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I have an AR that has an 11.5" barrel with welded flash suppressor to make it 16" legal. If it weren't for the flash suppressor it would definitely be an SBR.
I have found that the best powders for this rifle are powders that were developed for the AR-15/M-16 family.
I use AA2230 for most of my loads as it's fairly inexpensive in 8# jugs. Also have used H335 and know lots of people that use BL-C(2). Another powder that's being used by some AR shooters is TAC.
With these short barrels it's impossible to achieve the 3,000 fps and up that a 20" bbl can achieve. The biggest issue with any Direct Impingement Gas Operated rifle is the cleanliness with which the powder burns. For this reason I prefer the Accurate 2230 which in my AR's leaves the least carbon residue behind in the Bolt Carrier Group.
With the short barrel you'd be best off with 55 gr bullets. Accuracy at distance is not one of an SBR's strong points. Any of the above mentioned powders will perform well in your firearm with 55 gr bullets.
As a recommended load, try 25.5 gr of AA 2230 with either Hornady or Montana Gold 55 gr. FMJ-BT For primers I use the Wolf/Tula .223 primers which are designed to prevent slamfire's, just like the CCI #41 "Military" primers, but at half the price.
BTW: of the two powders you listed, only the IMR is "recommended" for a typical AR-15 load using a 55 gr bullet. These powders tend to build pressure quickly and might well lead to other issues with your firearm, regardless of barrel length.
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