5.56 or 300blk for pistol upper

[G.B.]

Putting together an upper and looking for thoughts from those with some insight on 300blk. It will be a pistol upper either 11.5 5.56 or 10.5 300blk. That’s as short as I can go and still put a comfortable length handguard. Requirements are 25-125 yds and can only use the cheapest fmj ammo. I can’t afford a stock pile of ammo at 1.50 a round. Already have a 5.56 rifle. Using only fmj is there enough difference to justify cost difference of ammo and adding a different caliber.

[Beat Trash]

If ammunition cost is an issue and you already own a 5.56mm rifle, then I’d go with a 5.56mm pistol.

Same ammunition for both guns simplifies your cost and logistics.

[themonk]

Putting together an upper and looking for thoughts from those with some insight on 300blk. It will be a pistol upper either 11.5 5.56 or 10.5 300blk. That’s as short as I can go and still put a comfortable length handguard. Requirements are 25-125 yds and can only use the cheapest fmj ammo. I can’t afford a stock pile of ammo at 1.50 a round. Already have a 5.56 rifle. Using only fmj is there enough difference to justify cost difference of ammo and adding a different caliber.

I would say no. The 2 main advantages of 300 is it can be very quiet with subsonic ammunition and its ability to have short barrels (9" and below) and still have the stopping power of 556 in a longer barrel. A 7" 300 blackout is a very handy and is very effective as a HD rifle within say 150 yards. Sounds like you dont have a desire for a short barrel. Also the expensive HD rounds are where you will get that maximum expansion and if your not using those just go 556.

A 10.5" MK18 with Hornady TAP SBR or other good ammo will have just as much of a devastating effect.

[TMS951]

I have 11.5" uppers and love them. Based on your ammo statement I'd say 5.56 all day. Especially since you don't mention surpassing it.

I have a 7" 300 BLK SBR and I run it 100% suppressed with sub sonic ammo. It shines in this role. Ammo costs twice as much.

[bamashooter]

5.56 based on OP's post. However, save your pennies and get a box or two of some better "defensive"-type ammunition if that's its primary role. You can still use cheaper stuff for range work, etc. I have found little to no difference in defensive ammos vs basic fmj stuff when it comes to accuracy and zeroing at under 100m. That was very surprising to me when I first messed around with a few and it was consistent.

[mack7.62]

I would not go shorter than a 11.5 5.56, as a matter of fact IMO the 10.5(.3) just needs to go away, it does nothing better than a 11.5 other than being 1" shorter and comes with a cost attached of harsher cycling and decreased reliability.

[themonk]

I would not go shorter than a 11.5 5.56, as a matter of fact IMO the 10.5(.3) just needs to go away, it does nothing better than a 11.5 other than being 1" shorter and comes with a cost attached of harsher cycling and decreased reliability.

Harsher cycling is completely dependent and gas port size and buffer configuration. Decreased reliability was an issue 10 years ago and is no longer the case or has been the case for some time.

[mack7.62]

I know they can be made to work, I just fail to see how decreasing the dwell time and upping the gas port size is worth 1" shorter and still believe a properly set up 11.5 is going to be better than a 10.5 long term and under adverse conditions. YMMV.

[MegademiC]

For your situation, 556. Barrel length depends on if you prefer shorter length or less blast.
Assuming you wont suppress it, 11.5 with flash hider or linear comp/flash can would be my recommendation.

[Warp]

Putting together an upper and looking for thoughts from those with some insight on 300blk. It will be a pistol upper either 11.5 5.56 or 10.5 300blk. That’s as short as I can go and still put a comfortable length handguard. Requirements are 25-125 yds and can only use the cheapest fmj ammo. I can’t afford a stock pile of ammo at 1.50 a round. Already have a 5.56 rifle. Using only fmj is there enough difference to justify cost difference of ammo and adding a different caliber.

This is very simple in my eyes.

You are concerned about the cost of ammo, so, 5.56 it is

11.5 is a good spot for this though good manufacturers have success with the 10 .5/3 as well

And I'd go ahead and buy a couple or few boxes of a good SMK/OTM or Federal Fusion to keep ready as well.