Velocity Differences between 9, 12.5, 16" AAC Uppers with Question

[chip12345]

The tax stamp on my "NOS" AAC MPW 9" SBR dropped last week. Incidental to jumping both feet into 300blk, over the last couple of years I ended up collecting the 3 barrel lengths that AAC offered in 300blk. This of course gives me the opportunity to run velocity comparison's while trying to figure out whether there is value in keeping the 12.5" upper.

At ~500' elevation about 60 degrees outside using a $100 chrony from Cabelas, what I found...

With the Remington 120gr Factory "L300AAC1" ammo over 5 shot groups, the velocity averaged:

9" 1974FPS
12.5" 2058FPS
16" 2166FPS

With the Remington 220gr Factory "L300AAC4" ammo over 5 shot groups through my SDN-6 the velocity averaged:

9" 1045FPS
12.5" 1063FPS
16" 1136FPS (which of course was mostly supersonic as is well known with this mix of ammo and barrel lengths)


I found it most interesting that L300AAC4(220gr) velocities between the 9" and 12.5" were closer than the 12.5" and 16" barrel - i.e. the extra 73FPS suppressed is unexpectedly higher. I have been reading a bunch, but is there already an authoritative answer as to why the 16" barrel seems to result in non-linear increase in velocity compared to the 12.5"? I presumed the different gas systems has no bearing to this, but I can also reason that is a possibility(pistol gas with longer barrel length bleeds pressure for a longer period of time).

appreciate any input!

[Guest762]

Have you ever noticed that when a gaggle of geese is flying over one side of the "v" is always longer than the other side. Do you know why that is?

[dellet]

Have you ever noticed that when a gaggle of geese is flying over one side of the "v" is always longer than the other side. Do you know why that is?

I would guess it relates to ease of flight, conservation of energy. I always thought it was because they didn't have enough fingers and toes to make a proper head count.

I just hope we get the answer before the morning so I can sleep

I am curious how it relates to velocity gains.

[bangbangping]

Have you ever noticed that when a gaggle of geese is flying over one side of the "v" is always longer than the other side. Do you know why that is?

One of my favorites.
(but I don't know how it relates, either)

[bangbangping]

I would expect more velocity gain in the 3.5" from 9 to 12.5 than the 3.5" from 12.5 to 16. My guess is that if you cut the 16 down to 9 or 12.5 it would still be faster the comparable barrel. Chamber size, bore size...something is different. What are the barrel materials?

[Guest762]

Have you ever noticed that when a gaggle of geese is flying over one side of the "v" is always longer than the other side. Do you know why that is?

I would guess it relates to ease of flight, conservation of energy. I always thought it was because they didn't have enough fingers and toes to make a proper head count.

I just hope we get the answer before the morning so I can sleep

I am curious how it relates to velocity gains.

It's because there are more geese on that side. My point being that some things are just the way they are,BUT if this is what makes you happy about shooting (figuring out the minute details) then more power to you. Personally I would buy a reloader and a good trigger and spend time and money on accurate shooting.
I does say "appreciate any input ! "

[dellet]

Have you ever noticed that when a gaggle of geese is flying over one side of the "v" is always longer than the other side. Do you know why that is?

I would guess it relates to ease of flight, conservation of energy. I always thought it was because they didn't have enough fingers and toes to make a proper head count.

I just hope we get the answer before the morning so I can sleep

I am curious how it relates to velocity gains.

It's because there are more geese on that side. My point being that some things are just the way they are,BUT if this is what makes you happy about shooting (figuring out the minute details) then more power to you. Personally I would buy a reloader and a good trigger and spend time and money on accurate shooting.
I does say "appreciate any input ! "

I knew it was because they couldn't count

[rimshaker]

Thanks for providing more proof that the 220gr UMC ammo is crap. Subsonic should be subsonic, even through a 16" barrel... which is the official test length for 300 BLK.

[Guest762]

I know I'm being a smart ass but to answer questions like these you would have to get down to scientifically controlled conditions just for starters thing like :

Barrel temp the same for every shot
Ammo WAY more preside than factory loads
Controlled lighting on the chrono
A way more expensive chrono
Inside with no wind
Exactly the same twist rate on the barrels(and I mean exact)
Controlled precise humidity inside and outside of the casings
Oxygen content of the barrel and silencer(i.e. First round pop)
And that's just a few things off the top of the head of a dumb ole country boy

[dellet]

Maybe clarify the question.

Are you asking about a non-linear velocity gain from barrel length?

Or why the suppressor added more velocity to a shorter barrel than a longer one?