115 DTAC in gas gun questions...

[Hellbender]

I'm getting a barrel built for an AR 10 in .243, will probably be running 2800-2850 fps w/ a 24" BBL.

What barrel twist for 115 DTAC's at this speed?

I have a choice of 7 or 8 twist, I'm leaning towards the 7 because of the 2800 fps velocity. But I have heard of spiky pressures from the faster twist, or is this from hotter loads only??

Any input or links to gas guns in .243 Win would be appreciated.

Thanks!

 

[cmillard]

it depends on how much free bore you want in the throat. if you can load the bullets out a little longer--you might be able to get a little more boiler room for powder to bring up the velocity.

 

[Hellbender]

I'll be limited by mag length, Max. OAL will be approx. 2.840 or so.

I hope to be able to seat the heel of the BT to the case neck junction and still have a little room to chase the lands as they wear.

I just ordered some bullets and brass to make up some dummies to send the bbl maker.

 

[HPMike800]

1:8 is to slow for 115's. The main stream thought is that 1:7.5 is best. I would be supprised if 115's will load mag length. The bullet has allways been desiged for single shot applications.

you might have to single load them. I assume this is an NRA match rifle.

 

[Hellbender]

It's actually going to be an AR10 in .243 Win. for Tactical long range competitions.

I wanted the 7.5 twist, but my barrel maker (Hart) only makes even 7 and 8 twist.

Thanks.

 

[rcw3]

Hellbender

I hate to say this but we went down that path with Hart barrels and bought the 1:7" twist Harts only to have bad pressure issues - don't go there! Go with a maker that will give you a 7.5" twist. I have two 6mm 1:7" twist Hart barrels I consider to be good for nothing. It has nothing to do with the length of the freebore as suggested by another poster here - it's that the twist is so tight it is more like a wall and less like a ramp.

Robert Whitley

 

[HPMike800]

What's long range in tactical? unless it's 800yds or more, you may be better served with a 6XC or 6-250 running 105-108gr bullets. They should allow better loading to mag length. Good proven loads too.

 

[spclark]

Tubb says the DTAC115 will work in a "... true 1:8" barrel but evidence shows that's on the ragged edge of what will work. I use 1:7.8 minimum for mine, 1:7.5 even better. 1:7 is too fast as RCW3 has noted.

Harts are awesome barrels but unless you match the twist to what your bullets prefer, any barrel will be a disappointment.

 

[Hellbender]

What's long range in tactical? unless it's 800yds or more, you may be better served with a 6XC or 6-250 running 105-108gr bullets. They should allow better loading to mag length. Good proven loads too.

I have a custom 243 WSSM on an AR 15 chassis now, shoots 87 gr. A-Max's into 1/2" @ 3050 fps (Can load up to 3150 fps) that is a hammer to 850 yards, but I am limited by mag length (and bbl twist) to 90 gr and less bullets, and they all run out of steam around 800-850 yards.

I'll shoot it when I can, but 3 or 4 big matches a year are out to 1100-1200 yards, and that 115 will do it, I believe, in a 243. If it won't work, the 105-108's are 2nd choice.

I'll know in a day or two when the 115's get here if they will load to mag length.

Looks like a Kreiger w/7.5 twist is in my future (with the extra 2 month wait over the Hart, bummer).

 

[Hellbender]

Hellbender

I hate to say this but we went down that path with Hart barrels and bought the 1:7" twist Harts only to have bad pressure issues - don't go there! Go with a maker that will give you a 7.5" twist. I have two 6mm 1:7" twist Hart barrels I consider to be good for nothing. It has nothing to do with the length of the freebore as suggested by another poster here - it's that the twist is so tight it is more like a wall and less like a ramp.

Robert Whitley

Thanks Robert, I was kinda afraid of that, good to get it from someone with real experience.

I have studied your site and guns in detail and you do some very nice work. I noticed you use the extended gas system on some of your uppers, I was going to do that on my rifle with an adjustable gas system.. (Hoping to lower port pressure with the slow burning powders)

Does the extended tube need a port location exactly 2" forward of standard?

Do you sell the extended tubes?

Thanks!

 

[watercam]

I have a 7" twist Lilja that works fine with DTACs and the .243. It's a really fast barrel and been shooting like a house afire for close to 2000 rounds.

 

[Hellbender]

I have a 7" twist Lilja that works fine with DTACs and the .243. It's a really fast barrel and been shooting like a house afire for close to 2000 rounds.

What velocity are you pushing those DTAC's?

What OAL? (hopefully under 2.85")

What load data?

Thanks for the info!

 

[rcw3]

Thanks Robert, I was kinda afraid of that, good to get it from someone with real experience.

I have studied your site and guns in detail and you do some very nice work. I noticed you use the extended gas system on some of your uppers, I was going to do that on my rifle with an adjustable gas system.. (Hoping to lower port pressure with the slow burning powders)

Does the extended tube need a port location exactly 2" forward of standard?

Do you sell the extended tubes?

Thanks!

We use the AR-15 extended gas tubes (2" longer than an AR-15 gas tube) in the AR-10 and it works out for porting to be 1.8" longer for an AR-10.

By the way - I also had a 1:7" twist Krieger that was problematic with pressures - I would avoid that twist like the plague. 7.5" twist is fine. Also Hart's 8" twist is many times 8.25" or thereabouts and you can find the 115's won't stabilize with that (been there, done that too).

There is no reason you cannot run the 115's magazine length out of a 243 Win AR-10. A fair amount of the bearing surface of the bullet will be back in the case, but you may find they shoot surprisingly well if you get the load dialed in right.

I have some longer gas tubes if you want one. Call me at 215-348-8789

Robert

 

[Hellbender]

Robert,

Thanks for the reply, I just called and left a message.

I've heard the DPMS's and the Armalite's (large frame AR10/LR) have different length gas tubes, is this correct?

I've had both, but got rid of the Armalite 10 years or so ago, their magazines really turned me off, I'm sure they are better now.

I've got a DPMS for this build.

I am just thinking out loud, but could an extended tube system (with the bbl contoured right at the chamber end) be set back the 1.8" and a standard length tube installed, to allow the barrel to be recut and rechambered when the throat gets shot out?

Or is there a way to shorten a gas tube, say by 1/2" to allow this? I guess you could just cut the tube and tap it for a small SS set screw in the end to plug it off and redrill the gas port.

Anyway, looking forward to your call.

Thanks again.

 

[rcw3]

Tried to return your call but line was busy - will try tomorrow.

If you are shooting a .243 and your barrel gets to the point you are thinking of setting it back, forget it, the barrel rifling will look like alligator skin for about 6" to 8" up the bore (not worth even trying a set back - shoot it until it's toast and forget it).

I have a couple AR-10/LR-308 barrel blanks in stock, both identical 6mm Pac Nor Super Match stainless 1:7.5" twist, polygon rifled. If you want to try one of them you would not have to wait for a barrel blank. I bought them for my own use but have not had time to chamber one up yet.

Robert Whitley

 

[rcw3]

By the way, you may want to check this out.

http://www.6mmar.com/Super_LR.html

The chamber for a DPMS LR-308 semi auto in 6mm like you want to build is the 6mm Super LR/S drawing at the bottom of the page.

Robert Whitley

 

[Hellbender]

That looks very interesting.

I'll get in touch tomorrow.

Thanks!

 

[flatlander]

I had John Holliger chamber & contour a 22" Bartlein 7.8-tw. in 6XC for my DPMS AR10 build. Going with a barrel that short, I didn't expect terrific velocity, but it's doing at least as well as a friend's modified 243 in a 20" Krieger on his DMPS. Results fireforming WW 22-250 brass with 105s & 107s have been very good, with excellent accuracy, and velocities in the 2815-2850fps range. RL17 gives around 2900fps with S107s in formed cases.

The only DTAC 115 load I've tested to date was with RS Hunter ball powder, which gave 2822fps, but was a tad too warm for my tastes. I intend to do some more shooting with 115s down the road, but with RL17 and perhaps N160.

I've got three 6x47Ls - two with 1:7.5 Kriegers, the other with a 1:8 Bartlein. They all shoot DTAC 115s really well, but the 1:7.5 bbls. start to show pressure at lower charge weights & velocities than the 1:8. Do not take that as a knock on Krieger - it's only logical that .236" bore 1:7.5 bbs. would show presure sooner than a .237" bore 1:8. The point I'm making is that a true 1:8 will probably shoot 115s just fine.

 

[Hellbender]

Thanks for the reply Dennis, good info there.

According to Quickload, Reloader 25 in the 243 size case with the DTAC's should give the best velocity, keeping pressure in the 55K range, I don't know exactly what I'll lose with the gas port (maybe 30-50 fps or so), but should be able to push it 2850-2900 fps w/ a 24" bbl.

We'll see in a few months, as I just ordered the bbl (7.5" twist Kreiger) and will be chambering it in Robert's wildcat 6mm Super LR posted above.

Thanks again!

 

[HPMike800]

Good choice of cartridges. Let us know how the perfomance is with 115's at mag length.

Robert:
Is the Super_LR the 6SuperX that's been discussed here? Looks like it.