7.5" AR-15 Upper for AR Pistol

[LSUTiger]

I have an 11.5" Upper and a 10.5" Upper and I am trying to convince my self to get a 7.5" AR-15 Upper for an AR Pistol. Besides a fun range toy is their any practical use for such an thing? What is the performance of 7.5" 5.56 close to?

From what I have been able to Google:
In terms of velocity (\(1,900\text{-}2,100\text{ fps}\)) and muzzle energy (roughly \(500\text{-}600 \text{ ft-lbs}\)), a 7.5" 5.56 closely mimics heavy defensive handgun cartridges like the \(.357\) Magnum or the \(10\text{mm}\) Auto fired from carbine-length barrels.Cartridge

Counterparts: The ballistic profile drops so drastically that its impact characteristics at close range are virtually identical to a \(.30\) Carbine (historically used in the M1 Carbine).

If this is true, then I would not mind having 30 rounds of .357 Magnum or 10mm equivalent on tap in a small AR package for Home Defense specific applications (25 yard max).

Thoughts?

[JRG]

Sounds like a pistol caliber carbine is in your future. I am researching one of those also at this time. I am thinking of 9mm though because the ammo is more cost effective. However, a 10mm PCC is certainly tempting.

Joe

[LSUTiger]

Sounds like a pistol caliber carbine is in your future. I am researching one of those also at this time. I am thinking of 9mm though because the ammo is more cost effective. However, a 10mm PCC is certainly tempting.

Joe

A PCC ain't for me. I am trying to go shorter, not longer. PCC's require a rifle length barrel/footprint to enhance a pistol cartridge performance. You give up the portability and maneuverability of a pistol for not much gain.

I am trying to go as short as possible and still take advantage of a rifle cartridge and the characteristics of the AR-15 with reasonable ( enhanced handgun) ballistic performance at 25 yds or less.

That is in the AR pistol with at least 3 points of contact (2 hands and cheek weld on buffer tube), use of a red dot optic, 2-30rd coupled magazines so a total of 60 rounds on the gun. If feel it would give me enhanced accuracy over a handgun at the outside distance of 25 yds and still give me good maneuverability at close up distances with greatly increased firepower.

The question is where do the lines intersect between a PCC and shorty AR? Since my dedicated application is 25 yds or less, the general research seems to indicate that the 7.5" AR would be sufficiently fit for purpose.

16" 10mm PCC vs 7.5" 5.56

These two platforms cross ballistically at roughly 50 to 75 yards.

Within this window, their velocities equalize and terminal performance flips, creating a distinct "crossover" point between a heavy-and-slow projectile and a light-and-ultra-fast projectile suffering from extreme velocity loss.

The Crossover Breakdown

Point Blank to 50 Yards: The 7.5" 5.56 dominates. Even out of an extremely short barrel, 5.56 (using 55gr–77gr projectiles) leaves the muzzle at roughly \(2,100 \text{ to } 2,400 \text{ fps}\). This creates massive temporary cavities and hydrostatic shock despite poor fragmentation. The 10mm will yield about \(1,200 \text{ to } 1,400 \text{ fps}\) at the muzzle with \(600 \text{ to } 700 \text{ ft-lbs}\) of energy.

The Crossover Zone (50–75 yards): This is where the lines meet. The 10mm round is relatively "fat and heavy" with a poor ballistic coefficient (BC). The 7.5" 5.56 round relies completely on high velocity to do damage, and from a 7.5" barrel, it sheds velocity at an aggressive rate. By 75 yards, the 5.56 SBR drops into the \(1,800 \text{ to } 1,900 \text{ fps}\) range. At this point, the 5.56 lacks the velocity required for the round to properly fragment. Concurrently, the 10mm (using heavy hard-cast or high-performance 200gr loads) carries roughly equal kinetic energy but uses mass and diameter rather than speed.

Beyond 75 Yards: The 10mm line takes over in energy retention, trajectory, and predictability. The 7.5" 5.56 round drops sub-sonic and behaves like a lightweight .22 caliber rimfire due to bullet instability and the inability to yaw/fragment. 10mm PCC rounds, while still dropping like a stone compared to rifle cartridges, will retain their energy better at longer distances (100+ yards) than a severely crippled 7.5" SBR.For reliable ballistic performance when comparing pistol-caliber carbines (PCCs) to rifle-caliber SBRs, consider using tools like the ShootersCalculator Ballistic Calculator or reviewing Everyday Marksman's breakdown on barrel length and trajectory.

http://www.ballisticsbytheinch.com/
7.5 AR about 700+ ft/lbs energy
16" 10mm 600-1000 ft/lbs energy
16" 9mm 400-600 ft/lbs energy