I ordered a lower now I just have to decide barrel length/caliber.  I am leaning towards a 5.56 10 1/2" 1:7 barrel any advice?

Quote from: 10-7 leo on July 18 2015 08:50:28 AM MDT
If you look at the velocity of a 55gr. .223 out of a pistol length barrel (7") and compare it to the velocity of a 124gr. 9mm from a longer than normal pistol barrel (7'), the 9mm actually has more energy than the .223.

No. Not even close. Use http://www.ballisticsbytheinch.com to find velocities and whatever calculator you want to find energy. I used http://www.shooterscalculator.com/bullet-kinetic-energy.php. The muzzle energy for the .223 is almost double that of the 9mm from the same 7" barrel.

Did the math:

ME = 1/2mv^2

ME = (Mass * Velocity^2)/450435   (the constant at the bottom is 7000 gr/lb * 32.1739 ft^2/sec^2 * 2, this normalizes weight in grains to mass in slugs and takes care of the 1/2 business by multiplying the denominator by 2)

So:

9MM 125gr Corbon +P @1329 from 7" bbl, hottest load on ballistics by the inch
ME = (1329*1329*125)/450435 = 490.149

9mm 124gr Federal Hydra Shok @1161 from 7" bbl, standard 124 grain load
ME = (1161*1161*124)/450435 = 374.061

Remington UMC 55gr @2190 from 7" Carbon 15
ME = (2190*2190*55)/450435 = 585.624

I should have all my parts in over the next couple weeks. I went with a 300blk with a 10.5" barrel. It seemed like a better candidate to suppress in the future.

Sounds fun. Hope you enjoy it.

Quote from: Brian B. on July 21 2015 08:25:28 PM MDT
( Did I mention my other pistol is a Hi-Point CF380?)

I pity da fool! 

Nah, just jerkin your chain....

what ever you do make sure you test a bunch of different brands and bullet weights of ammo, ar pistols are really picky about their ammo my son and I have 2 of them built identical and mine is partial to 55 grn. 5.56 and higher velocity loads and his will eat anything in 62 grain and don't like 55 grn. unless it is the cheap steel case or lower velocity .223

This has a lot to do with the gas system and barrel length. 11.5" barrels with carbine gas systems usually do pretty well because they have adequate dwell time. 10.5" barrels with carbine gas systems can be more troublesome and folks have done back flips to get 7" barrels to run right. My 11.5" SBR runs everything except cheap steel case stuff when the can is off. Without the silencer, it short strokes about two to three times in a magazine and never locks back on an empty mag. With the silencer, it runs like a raped ape. With or without the can, it runs any other ammo just fine. If it was an issue and I really wanted it to run that type of ammo unsuppressed, I could have the gas port opened up, but that would increase bolt carrier velocity for all the other loads, too, which increases wear.