
The afformentioned rifles .308 through .321 diameter.
I've also shot quite a few head of game and seen many more that were badly wooded by poorly performing bullets, most of them in the smaller caliber range. I've only been hunting since 1969 so I don't know about the 1930's. JMOFWIW

.30 cals did not win WWII, tons of bombs & artillery did. When you look at how late our guys hit the ground in the European theater, the amount of thermite that we dropped on Germany and Japan, and the fact that the majority of the Wermacht was kicking the crap out of Russians in Eastern Europe, the role that small arms played was one of a more cleaning crew after mass devastation.  It didn't really matter what bore diameter Joe tentpeg had, except for the massive logistical strain.  The US Military clutching to .30 cal was and still is a huge mistake, in my opinion.  Garand had it right before the Army "fixed" it, as they often do, with the .276 Pedersen, but we know how that went.  It even had a 10rd clip in the original Garand, with a flat-shooting, low-recoiling cartridge system.

As to the OP's original question, I personally would not barrel a .260 Rem in anything less than 20", although 18" would be the least I would want to go.  GAP did at least one for Sniper's Hide and it was still a great 1000yd gun, but it was a bolt gun.  I have a 22" GAP AR10 for long-range, which is an excellent rifle, as well as a 16" Grendel for hunting.  There is nothing wrong with 6.5mm projectiles and medium game, as the picture above shows, even at lower velocities.

6.5 x 55 Swede is a highly regarded in Scandinavia for moose hunting.  The 260 is compares pretty well with that round.  At 200 yards I'd not worry about barrel length but probably would go with the 6.5G simply because it comes in a lighter overall package.  I use a 6.8 for much the same purpose but don't have the need or desire to shoot out to long ranges for target practice, if I did I'd probably consider the 6.5G.  

I'd like a 260 in a bolt gun.

I'm actually NOT trying to be argumentative. But I just think you totally missed the simple fact that in the 1890s
countries either adopted 6.5mms or .30/8mms. 6.5mms were widely used and saw heavy and extensive combat.
Sometimes the 6.5s won
Such as in 1904/05 when the Arisaka toting Jappers beat the 7.62mm Mosin carrying Russians.

Sometimes they lost.

But the simple fact is tens of millions of 6.5mm rifles were fielded. Even to the point of huge quantities of 6.5mm Arisaka rifles
being bought and fielded by both the English and Russians during WWI.

The Norwegians and Dutch (especially in the East Indies against the Jappers) fielded 6.5mms as did the Greeks
during World War II. Swedish volunteers fought in Finland against the Soviets using their 6.5x55s. The Italians fought
on many fronts, including the Eastern Front against the Soviets with 6.5mms. The Japanese conquered most of Asia and
went head to head with Springfields, Lee Enfields and Garands with 6.5 Arisakas.

The .30 cals and 8mms were not 'better'. They were actually deemed to be overly powerful and a waste of raw materials
and resources by both the Germans and the Soviets during the war.  

If you take post war trends to their logical conclusion of shorter, lighter more compact rifles with easier to control recoil
and lighter ammunition then the 6.5mms were ahead of their time.

I also believe far more game are wounded by poor shot placement than by poorly performing bullets.

Agree with gun, for hunting uner 300 260 would be no prob

If you want something light and maneuverable for the deer stand, a 16"-18" Grendel would be just the ticket, I would say, even over the .260rem or 6.5creedmore, just due to the fact that the shorter barrel length is more efficient with the smaller cartridges. You can burn all the powder completely and get decent velocity (in relation to the same load in a longer barrel), where a larger case capacity round needs the longer barrel to burn all of its powder completely. You may actually come out with about the same performance from a shorter barrel, you will just have a bigger blast and more flash with the larger case.

And if target shooting at longer ranges is your thing, that same carbine/spr-length Grendel would perform admirably. You aren't really worried about terminal performance at that range (even though the Grendel can make ethical kills out to around 400-500 yards on medium/large game, and further on varmints), your main concern in wind drift and bullet drop, which are a product of ballistic coefficient and projectile velocity retained. In both areas, the Grendel excels. It is, after all, basically a benchrest cartridge, with its fat case and squared shoulders, pushing an extremely long and slippery bullet.

Don't get me wrong, I would love to have a 6.5 Creedmore built in a LaRue OBR with a 20"-22" barrel, a PRS or UBR and a Nightforce 2.5-10x, but when it comes to shortening the barrel a good 6 inches, I'll stick with my 6.5 Grendel Middy for efficiency and economic reasons.

Here is a thread comparing the 24" and the 18" DPMS 260 uppers.  Velocity and accuracy comparisons are given.

http://www.longrangehunting.com/articles/dpms-panther-lr-260l-review-1.php

Let's not go trying to throw logic around,  Grampa said .30 was the minimum and grampa was always right.

I have a DPMS REPR in .260 with a 24in bull bbl.
what i can tell you is when you see how flat the VLD .260 rounds fly, your gonna
start looking for all the speed you can get. its an addicting round.

Depends on what sort of energy you're looking for. Somebody, somewhere, sometime ago suggested a 1,000 ftlb energy for whitetail. I seem to remember some states regulating handguns for whitetails to 500 ftlbs energy. About .357 Mag levels. Just parameters to start w.

I think today's better bullets and accuracy make lower power less of an issue. I believe the barnes TSX & TTSX are one of the best performing projectiles out there. On our tough hogs, I've seen 223 barnes perform better than I could've imagined.

Anyhow, if you load Barnes 120g TSX conservatively into 6.5 Gendel, and Rem .260, here is what you can expect:
16" 6.5G muzzle=2300fps,1,400ftlbe, 1,000ftlbe=200yds, 500ftlbe=500yds
16" .260 muzzle=2,650fps, 1,900ftlbe, 1,000ftlbe=300yds, 500ftlbe=650yds
24" .260 muzzle=2,980fps, 2,360ftlbe, 1,000ftlbe=450yds, 500ftlbe=800yds

Looks like they would all meet your requirements as an adequate 200yd hunting rifle and longer-range target rifle.

As for target shooting, it depends on how far out you want to play. The .260 will allow you to use the heavier 6.5 projectiles that give you the much higher BCs (ex:Hornady 140g AMax=0.585BC will stay supersonic to 1,000 yd from a 16" .260).  The Grendel just doesn't have the case capacity to launch the better BC bullets at the higher velocities they need to reap the BC benefits they are capable of.  Using the heavier, higher BC target bullets, the 260 is easily a fun 1,000yard gun. I don't think a  16" Grendel would do that.

I've shot my 18" grendel at 1000 before with no major issues.  Was hitting roughly torso-sized plates.  Nothing scientific or anything.  I was also shooting 4500 ft above sea level.

I would not want to tote around a rifle with a 24" tube for hunting.
I think the 18" 260 would work well for both of the OPs needs.

For long range work with the Grendel, the best option is the 123gr Scenar with a .527 BC. A 16" barrel Grendel is capable of launching 123gr pills at 2550fps (I have chronoed my rifle on many occasions with several 123gr and 130gr pills and come up with those numbers for my own rifle, so I know it is possible). With those numbers, the Grendel will keep pills supersonic out to 1100 yards give or take, from a 16" barrel. That also keeps varmint energies out to those distances. Hits at 1000+ are definitely possible with the Grendel.

why not?