Anyone running a 7in barrel on their ar15?
 

Is anyone running a 7inch barrel on their ar15 in 300 whisper/fireball ? If so what loads are you using? Do subsonic loads cycle with a barrel this short? If you know what size gas port you are running please let me know.



Thanks.

My only 7" ARs are in 223. My shortest 300 whisper is 10.5 with the gas port at the same position as a 7" pistol. No doubt you can move the gas port closer to the chamber and still use a reasonable length gas tube to prevent early bolt opening. The gas impulse from the gas port is at the average pressure from the time the bullet passes the gas port until it exits the muzzle. The energy available is the time x pressure product. gas port area is a multiplier. Sure a larger gas port helps, but 300 Whispers have about 1.9 times the bore volume and about 1/3 the propellant of a 223. The 300 Whisper propellent is (usually) much faster burning so the pressure drops very rapidly with bullet travel down the barrel. The slower bullet plugging the barel helps the time factor, but it's still won't be as much available energy at the port as a 223.

Ever look at a Desert Eagle gas system? Their gas port is about 1 mm in front of the case mouth. The gas still travels the length of the barrel (in a small tube parallel to the bore) to reach a gas piston near the muzzle.

If you're planning on suppressing and just want the overall length reduced you might consider this Finnish design where over half of the suppressors expansion volume wraps around the barrel instead of sticking out in front.
http://guns.connect.fi/rs/tx4cut.jpg
http://guns.connect.fi/rs/ingram.jpg

Reason why I ask is that I was going through some of my old parts and came across a barrel I had picked up about 6 years ago. Its a 300 fireball 1x8 twist, 7 inches long and threaded 1/2x28. I found enough parts to build a complete upper including a new bolt but don't have a upper receiver. I was wondering if it would run and be worth buying the upper to build it. Any thoughts on only 7 inches stabilizing heavier bullets ? I already have a 16 inch and 10 tp555 running and have a sbr and pistol lower so a shorty would be cool if I could get it running.


Thanks.

I've made them down to 8 1/2 but the guy said it wouldnt cycle without the can on... .120 gas port in the pistol position.

my gas port is .125, 8.5" Bartlein tightbore, and it will not cycle properly without the can in place

Sounds like you need a custom length gas system.

What you can do is make a high gas block, take a rifle length tube, cut it and turn the end and have a straight tube. I actually used a piece of 3/16" brake line on my TOK, the gas system is less than half the length of a pistol gas system.


Jon

Great idea, any chance we could get some visual aids.... for the creatively challenged :o ?

Yes, I was thinking about moving the gas block back some. Any one have any ideas on how far or what size gas port to start out with ? It will take me a little while to do this but I'll report back on how it goes.

Visual aid added.

Also in the picture is a rifle length gas tube, cut it off at the length you need less than pistol length and profile it, along with a taller gas block you can use a straight tube.

I wonder if there is a demand for tall gas blocks, I could have some real nice ones whipped up at work.

See this pressure curve, if you could capture at the peak or just after and regulate the port size(start at about .060") you could probably get a really short barrel out of it(5-6").
http://stuff.jfettigmachines.com/240smk.jpg


Please don't post the pic below on other boards.

Jon

Jon, Thanks, this is good info. I am not sure how you did your chart but could you do one of what a full charge .223 looks like. I know if I try to unlock the bolt with the pressure to high I'd have extraction issues. I am trying to figure out at about what pressures a .223 does it.


Thanks again for the info.

http://ar15barrels.com/tech/223plot.gif
from ar15barrels.com

The port size determines the pressures your system sees. Time under pressure is another factor too.

Jon, nice gas block, will it fit under a float tube and how much would you make them for? I've wanted to try a 30 luger with 208 Amax and that looks like it may be what I need.

A tall gas block would not fit under a standard FF tube, it would fit under glacier guards though. The one pictured would fit under a FF tube. I have a similar one on my 300/221.

I don't think I'd be making them in my shop if I did some, I'd have to get a quote from work.

Spook, some guys are running large bullets in 7.62x25 TOK uppers. I haven't read much into it though. They run them from Pmags. AR15.com pistol cal and variants sections have a few posts in there.


Jon

I moved the gas port back 1 1/2 inches and made a gas tube by cutting down a rifle length tube and forming it to the shape I needed. Its running with a .100 size hole. So far its running with 110 - 150 - 240 grain bullets, the powders were lil gun, reloader #7 and h335 as these were the powders I had on hand. All but the 110 grain bullets locked the bolt back on an empty mag, it did feed the next round though. This was just a function test so I did not clock the rounds or check for accuracy. They were all subsonic though, I have shot some of these same loads out of my 10 inch barrel and they ran about 1000 fps. Barrel length is just over 7 inches and these were run without a suppressor.

mak, very cool :smile::grin:,how was the accuracy?

Loooks pretty close to what I'm about to do soon with a barrel I have sitting here :)

Spook, Not sure on the accuracy yet as I did not have the opportunity to bench shoot this yet. I did shoot some 240 grain smk at point blank range (about 2 feet from the muzzle) through a cardboard target to see if the bullet was stabilized or not. The holes were nice and round so it looks like stability is not an issue with a barrel this short and the heavy weight bullets.

JFettig and mak, I wont be too surprised if these short barrels end up shooting moa out to 100yds.
Good on you guys for going where no whisper has gone before :smile:

Hopefully testing tomorrow :grin:
http://stuff.jfettigmachines.com/300w/tn_IMG_2000.JPG

-edit- shot flawlessly, hit the gas port diameter first try, locked back positively without too much gas.

Jon

No accuracy results yet but I did clock a few loads. Reloader #7 12 grains with a 150 grain bullet ran 880 -910 fps, the brass showed some slight marks from the extractor, makes me think I might have opened up the gas port a bit much for this powder or the port is to close to the chamber for the powders speed. Lil gun 7.8 grains with a 180 grain bullet ran at 934 - 967 fps brass looked ok. These are just the loads I had on hand. Does anyone know of powder that is a little faster than Lil gun (69) that will take up more volume by weight ? I have Accurate #9 but it seems to have a bit less volume by weight. I am looking for a powder to use with the 150 grain bullets that will take up more case volume. I don't like how much space is left in the case with the subsonic 150 grain bullets and these powders that I have.

couldnt resist
 

http://usera.ImageCave.com/Spook/1bullethose.jpg
I couldnt resist so I spun one up this afternoon...8.5",8T, set up for a .940 gasblock...It'll probably end up reamed to 308 X 39 but in either case it'll be a hose. Left it heavy to discourage muzzle climb.

Looks good Spook,

Keep in mind that the shorter gas system is probably not optimal for supersonic high pressure rounds, I haven't run quickload on it but I bet it would have too high of pressures.

Jon

Jon/mak, Had it out today with 220 rn's over 9 gr VVN 110...ran like a sewing machine...the accuracy was right in there with the 10.5", which was about what I had figured.

Is this the evolution of the whisper barrel?

Nice work Spook. That high mount upper is my favorite for optics.

I think it is an evolution, not being able to shoot supersonic rounds will be a turn off for some. I'll be making up a fixture for making gas tubes soon.

Jon

You have to wonder if an adjustable gas block with a "low" setting for supersonic and a "hi" setting for subsonic......

Kind of the opposite for what it is when you use one for the pistol position?:smile:

I didn't have much to go off of with quick load but it looks like pressures will be at least 1.5x on a 125gr bullet traveling 1600fps out of a 7" barrel.

I have a bunch of 160gr bullets that maybe I'll work up a load slowly, from 1000fps to as fast as I am comfortable with.

Spook, your labeling is backwards ;) even if it was pistol position the port would decrease for high velocity.


Jon

:grin: Jon ,you're right...must be AADD :o best get back on my meds.

Funny you guys would think about an adjustable gas block. I ordered one the other day for my 7 inch barrel. Its a PRI low profile adjustable gas block. I got this one because it is the exact size of the non adjustable one I have on it now so I should just have to pop out the gas tube I have already modified from the old one and use it on this one without any more mods.

Now that I think about it some more, It should be feasable with an adjustable block but it might be finicky. I mean think about how finicky 5.56 is through a 7.5" barrel.

If all goes as planned, I'll be shooting some 165gr and more 220gr subs through my barrel tonight.

Jon

I just want to confirm my understanding on these threads. 300 whisper with gas block in pistol position will not cycle reliably without the can with barrel lengths in the 8-8.5 inch range? I was thinking of shortening my 12 inch barrel down to reduce overall length. I do have a PRI adjustable gas block so maybe I could open up my gas port more for unsuppressed. I rarely shoot it without the can though.

Medic650, My barrel was 7 inches to start with and I move the gas port in 1 1/2 inches to get it to run with or without a can. From what others have said the 8 - 81/2 inch barrels do not run well without a can with the standard pistol position gas port but I have no personal experience with that length. I do not know what loads they were running but my setup will run subsonic with bullets as light as 110 grain. I've not tried any super loads yet as I am still working things out with it but it does show promise.

I'm waiting for a update from you and Spook as how well the
7" and 8" barrels with the shorter position gas blocks are cycling with
the heavy lead sub ammo.
I'm thinking maybe a 7" to 8" barrel for a form 1 integral I have
plans for and the shorter the gas tube means the longer the can :wink:

rtd, I've got no complaints thus far shooting 220's right at 1050fps. You cant tell that you're not shooting a 10+" barrel.:smile:
I'd like to hear maks accuracy results and Jon had pm'd me with really good groups from his "evo" barrel.
My next step is an adj gas block, or a more restricted port for the high vel loads.
I've been thinking some 110VMax's loaded up screaming fast would make a pretty decent personal defense weapon set up as an AR- pistol w/red dot optic? I've got one of those old ChiCom 100+ round drum mags around here somewhere........

Sorry Spook no accuracy reports yet but here is a picture of the little guy. I've opened up the gas port to .120 and put on a adjustable gas block. I've run 110/147/150/175/180/200/220/240 grain subsonic loads using 5 different powders that I have on hand. Adjusting the gas port 2 - 3 1/2 turns (depending on the load) and all the loads cycle with no issues with a 9mm can. This is roughly 1/2 to full open on the gas port. For the heavy loads of the powders that I have the best load seems to be 240 smk with 8.5 grain accurate #9 running right at 1000fps and making the least amount of noise. Lil Gun coming in second for the heavy bullets. For the real light bullets I tried IMI sr 4759. It is slightly slower than lilgun but takes up much more room in the case. It seems to work well so far but looks to be not as clean.

Here is mine :grin:

http://stuff.jfettigmachines.com/300w/tn_IMG_2063.JPG

Damn this has me wanting spook to build me another whisper, I have a sbr lower and a dias, but dont wana run it on fa with my 30 cal can. Possibly a .308x39? Wonder about reliability with that round with the mag and bolt problems everyone had in the past. I heard some issues were fixed but have heard mixed results with the mags jamming and having to tune them along with the boltfaces breaking off still.

Oh I emailed SWR if anyone is interested and he said ok aslong as I understand my can life may decrease some more than normal.

I may just fins a howa 223 and flip it into a whisper or a 308x39 instead especially with the cdi and ai mags out now.

My friend saw mine and begged me to build him one :grin:

http://stuff.jfettigmachines.com/300w/tn_IMG_2094.JPG

We're just waiting for his gas block to get in so I can fit it to give him a nice tight seal. I am supposed to have it tomorrow.

I've now shot mine with the gas system in 2 different locations, both worked flawlessly.

Nice work JF :smile: ........has anyone figured out how short they can go yet?

mak is that a Krink flash suppressor on your SBR?

Spook, that is just a 1/2x28 - 10x3/4 thread adapter. I was using my m11 9mm can on it while I waited for advanced armament to make me a custom 18t 1/2x28 -308 adapter for the 7.62SD can I have of theirs. I originally got the 7 inch barrel from M1S years back when they were threading them 1/2x28.