[Ananael]

First off, thank you for taking the time to answer my questions!

I live in an urban environment (near Portland, OR), so I'm thinking that an SBR 5.56 might be a smarter way to go for me in a defensive rifle than, say, a FAL, but I just don't know that much about long guns.

I'm looking into ARs and other defensive rifles(I'm a total newb to them) for defense, just plain old fun, and SHTF, in that order, and I'm having a hard time getting a straight answer or three on the following:

When you drop barrel length below 16", how much effective range do you lose at what rate with a 5.56mm?

How does changing to 6.8SPC or 6.5 Grendel change this? In effect, is 6.8 the way to go below 16", and why? Does 6.5 out of an SBR even make since, or is it just a long range round that should be left to DMRs?

What about 7.62? Does it also lose a ton of range/power out of a shorter barrel? Would going to something like a D S Arms SBR be the way to go or a waste of a good cartridge?

While you're here, answering a ton of questions for me out of the kindness of your heart, what do you think of the Robarms XCR? They look cool to me, but it's totally possible that I'm just dazzled by the shiny. A pro's advice would be very appreciated!

[Constructor]

You will loose apx 20-30fps/inch around 16" but, the shorter the barrel the more velocity is lost /inch, effective range depends on what you use it for. Everyone has their own ideas about the required terminal performance for game or zombies and if those zombies are wearing body armor, skinny, fat etc.
If shooting deer sized game/zombies and want reliable expansion of the bullet considering it is an expanding bullet you need apx 1700fps for most bullets.
A 5.56 55gr soft point of fmj will still be traveling 1750fps at 400yds if it started at 3000fps, dropping the starting vel to 2800fps it is still traveling over 1800fps at 300 yds so IMO you would loose less than 50yds of terminal performance for every 100fps lost. The 6.5 G would act similar to the 5.56 as far as loss/inch, it's best quality is long range performance with match bullets, a DMR as you said.
The 6.8 is a very efficient cartridge due to the case capacity to bore area ratio, there is a lot of room in the barrel for expansion of the gases. The 6.8 works better than most in a SBR and when handloaded can hit great velocities when compared to other calibers, handloaded 85gr Barnes TSXs can hit over 3100fps from a 16" barrel, pretty close to 243 performance .
When you step up to a 308 sized rifle the weight of the firearm increases. A light projectile in a 308 will produce more terminal performance than any of the others but IMO not sure it is worth the extra weight, an AR10 commonly weighes 2lbs more than a AR15, also when rapid firing it is not as quick to get back on target as the 5.56 or 6.8 would be.
Ammo is something to consider, 5.56 and 308 will always be around for barter or whatever with the 5.56 being the cheapest but in most states you can't hunt big game with a 5.56, the 6.8 is becoming a very popular hunting round for hogs and whitetail and legal in most states but still no surplus ammo like the 5.56 and 308.
I like the idea of the XCR but I have not shot one so I really can't say.

[Ananael]

Thank you very much! I'll be able to figure out everything I need to know from this!