I'm trying to decide on how I want to build my first pistol, I was set on a 12.5 5.56, but someone mentioned building a 10.5 300blk and  now I'm undecided.

I would like to run a suppressor but that will likely be years down the road and I'll probably have both uppers before then.  

If you don’t have a can skip the 300blk.

That’s the only thing that makes it worth the added cost per round.

This.  But if you are open to another cal, I'd look at 762x39

If you end up building a .300Blk I'd suggest a shorter barrel than the 10.5". It will be very ballistically similar from an 8.5" barrel and the 10.5". I wanted an 8.5" but I got a screaming deal on a 9.5" so that's what I'm rocking lol.

I'm not really sold on the 300blk, but I do like the 10.5, how does the 556 in a 10.5 compare to the 12.5?

I like variety, so if you already have a 5.56 rifle, I'd say 300BLK 10.5.

Then again I'm biased as I love my 300BLK. Planning on ordering a Surefire can here in the next year, so I can say itll be worth it for me in the long run.

Otherwise, for 5.56 - stick to 11.5 or 12.5" The 12.5 will probably be better off for you, if suppression isnt a option that is in the near future.


A reference for a 10.5" 300BLK

I would go with 11.5 or 12.5 556 if you don't have a can. No reason to add another caliber.

I have two 10.5 and a 11.5 AR. Ill probably end up with a 12.5 as well.


The 11.5 is my favorite of the short barreled guns. Its a good balance between shortness, handguard length and dwell time.

Poll fail, no 6.5 Grendel

I have .300blk in 8.3", and 9". (And an old 8.5" barrel).
I have 5.56 in 10.5", 12.5", 16" and 20".


In terms of short barrels, my 10.5" never gets touched.
My 12.5" is basically a host for a 22LR conversion bolt, but 12.5" is indeed a good length.

I have no clue as to why people say .300blk is "only" good with a can.  (Or sometimes it is "only" good shooting subs).  That is incorrect.
.300blk excels with short barrels.  Can or no can.

So this is going to come down to how you plan on using this weapon.  My personal take is that anything under a foot gets into .300blk territory if you are using the weapon for HD/SD.
My 9" .300blk using factory 110gr Vmax (2190 fps) falls between 11.5" and 12.5" 5.56 using IMI 77gr (2520/2620 fps), energy wise at short distances (out to about 60 yards).  I'm using those bullets in particular because they are both excellent fragmenting rounds (so "energy" is a somewhat useful comparison), and this is what I've stocked up on.  Energy wise, the 9" and 12.5" are basically identical at the muzzle, but the 5.56 keeps pulling away.  The 11.5 starts out weaker, catches up to the .300 around 60 yards, and pulls away after that.  Then again the barrels are 2.5 and 3.5 inches longer.

Get both

My first AR pistol build was also my first 300BLK build.  It's a 10.5" barrel & I like it, but it's mainly just a fun toy for me.

For me .300blk without a can doesn't offer enough of an advantage to offset the price of ammo or adopting another caliber. Where it shines is short range, short barrels, subs, and a can.

556 M193 is cheap and offers a lot for the price if you can keep it around frag velocities.  

If were talking about more expensive rounds then I would rather have a 62gr bonded soft point for defense like the Speer GDSP or Fusion/MSR. Those bullets will expand down to 1800 FPS. Which extends your range and keeps your bullet doing whats its supposed to do at further ranges.

If were shooting at extended ranges then that's when I would look more towards the 77gr stuff.

I own a 10.3" urgi and 12.5" centurion and the centurion barrel is better in every category,  smoother, better balance,  more accurate and much better to shoot. The 10.3 has alot more snap to it . I haven't ventured into 300 blk just because of the price of subs , thats the only reason id want 300 blk .

I think I'm just gonna stick with 556, now I'm torn between a 11.5 and a 12.5 but I'm leaning towards the 12.5

Here is the issue.  For M193 to do it's thing, it needs to be above 2700 fps.   By 2500 fps, there is no fragmentation.    Since we are talking short barrels, this really isn't the best area for M193.  So we move on to more expensive ammo no matter what.
Plus 5.56 is really loud and blasty out of short barrels.

For expanding rounds:
I'll take 110gr .308 Tac-tx over 50gr/62gr/64gr .224 rounds.     Tac-tx for .300blk expands down to 1300 fps (factory listed).  Out of a 9" barrel, that is +350 yards.

For fragmenting rounds:
I'll take 110gr .308 Vmax over 75gr/77gr .224 rounds out of short barrels.   Vmax for .300blk is "supposed" to fragment down 1600 fps (factory listed), although I would add quite a bit of cushion to that. (Out of a 9" barrel, 1700 fps is 190 yards. 1800 fps is 150 yards).

But all of that is subject to purpose and barrel length.

My 12.5" 5.56 is a good barrel length, it just gets squeezed from both sides.  At really short ranges, I'm not getting much of a benefit out of 3.5 more inches of barrel over my 9" 300blk.   And once you get into that 12.5" barrel length range, there are a lot of other small frame calibers that start to make sense as well (6.5, 6.5, 300Ham'r, etc).  It's probably why my 12.5" spends a lot of time what that 22LR bolt it in it.

Right on. To each their own. I'm happy where I'm at with 5.56. Ill take the expanding 62gr bullets and cheap ammo. I've killed plenty of things with success with .223/5.56. I have no use for a 300blk.

Hard to go wrong with either.  Pick which rail you want, and then pick the barrel length you think looks best with it. I'd probably go 12.5" with ~11" rail.

For short barrel, I prefer the 300 due to ballistics. However, many people here have strong support for 5.56 and based on their personal experience seem to have good reason.
On paper, the 5.56 needs the speed to do its thing, that is on paper, and an Engineer can "prove" anything on paper.

With the 300 I can go subsonic or supersonic and have a great variety of bullet weights. Just my feelings on it.

With a can in your future, go Blackout but shorter.....think 9”