AR Build Gas Tube

[Kettle93]

Hello all, I'm new to THR and sorta new to the whole Ar building thing as well, only tinkering and such with mine up until now. Here recently I have been seriously looking around at what parts to get for my first build, and one of the biggest things that I've been thinking about with it is were to put the gas block/tube at. I'm almost possitive I'm gunna get the White Oaks SPR 18'' barrel (which has a 1:7" twist which I'm not to sure about either so if you have input about this twist ratio help me out as far as accuracy goes for the twist) but haven't been able to decide on whether to get the barrel with a Mid or Rifle length gas system. If someone could chime in on what the pro's/con's of each are as far as reliability and accuracy it'd be VERY appreciated. Thanks for any imput.

 

[otomik]

for 18 a rifle length gas system is better. it will be a little softer shooting but still have enough dwell time to do the job reliably unless you're shooting some pretty weak loads.

one example is the mk12 SPR which uses an 18 inch barrel and rifle length gas system.
http://en.wikipedia.org/wiki/Mk_12_Special_Purpose_Rifle

 

[MrCleanOK]

Rifle gas will be softer shooting, and middy gas will will probably ensure better function across a wider range of loads. If you are concerned about accuracy, I would get the rifle gas, and especially if you hand load or don't mind passing up weak commercial ammo.

 

[KansasPaul]

the question about gas tube position is covered so I'll add input regarding barrel twist:

1/7 twist should allow you to shoot anything from 55 grain to 70 grain bullets accurately. The heavier bullets typically benefit the longer shots. Heavy bullets usually vary in bullet length and the .223 accuracy is sensitive bullet length. Stick with bullets in this range and you should be good to go. If you are a handloader there is a myriad of bullet/powder combos available.

Blessings,

Paul

 

[HOLY DIVER]

I have a complete upper just what you want 2 build check it out its on the buy/sell/ trade sec of the forum.except its put together and ready 2 shoot.

 

[Kettle93]

Wow, thanks to all for replying, definately going with the rifle length gas now, after all the weakest that I'm gunna be shooting out of it is some American Eagle 55gr 5.56 stuff which I don't believe is too under powered. As far as the twist ratio, I wasn't too certain on whether or not the twist was going to be too much for a longer barrel like that but I guess not. Looks like I'll be calling WOA before too long.

 

[mshootnit]

1/7 twist is too fast for any rifle barrel in my opinion. It can cause 45 gr bullets to fly apart, and hence cant be too good for 50 gr. loads either. The only way I would opt for 1/7 would be if I was going to for sure be shooting A LOT of 75 gr ammo or heavier. My old 1/12 twist ruger is plenty enough twist for anything up to 60 gr.

 

[ugaarguy]

1/7 twist should allow you to shoot anything from 55 grain to 70 grain bullets accurately.

You can shoot conventional jacketed lead 77 gr bullets in a 1:7 twist .223" / 5.56mm bore (i.e. Mk262 Mod 0/1 ammo), and even the equally short 87 grain powdered tungsten core bullets developed by the now defunct Powell River Laboratories. DRT is making bullets using the technology developed by PRL, but they aren't currently offering anything quite that heavy.

1/7 twist is too fast for any rifle barrel in my opinion. It can cause 45 gr bullets to fly apart, and hence cant be too good for 50 gr. loads either. The only way I would opt for 1/7 would be if I was going to for sure be shooting A LOT of 75 gr ammo or heavier. My old 1/12 twist ruger is plenty enough twist for anything up to 60 gr.

The fast twist can make light construction bullets come apart at velocities exceeding 2800 fps. True, most of these light construction - i.e. thinly jacketed - bullets are 50 grains and lighter. I would counter the above point and recommend 1:8 or 1:7 twist, unless you specifically plan on shooting light construction bullets. Speer recommends 1:10 of faster just for 62 grain bullets. 1:12 twist wont even stabilize the Winchester 64 gr SP, which is one of the best all around use .223 commercial loads available. My opinion is that you're limiting yourself with the slower twist rates.

I've posted this recently in another thread, but let me reiterate it here. Bbl length has NOTHING to do with required twist rates. Also, 1:12 twist shoots 55gr ball (M193) no better than 1:7 twist.

FM 23-9 p. F-15 (p. 210 of 305 in the PDF) http://www.ar15.com/content/manuals/FM23-9.pdf.

The M16A2 rifle with its 1:7 twist fires both types of ammunition with little difference in accuracy out to a range of 500 meters. The M16A2 and its ammunition are more effective at ranges out to and beyond 500 meters due to a better stabilization of the round.

The two 10-round shot groups in Figure F-15A were fired by a skilled marksman at a distance of 274.2 meters, using the same M16A1 rifle. The 25.4-cm shot group on the left was fired (and zeroed) with M193 ammunition. The 6-foot shot group on the right was fired after substituting M855 ammunition.

Figure F-15B shows two 25.4-cm shot groups fired by the same skilled marksman at a distance of 274.2 meters, using an M16A2 rifle. The shot group on the left was fired (and zeroed) with M855 ammunition. The shot group on the right was fired after substituting M193 ammunition.

Note that at 274.2 meters, both the M16A1 (1:12 twist), and M16A2 (1:7) both fired 25.4 cm groups with M193 ammunition. The M16A2 fires the same 25.4 cm group with M855 ball at 274.2 meters.

Take a look at the information presented, and decide what will work best for the intended use.

 

[Kettle93]

Well, all I have/ probably ever will shoot is just going to be in the 55-70gr range anyhow so I shouldn't haved to worry about the round frag.ing on me.