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Posted: 3/10/2006 8:15:10 PM EDT
what the optimum barrel length for a 9mm SBR ?

Still debating between TROS (5" , 7.25" or 9.25") and RRA 7.5", 10.5"

It will eventually be used full auto as well
Also suppressor in the future
Link Posted: 3/11/2006 6:21:57 AM EDT
I have a 10.3 for sale, but I would reccomend the 5" if you are planning on a suppressor.

I just think the shorter the better, I would actually reccomend a 3.1inch barrel as that would make the standard ammunition subsonic. As long as your suppressor will fit uner the rail you want to use that would be a good plan.
Link Posted: 3/11/2006 8:15:13 AM EDT
will there be a loss in velocity between a 5" and 7" ?
how long is a MP5 barrel anyways (for comparison) ?

thanks
Link Posted: 3/11/2006 8:40:09 AM EDT
I just put together a 9mm SBR recently. I went with a 5" Tros 3 lug. I have an SWR Trident Tucked into a DD rail. Overall length of barrel and suppressor is 11". Very Cool


Bomber
Link Posted: 3/11/2006 8:47:28 AM EDT

Originally Posted By thebomber:
I just put together a 9mm SBR recently. I went with a 5" Tros 3 lug. I have an SWR Trident Tucked into a DD rail. Overall length of barrel and suppressor is 11". Very Cool


Bomber



thanks. Yeah, after seing your SBR I decided to go that way
why not a 7.25" barrel?
Link Posted: 3/11/2006 8:53:51 AM EDT
If you are going to use a supressor, there is no reason to have a longer barrel. You don't want the extra velocity. Ammo needs to be subsonic.
Link Posted: 3/11/2006 8:56:35 AM EDT
I chose the 5" for several reasons. The pragmatic one was I wanted a 3 lug barrel and TROS only makes 5" and 9.5". I could have purchased a threaded barrel and had a 3 log adapter put on but it wasn't worth the trouble.

Since I'm going to be running a suppressor full time I wanted to be sure that 147 gr ammo would stay subsonic. The 5" barrel pretty much guarantees that. I wasn't certain t he 9" would or not. The 9" barrel with the suppressor is too long for what I wanted to do. With that said, if I wasn't going the suppressed route, I would have opted for the 9". In fact I am considering buiolding another upper with a 9" barrel.

Bomber
Link Posted: 3/11/2006 9:32:24 AM EDT
Thanks guys.

in this thread:

www.ar15.com/forums/topic.html?b=7&f=118&t=331584

Talon Arms seems to be ok with ordering a 7.25" from TROS ? does such a thing exist ?
if not I will probably go with a 5" then
Link Posted: 3/12/2006 11:21:34 AM EDT
Talon is reputable. If they say they are available then they must be. I was only aware of the the 5" and 9.25" barrels. Maybe TROS will do custom lengths.

Bomber
Link Posted: 3/12/2006 5:16:12 PM EDT
I think the 7.5 inch is a special order, and Talon has exclusive rights for 6 to 12 months or something like that.
Link Posted: 3/13/2006 2:49:55 PM EDT
I had a big problem with factory loaded 147gr hollow points going over 1100 FPS out of my 7.25 inch barrel 9mm ar15 pistol. I built a 5" TROS barreled upper built to use with my suppressor. Go with the 5" so you don't have to do it over again!
Link Posted: 3/13/2006 3:45:43 PM EDT

Originally Posted By terry1:
I had a big problem with factory loaded 147gr hollow points going over 1100 FPS out of my 7.25 inch barrel 9mm ar15 pistol. I built a 5" TROS barreled upper built to use with my suppressor. Go with the 5" so you don't have to do it over again!




I have a 5" and I'm not convinced the "subsonic" I have is actually subsonic. Unfortunately, my chrono was damaged during use , so I don't have a way to verify at the moment. I wish we had some data which showed the velocity of common ammo in a 5", 7.25 and 9.25 barrel.

Bomber
Link Posted: 3/13/2006 6:14:47 PM EDT
If you go with 147 gr ammo U (should) be able to stay under subsonic speed with a short barrel. UMC is at 990 FPS with a 4 inch barrel. Since the speed of sound is around 1130 fps depending on the air and temp around U. I dont know how much more speed U will get out of a longer barrel .
Link Posted: 3/13/2006 6:26:53 PM EDT
Just because it's subsonic doesn't mean it's going to be silent

Bullet flight noise gets much louder as velocity reaches the speed of sound (transsonic). IIRC bullet noise gets dramatically louder somewhere between 50-100 fps below the speed of sound.

I think the rule of thumb is below 1050 fps to be sure of "subsonic" performance.
Link Posted: 3/13/2006 7:14:38 PM EDT
Ar 15 pistol 7.25" barrel

Winchester Ranger 147gr SXT HP = 1100 to 1150FPS
Hornaday 147gr XTP HP = 1080 to 1140 FPS
Winchester 147gr Silvertip HP = 1000FPS average
American eagle 147gr FT FMJ 980FPS average


This was my resultd through my Chronograph over two testing days about a week apart. The problem for me is I have alot of the Ranger and XTP ammo two of my favorite loads and both were right at or above supersonic speed. I ordered a TROS upper but it's mostliky not going to be here for 2-3 weeks and I am waiting on a pistol lower to before I can test the 5" barrel and same loads.
Link Posted: 3/13/2006 8:16:43 PM EDT
very interesting guys. Thanks.
My TROS barrel will be used with suppressor too so I'm looking for perfect length. seems that the 5" fits the bill.
Link Posted: 3/14/2006 4:01:22 AM EDT

Originally Posted By exocet:
very interesting guys. Thanks.
My TROS barrel will be used with suppressor too so I'm looking for perfect length. seems that the 5" fits the bill.



Here's my personal set-up:




5" TROS with an SWR Trident tucked into a DD carbine length rail. No, the Mag clamps don't work with a side charger, it's a photo prop quite literally

Bomber
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