What stock you use on what gun really comes down to personal preference and personal preference differs from person to person and is not something that can be easliy explained or decided over the internet.
Something I have learned about AR's over the years is, it's a game of matching the right part with the right gun. You may try a stock on a 10.5" SBR and hate it, and then try the same stock on a 16" mid-length carbine and think it's the best stock on the market.
1) help balance out a rifle
2) or if the stock too big or heavy can dominate the gun
3) and if it's too small or too light can make the gun feel muzzle heavy
Magpul CTR with complete receiver extention (no buffer or buffer spring): 14.0 oz.
LMT (Lewis Machine and Tool) SOPMOD with complete receiver extention (no buffer or buffer spring - older model): 15.4 oz.
Magpul ACS with complete receiver extention (no buffer or buffer spring): 1 lbs - 2.6 oz.
Magpul UBR complete (no buffer or buffer spring): 1 lbs - 9.4 oz.
Vltor EMOD with complete receiver extention (no buffer or buffer spring): 1 lbs - 2.8 oz.
(all Magpul stock had the Magpul rubber buttpad on them when they were weighed)
After trying several different configurations over the year, at the current time I have found that I like Magpul CTR's on short barreled rifles, some 14.5" M4's (with rail lengths from 7.0 to 11.0), and some lightweight or government profile 16" mid-lenght barrels (with a 9.0 rail). The Magpul CTR is a light weight stock and doesn't balance out a front heavy carbine well, such as a 16" barreled mid-length carbine with longer rail systems (11.0 - 13.2).
LMT SOPMOD (for me) balance perfectly on a 14.5" M4 with a 7.0 rail, but can also balance most SBR's and mid-lengths out fairly well. I have used the SOPMOD on everything from a LaRue Stealth 12" with 10.0 rail to a 16" mid-length with a 12.0 rail.
As a general rule I use:
-Magpul CTR stocks on short barreled rifle, some 14.5" M4 carbines (with 7.0 - 11.0 rail systems), and some government profile or lightweight 16" mid-length carbines with a 9.0 rail system and standard front sight base.
-LMT SOPMOD stocks on 14.5" barreled M4 carbines, some SBR's with longer rail systems, and some government or lightweight profile 16" mid-length with a 12.0 rail
-Vltor EMOD stocks on 14.5" M4 carbines with longer rails (10.0 - 12.0) and 16" mid-lengths.
-Magpul UBR stock on 14.5" - 16" barrels with longer rail systems (10.0 - 13.2) (example shown below of a UBR on a Noveske with a 14.5" barrel and a 11.0 rail system), some 16" mid-lengths with a 9.0 rail system, and I also recently had the chance to try a Magpul UBR on a 20" rifle and the UBR balanced out the rifle extreemly well.
Magpul CTR on a 11.5" Short Barreled Rifle:
Magpul CTR on 14.5" M4:
LMT SOPMOD on a 14.5" M4:
Vltor EMod on a 16" Mid-Length:
Vltor EMOD on a plain 16" Mid-Length (lightweight barrel profile):
Vltor EMOD on a 14.5" M4 with LaRue 11.0
Magpul UBR on a 14.5" Noveske N4 barrel with LaRue 11.0 rail:
Magpul UBR on a 16" Noveske N4 RECCE with LaRue 13.2
Magpul UBR on 16" Mid-Length:
Magpul CTR on 16" Mid-Length
Magpul ACS on 16" Mid-Length with LaRue 12.0 rail
I recently put a Magpul CTR on a BCM 16" Mid-Length (govt profile barrel) w/ LaRue 9.0 rail, Surefire x300, and Aimpoint T1 Micro. The carbine balances well for me. I had tried a Magpul CTR on this gun in the past and it didn't feel right. I don't know what changed but tried it again recently and it feels good. Sometimes our personal preferences change over time.
By changing one item on a carbine it can drastically change how the carbine feels and handles (weight and balance). If you replace the Surefire x300 light (3.8 oz) for a Surefire M961 light (10.4 oz) and the CTR may no longer balance the carbine well. However, a UBR and EMOD may balance this gun out nicely.
Many times you can chage out one accessory (light, optic, stock, etc.) and it can drastically change how the gun feels and balances. Sometimes for the better, sometimes for the worse.
The ultimate goal is to select accessories that complement each other so you end up with a well balanced carbine that doesn't feel like a boat anchor.
(I don't have enough trigger time on the Magpul ACS yet, but will update this article after I have used the ACS on several different configurations)