Different Blast Between 7.5" Rifle and 7.5" Pistol?

Beretta686 · August 9, 2014, 02:08 PM

I have a 7.5 inch .44 Mag and a 7.5 AR-15 in 5.56.

Shooting the .44 Mag has quite a hefty report, but not something that will blow your eardrums out after just one round without earplugs. However shooting the 7.5 AR will blow your eardrums out.

Why is this? Clearly the .44 mag is pushing quite a bit more energy than the 5.56, but it's nowhere near as loud as a shorty AR shooting the relatively anemic 5.56. Clearly there's a reason for this, but I'm not sure why.

FrankenMauser · August 9, 2014, 03:58 PM

There are many factors at play, not the least of which are:
Position of the firearm. -Most SBRs and AR pistols are fired with the 'handgun' closer to your face than the same person holds a revolver to fire it. As such, the muzzle blast and report seem much more pronounced (due to less distance over which to dissipate).

Powder burn rate. -Most handgun powders reach peak pressure in less than 6 inches, and are then dropping by the time the bullet exits. Yet, most rifle powders don't reach peak pressure for more than double that, and are often at or near peak pressure when bullets exit short barrels.

Operating pressure. -.44 Mag runs at 36,000 psi map (maximum average pressure). .223 Rem runs at 55,000 psi map. That 19k psi makes a huge difference in muzzle pressure and perceived muzzle blast.
And there are more....

I know many people find a 7.5" AR to be incredibly offensive, but I must note:
No one that has shot mine (5.56x45mm) has really complained about muzzle blast or the report. Some people have commented that it's definitely more noticeable than certain other firearms, but none have voiced a problem with it -- even people that very much dislike my .327 Federal revolvers (45k psi), for the perceived offensive muzzle blast and concussion.
(Even there, I'm not really in agreement - I think .357 Mag is worse...

)

Beretta686 · August 10, 2014, 03:42 PM

Thanks for the info. I guess that also helps explain why my 5.7 is so damn loud, even though it's not really doing much.

Though I'll have to disagree on the 7.5" upper being annoying. Everytime I pull off the KX3 indoors, people stop shooting and look at just what you're lighting off. Outdoors, it's not too too bad, but if I shoot it under an overhang, it rattles the roof.

It really shows off it's blast at night without the KX3. As soon as my M16 comes in, I'm going to slap that 7.5" upper on it and do a 60rd mag dump to light up the night.

Mobuck · August 10, 2014, 08:10 PM

I can't imagine a 7.5" AR since my 11.5" is so obnoxious I hardly ever shoot it. I keep it loaded as a bedside gun mostly because it easily supports a high output light and laser and is not much harder to manipulate than a similarly equipped handgun.

August 9, 2014, 02:08 PM	#1
Beretta686 Senior Member Join Date: May 11, 2005 Location: Texas Posts: 761	Different Blast Between 7.5" Rifle and 7.5" Pistol? I have a 7.5 inch .44 Mag and a 7.5 AR-15 in 5.56. Shooting the .44 Mag has quite a hefty report, but not something that will blow your eardrums out after just one round without earplugs. However shooting the 7.5 AR will blow your eardrums out. Why is this? Clearly the .44 mag is pushing quite a bit more energy than the 5.56, but it's nowhere near as loud as a shorty AR shooting the relatively anemic 5.56. Clearly there's a reason for this, but I'm not sure why. __________________ "Our contract called for 16 cases of rifles and ammunition for $10,000 dollars, not a machine gun...........That is our present to the General"-Pike Bishop “When fascism comes to America, it will be wrapped in the flag and carrying a cross.”

August 9, 2014, 03:58 PM	#2
FrankenMauser Senior Member Join Date: August 25, 2008 Location: In the valley above the plain Posts: 13,424	There are many factors at play, not the least of which are: Position of the firearm. -Most SBRs and AR pistols are fired with the 'handgun' closer to your face than the same person holds a revolver to fire it. As such, the muzzle blast and report seem much more pronounced (due to less distance over which to dissipate). Powder burn rate. -Most handgun powders reach peak pressure in less than 6 inches, and are then dropping by the time the bullet exits. Yet, most rifle powders don't reach peak pressure for more than double that, and are often at or near peak pressure when bullets exit short barrels. Operating pressure. -.44 Mag runs at 36,000 psi map (maximum average pressure). .223 Rem runs at 55,000 psi map. That 19k psi makes a huge difference in muzzle pressure and perceived muzzle blast. And there are more.... I know many people find a 7.5" AR to be incredibly offensive, but I must note: No one that has shot mine (5.56x45mm) has really complained about muzzle blast or the report. Some people have commented that it's definitely more noticeable than certain other firearms, but none have voiced a problem with it -- even people that very much dislike my .327 Federal revolvers (45k psi), for the perceived offensive muzzle blast and concussion. (Even there, I'm not really in agreement - I think .357 Mag is worse... ) __________________ Don't even try it. It's even worse than the internet would lead you to believe.

August 10, 2014, 03:42 PM	#3
Beretta686 Senior Member Join Date: May 11, 2005 Location: Texas Posts: 761	Thanks for the info. I guess that also helps explain why my 5.7 is so damn loud, even though it's not really doing much. Though I'll have to disagree on the 7.5" upper being annoying. Everytime I pull off the KX3 indoors, people stop shooting and look at just what you're lighting off. Outdoors, it's not too too bad, but if I shoot it under an overhang, it rattles the roof. It really shows off it's blast at night without the KX3. As soon as my M16 comes in, I'm going to slap that 7.5" upper on it and do a 60rd mag dump to light up the night. __________________ "Our contract called for 16 cases of rifles and ammunition for $10,000 dollars, not a machine gun...........That is our present to the General"-Pike Bishop “When fascism comes to America, it will be wrapped in the flag and carrying a cross.”

August 10, 2014, 08:10 PM	#4
Mobuck Junior member Join Date: February 2, 2010 Posts: 6,846	I can't imagine a 7.5" AR since my 11.5" is so obnoxious I hardly ever shoot it. I keep it loaded as a bedside gun mostly because it easily supports a high output light and laser and is not much harder to manipulate than a similarly equipped handgun.