7.5" Barrel 5.56/.223 Ammo

[GreaseMonkeySRT]

Anyone have any hands on experience with ammo selection for a 7.5" barrel AR? I made an impulse buy, picking up a S&W M&P-15 5.56 pistol and now I'm second guessing myself on why I didn't just build myself a .300BLK especially since I already have a 10.5" setup in .300BLK.

Anyway, I watched this guy's playlist of different ammo performance: https://www.youtube.com/playlist?lis...KrFBUsJjo5RWPd
Unfortunately he didn't put his info together into a nice simple format. One would have to watch all the videos and take notes.

So does anyone have any first hand performance data or use on ammo in 7.5" 5.56 barrels?

[Nightshade1]

Honestly why not just replace the 556 barrel with a 7.5 300blk barrel.and then you have two super compact 300blk weapons? Honestly a 7.5" 556 weapon really only has one use and that's clearing out your sinuses. Technically yes it will put holes in someone at close range but man alive whatever bodily harm or death you prevent by firing it without hearing protection may indeed be better than the alternative.

[GreaseMonkeySRT]

Honestly why not just replace the 556 barrel with a 7.5 300blk barrel.and then you have two super compact 300blk weapons? Honestly a 7.5" 556 weapon really only has one use and that's clearing out your sinuses. Technically yes it will put holes in someone at close range but man alive whatever bodily harm or death you prevent by firing it without hearing protection may indeed be better than the alternative.

I ended up buying a PSA 7.5" .300BLK upper to right the wrong here. I'm going to do a little range testing with my chrono, and maybe get some ballistic gel if I can between both uppers and calibers.

[slingshot77]

That 7.5" 300 blackout is a handy size but pretty anemic with most factory ammo as most factory ammo is loaded with powders a bit too slow for a 7.5" barrel.

With decent hand loads it can be loaded to shoot subsonic in the 1000-1050 range with good accuracy and a decent muzzle report.

It will still be limited with supers (especially with factory ammo) but with good faster powder handloads it will surprise you for only a 7.5 inch barrel.

The 7.5" 300 BO really shines when suppressed with subs, Hornady 190 sub-X gives decent expansion at sub velocities but again, it takes handloads to shine as the factory loaded 190 sub-X is loaded for slightly longer barrels.

A suppressor really tames the muzzle blast on that short barrel but if that isn't an option then at least use some sort of a blast-forwarder device to force the muzzle blast down range. Using a brake on a 7.5" super gun is a double hearing protection gun. A non-suppressed, short barrel, super load, 300 BO BO without a suppressor is not something you want to fire inside your house if you have any other option.

On that 7.5" .223/5.56 barrel, with real hot 5.56 loads & under 50 yards it can work but not great. A 10.5" barrel is the shortest I will use on a .223/5.56 gun with my favorite being an 11.5" (with an 11.5 it can get decent velocity and a reasonable muzzle blast.

A 10.5" is easier to meet Michigan pistol regs (depends on how you interpret "shortest fire-able condition".

[GreaseMonkeySRT]

That 7.5" 300 blackout is a handy size but pretty anemic with most factory ammo as most factory ammo is loaded with powders a bit too slow for a 7.5" barrel.

With decent hand loads it can be loaded to shoot subsonic in the 1000-1050 range with good accuracy and a decent muzzle report.

It will still be limited with supers (especially with factory ammo) but with good faster powder handloads it will surprise you for only a 7.5 inch barrel.

The 7.5" 300 BO really shines when suppressed with subs, Hornady 190 sub-X gives decent expansion at sub velocities but again, it takes handloads to shine as the factory loaded 190 sub-X is loaded for slightly longer barrels.

A suppressor really tames the muzzle blast on that short barrel but if that isn't an option then at least use some sort of a blast-forwarder device to force the muzzle blast down range. Using a brake on a 7.5" super gun is a double hearing protection gun. A non-suppressed, short barrel, super load, 300 BO BO without a suppressor is not something you want to fire inside your house if you have any other option.

On that 7.5" .223/5.56 barrel, with real hot 5.56 loads & under 50 yards it can work but not great. A 10.5" barrel is the shortest I will use on a .223/5.56 gun with my favorite being an 11.5" (with an 11.5 it can get decent velocity and a reasonable muzzle blast.

A 10.5" is easier to meet Michigan pistol regs (depends on how you interpret "shortest fire-able condition".

My goal was something compact enough to fit in a normal looking back pack that won't take up much space in the truck under a seat or behind it. Just to have something with more oomph than a 9mm pistol. I already own a 10.5" 300BLK, and could always use that if needed but having something cheaper, lighter, smaller was kind of the goal.

But, looking into terminal performance of 5.56, overpenetration is a distinct possibility without hitting the required velocity on rounds designed to expand or fragment. Honestly since there isn't much data out there for shorter barrels I might just spend the money on ballistic gelatin and try a few rounds between both calibers.

[slingshot77]

If you want better than pistol performance and are willing to spend some testing time then there an option I have been kind of playing with and thinking about using that option in my 7.5" 300 blackout gas gun. That gun rests so darn nicely inside a soft tennis racquet case.

There is nothing that says you can't run subsonic expanding bullets faster than the speed of sound. (well except for the laws of physics)

Hornady shows you can shoot the 190 sub-x at up to 1200 ft sec & they should still stay together (probably not much faster though). (you won't get near 1200' sec from a 7.5" barrel but you can get well into supersonic speeds

I had that thought last fall to try the 190 sub-x over 13.0 of CFE black (book says it can go as high as 13.2 CFE black), I used 2 layers of denim in front of a row or water jugs at 25 yards. It blew the first water jug up in a huge explosion, split the second water jug in half, I found the expanded bullet inside either the 3rd or 4th water jug (I can't remember which one but probably the 3rd).

The bullet pedals were folded back against the base but they must have cut a pretty big path as they opened up and were folding back. (would have liked to see that in a gel block). But the bullet stayed together and retained all it's pedals.

I only shot one 190 sub-x at that powder load (lot more testing required to fully vet it), I didn't even have my chronograph with me but it did have a solid supersonic crack so it was definitely up into the super range.

I was going to do more testing & chrono work but the winter weather and roaming winter Coyotes put it on the back burner.

You mentioned gel block so you might test some 190 sub-x at supersonic speeds from that 7.5" gun.

It sure won't be a 200 yard performer but at self defense ranges it should definitely beat a 9mm handgun by a healthy margin.

If it wasn't for the expense (it needs a locked breech of some sort so cheap blow-back is out) then a 10mm 7.5" or 8.5" PCC would be a great truck gun.