I currently have a 8.5" 300 blk pistol that I shoot suppressed with a thermal. I want to build another pistol that will have a 6 power LPVO on it, because 2 is one, and 1 is none. I have a 7.5" AR upper that I never shoot anymore, so I was going to swap the barrel over to 300 blk. Should I look at other barrel lengths?

Secondly I want to build a 9mm AR pistol. What barrel length should I be looking at for that. My only current PCC is a SBR's CZ Scorpion Evo.

10.5 for 300BO

9 mm ….. 16 in barrel - S&W folding pistol carbine rather than the AR platform.

9mm 124 gr HST +P+  1450 FPS. Energy is 578 Ft Lbs out of a S&W FPC

I've had 7.5, 8.0 and 8.5" barrels on 300blk pistols. Stuck with the 8.0 and 8.5" barrels. It's the sweet spot for 300blk.

Weaver v3 1-3x only weighs 8oz  on my pistols . I don't need any more magnification for 100yds or less they are shot at and lighter than anything else besides a reddot sight. My astigmatism doesn't do well with a reddot.  

I went with the CMMG 9mm RDB 8" barrel and BCG. MP5 uses an 8.9" barrel and 8" was close enough for me.

I have 300blk barrels in 7.5", 8.3", 8.5", 9" and 10.5"

If I had a single 8.5" and was going to add something with an LVPO, I would probably look into a Wilson Combat 11.3" or something like that.

If I was only going to have one 300blk AR, it would be 8.5" or 9".

But if was going to have two, it would "longer and shorter" for the caliber.    I've been eyeballing the new SIG 6.75" upper, and a WC 11.3" barrel or maybe even a BCM 12.5" barrel if they ever come back.


Sorta like if you only own one 5.56, it will probably be a 14.5/7" or 16".   But if you own two 5.56's, then you would want something in the 10.3"-12.5" range and something in the 18"-20" range.  

As for 9mm, I don't know, seems like the curve starts to flatten out in the 7"-9" range, especially when you click on the muzzle energy link.   http://www.ballisticsbytheinch.com/9luger.html

Although 300BO can be pretty accurate, a 6x will be wasted. 400 is a long shot for. 300BO. I'd go for something small and light. I put an ACOG on mine.

It's suppressed.

I have it sitting around and it has the 7.62/.300 reticle so I am going to use it.

If you're looking to build a 9mm I'd highly reccomend a Macon armory builders kit. I got one with an 8.5 inch barrel and it shoots like a dream. Rudy is a great guy and a member here as well.