I've got an SBR lower ready to go, but I'm divided on what to put on it.

I have an AUG, so 10.5 or so doesn't hold a ton of appeal to me outside of Mk18 Mod 0's being awesome looking.

So I figured shorter might be fun. I really liked the LWRC PSD and PDW a local gunstore had, but it's kind of expensive and I didn't really want to go piston (my original plan was to do a clone of those in .300 BO but I'm sticking with 5.56).

Those have an 8.5" barrel (I also note the HK53 has a 8.3) Is there any "value" at all with going to an 8" barrel or 8.5" over a 7.5" performance wise? I'm aware of the obvious limitations of these short barrel lengths, I just wondered if added .5 or 1" to what seems like the "standard" 7.5" barrel does anything at all or if it doesn't make a difference.

ETA: Damnit, this was supposed to be in the SBR forum. Can a mod move it?

ETA2: Thanks HardShell

Topic Moved

Tagging along for replies, because this becomes even more important when attaching a suppressor.

In my experience it will make a difference, but you'd need to chrono some rounds to see it (velocity wise). Once upon a time I had a 7" RRA factory AR pistol. That's long gone now due to the crazy stupid prices they were going for on GB after Newtown but I have a 10.5 SBR that is much more fun to shoot than that pistol.

Going from 7.5 to 8.5 will be a smaller difference than 7 to 10.5 obviously,  and I'd assume you'd still get a HUGE fireball off that 8.5 but velocity would be increased somewhat over a 7.5. How that relates to a given load will require research.

You'll also have to be a little more careful selecting a suppressor in case you want to suppress it. A lot of companies rate their suppressors down to 10.5 but there are also a bunch that go lower than that. Not sure if you're even concerned about that but just something to think about.

I am thinking about a suppressor (ones rated for so short), but again, dunno if the barrel length difference of .5 to 1" does anything value added for that either.

My wife and I own an Aug A3, an 8.5" SBR, and a 10.3" SBR (Block II CQBR cloneish thingy)

In all honesty, I hate shooting the Aug.... Cheek wield is horrible, not comfortable, etc.
the 8.5" 5.56 is "ok" to shoot... I'm not a huge fan of it though unless she put a can on it. We've shot it with a friends older Halo on it and actually bought the Halo from him (not because it blew up)
of all 3, the 10.3 is the most "pleasant" to shoot. Mild recoil/ softer shooting than the 8.5, but much more comfortable to shoot than the AUG.

ETA - The fireball from her 8.5 really wasnt as bad as I thought it was going to be. She is using a PWS FSC556 on it. I had a much more violent fireball out of a dynacomp on a 14.5" FN barrel.

With all this being said.... I think I'm going to build an 11.5" upper and may strongly consider selling the CQBR upper if that upper works well for me.

YMMV.

No scientific numbers, just my opinion but I think there would be a little less pressure on the suppressor in an 8.5 vs a 7.5. If I were to suppress a 5.56 barrel in either of those lengths, I would definitely want a muzzle brake to act as a sacrificial blast baffle though. That said though, I have a FH on my 10.5 running an Omega and (soon to be) Hybrid or Specwar K.

if your going to put a can on it there is plenty of "added value" for every inch of extra barrel , the added value being extended service life of all the parts and lower peak pressure for your can.  sub 10" barrels in 5.56 are brutal since your limited to a pistol gas system , 10" + barrels with a carbine gas system are much better.  if you want to put a suppressor on it a 10" or longer barrel with a carbine gas system would be the way to go.  with that being said i do have an upper that i went from a 7" barrel to a 9" barrel to run a can on.  i ended up with a 9" barrel so i could clear the rail with the mount on an 8" rail , i wanted to use a noveske 8.5" navy barrel but the mount wouldn't clear the rail.  the 9" barrel was also much easier to tune to run both suppressed / unsuppressed without adjustable gas systems than the 7" barrel.  if your not going to put a can on it a 7" barrelled 5.56 upper can be a great range blaster though
here's how mine ended up with the 9" barrel


 
 
 
 
 
 
 
 
 

One big advantage IMO of the 8.5" barrel over the 7.5" barrel is that a standard carbine length handguard will allow you to use most of the popular suppressor mounts (SF, )AAC, SiCo).

The thread shoulder on the 7.5" barrel will most likely end up under you handguard of choice.  Going with an 8" or 8.5" will solve that problem.  

I've got an 8" BA barrel paired with a 7.0 lite rail.  The shoulder of the threads is just past the end of the rail making adding a suppressor possible, if it doesn't mount over the muzzle device.  Dead Air cans should be GTG.

This is not an issue at all with a 7.5" barrel.  You just have to pick the right rail.  Here's my 7.5" with 6.7" SLR rail.

That SLR 6.7" rail is one of the few options out there for a 7.5" barrel.

@ Green Goose


OMG.  That is almost exactly the same build I am in the middle of right now.  Instead of hijacking this thread you have PM's incoming.