So, while doing research i see that the longer barrel is best for good powder burn and ballistics of the 5.56 round. Will having a 11.5 inch barrel with a silencer add the extra length needed to get similar burn from an unsilenced barrel like a 16inch?
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@Edward316 Welcome to our community, we are glad to have you with us. 
Short answer… No. 
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Welcome @Edward316!
The only reason IMO for the short barrel is CQB, why not get a gun designed for CQB that has a full length barrel?
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@Edward316 Welcome to the community!
Bruce has it right. No. The suppressor disburses the gas to muffle the sound. The pressure pushing the bullet ends at the end of the actual barrel, the rest goes into a “muffler” as on your car. It’s lost energy. Check out a site called “ballistics by the inch” which is very informative. Another consideration is simply accuracy. In my opinion having a few extra inches of barrel to gain long range accuracy and velocity is well worth it.
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No, because the expansion of the gas will be wasted expanding into the suppressor instead of driving the projectile. If you want to get the most out of 5.56, use the 20’ barrel it was designed to be fired from.
The short barrel is great for negotiating entrance ways and room clearing. Just buy or assemble two uppers. Cheaper than buying two complete rifles by a long shot
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There are many parts of the question you left out.
What is the purpose of the platform?
Are you optimizing for long range?
Are you optimizing for boolit weight?
If you add a 6" can to an 11.5" barrel you will effectively get a 13.5" barrel velocity wise give or take the can. Your Gov’t determined that a 14.5" tube twisted 1-7 with a flash hider would optimize the 62gr M-855 boolit out to over 500 meters. Running 55gr M-193 in the same tube produces less than optimal results at distance but it is still battle effective inside 300 meters (ish). A 1-9 twisted 24" tube (M-16 A2/3/4) will run M-193 fine to about 800 yards the M-855 not so much.
There is a whole lot of science about shooting “Service rifle” out to 1K depending on chambers twist rates and boolit weights. It’s a deep rabbit hole.
Personally if I’m running a can on an SBR or pistol it doesn’t really matter as it is a 200 - 300 yard combat effective weapon. Run what you brung, know your limitations and carry on.
Cheers,
Craig6
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It depends upon how you are planning to use it.
Shorter barrel is adequate for home self-defense while the 20-inch barrel will give you a distant long-range accuracy. Are you planning on using it around short distances or further out from 200 to 800 yards? That would be the difference.
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Ok, so the plan i was thinkn was to build a shorter barrel 5.56 upper for the focus of CQB training. The idea was to get a 10.5 barrel with a 6inch can however i see from research that simce the can doesnt have refiling it wont help as i thought it might. Looks like ill be better off building it as a .300 blk out instead.
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