Will this cause problems with too much gas due to the dwell time?  On 8541 Tactical (YouTube channel - https://www.youtube.com/watch?v=nFItXuEVWqo) they have been showing a build very similar to mine (exact same barrel), but I have just a regular gas block, not an adjustable one.  They refer to concerns about the dwell time of the bullet in the barrel between the gas port and the muzzle (~11 inches of travel).  I have a standard weight LR-308 BCG, and rifle length buffer system.  

Do you think this is ok, or should I replace the standard gas block with an adjustable gas block?

My 24" Creedmoor is pretty tough on brass when I run the factory Hornady 140 Amax load--it almost always exhibits ejector swipes.  I have an adjustable gas block on mine now, though I still don't have it tuned to my reloads (123 Amax over H414).  I can't decide whether I want to drill another gas port and use an extended length gas tube, which I think should be the standard gas system length on a 24" barrel.

If SCW sees this thread, he can chime in regarding his bolt and buffer setup.

That's definitely a lot of dwell, but I don't know that an adjustable block will make much difference.  An adjustable block only limits the volume of gas, which may slow down the peak pressure to the bolt somewhat, which may keep the bolt locked slightly longer, which may help, but a longer distance to the gas port will definitely reduce pressure at the port and bolt, and definitely delay unlocking.

I used the heaviest carrier and buffer I could find, and then used the longest and stiffest spring I could get in order to slow the unlocking of the bolt as much as possible.  It helps, but I'm still pushing my limits on it.  I'd like to have a longer gas system, and I'm using a 22" barrel with the gas turned down as much as possible to keep reliable cycling.


Look at it this way, what are your priorities?  On my lightweight AR in .223 I wanted fast cycling and the lowest overall weight I could get.  I used a very stiff spring, removed two weights from a standard buffer and replaced them with a light-weight filler and bought a lightened carrier.  It's fast and reliable for .223 where I really don't care about the brass.  I wanted reliable, light and fast in that order, and that's what I got.  I am beating the piss out of the brass in exchange, but .223 brass is cheap.

On the 6.5 I was more worried about brass life so I wanted chamber pressures to drop as much as possible before the bolt began to unlock.  The next barrel I buy will use a custom gas length out another couple of inches past the RLGS because it's still hard on brass and I've maxed out what I can do to limit abuse to the brass.  It's heavy, but the gun is over 14lbs and the weight in the back is actually helpful for balance, but overall weight takes a backseat on this rifle to brass life and reliability.  I'd much rather have a large gas port wide open way down the barrel than try to limit gas volume in a poor attempt to delay unlocking with a shorter gas system.


Another trick that I learned from LRRFP52 is to really give a good bevel to the ejector so that it doesn't cut into the face of the case while the bolt is trying to unlock with chamber pressures still pushing the case back into the bolt face.  This will also cut down on little shavings of brass that tend to get stuck in all sorts of places.  

And lastly, make sure you are using a small diameter firing pin.  This simple change made a huge difference for me in every single load I tried.

Short story:  You need to hand load for this and other 6.5-08 type cartridges in the AR10.

My buddy built an extended length gas .260 Rem on an ArmaLite Inc. rifle, and it will handle factory loads well, but he also did a lot of work polishing and tweaking every aspect of the bolt he could to facilitate gentle treatment of brass in feeding, chambering, extracting, and ejecting.  It has at least a 26" barrel IIRC.

I personally am interested in 22" barrels or less for maneuverability.

Thanks for the input, all. I am planning on reloading and want to be gentle on the brass.  

One thing I forgot to mention is that the gas port appeared to have a very small diameter.  I should have measured it.  I have only seen 5 AR barrels before (my .223/5.56 builds) and I have measured a couple of those.  When I got this barrel I remember seeing the gas port and thinking it looked smaller than those on my AR-15s.   I thought I might have to open it up because it looked small.  

Hopefully I'll get to shoot it Wed. evening, and I'll post a report here.

UPDATE - Took it to the range today and shot it for the first time.  Everything worked well; cycled properly, held open after last round every time, and I even had a couple 1 moa groups (thank goodness, after what I've spent putting this together).  

One issue:  The first few rounds had signs of overpressure on the primers.  The fifth round I shot blew a hole in the primer right where the firing pin hit it.  I shot 40 rounds total, and by the end the signs of overpressure were greatly diminished if not gone.  The ammo was Hornady 140 gr A-Max.  

Very happy it's running well.  I expect the group size will tighten up a little more as things get broken it, and it would probably help next time to make sure my scope mount is tightened properly (it came loose since I had only hand tightened it).

Where do you get a small diameter firing pin?  Armalite that is.

Yeah, I either let everything rust together or use copious amounts of blue loc-tite when I put my stuff together.  A week ago it was about 50/50 rust and loc-tite.  I'm hoping it's a little better now in favor of loc-tite, but I won't know till I get the rifle back.  Someday...

Your problems with pierced primers is likely due to the larger DPMS-sized firing pin.  You can either get it drilled/bushed like I did by a member of this forum, get a JP High-pressure bolt or switch to an Armalite bolt and carrier.  I was getting pierced primers on moderate loads and even my Hornady factory load duplicate, but now my brass life is better and no blown primers.  I consider it mandatory for the 6.5.

Pics?





See above, modify a DPMS, buy a JP bolt or get an Amalite bolt and carrier.

While it is counter intuitive I have had better results from using the lightest buffer/action spring that will give reliable feeding.

The action spring preload does not seem to have a noticeable effect on unlocking timing but a high spring rate does require more gas to fully drive the action open.

I would suggest experimenting with lighter springs if you can adjust the gas down, you might find you can run on less gas with the lighter spring.

A side benefit is the lighter spring makes the action easier to manually operate.

Currently I am having good luck in a .308 DPMS G2 with the Springco AR-15 green, Rifle extension, and a 10 oz buffer.

SCW's 22" Krieger barrel has ArmaLite RLGS, .086" gs port, bushed firing pin, adjustable JP gas block on a .936" journal, and the gas is turned halfway down occluding the gas port significantly.

He also has a Slash 6.5oz extra heavy DPMS-length carbine buffer for the LR-308/AP4 carbine.  His recoil spring is just short of 10.75".  BCG is NiB.

With that arrangement, his hand loads run well through the gun.

Aaaaand???  What color might those parts be?

To add to that, I have also used that buffer and spring in an A2 stock with a spacer that Slash sells and the handloads I've settled on are the Hornady factory duplication load with H4350.  I've tried a slew of other powders, but that's been the best load since I had Ziarifleman turn the firing pin and bush the barrel.

Very Interesting.

I am collecting the parts for a similar build. I have a 24" Lilja SS 7mm barrel and would like to shoot the 284 cartridge. This thread sounds like a good place to chime in. Hope that is OK.

I  cannot find a measurement for the location for the gas block on a rifle gas system. I have ordered a Bat barrel extension (DPMS).  I have searched to find many measurements from many different places.

Could I get some advice on the gas system and opinions on the 284?

Brucifer, I'll be happy to offer an opinion.  

The 284 is similar to the Creedmoor/260 Rem in that it's a relatively large case volume vs bore area, so the pressure curve is longer than a 308, for example.  Consequently, I think you'll have a much smoother running gun if you go the extended length gas system like the one offered by WOA.  It's two inches longer than a rifle length gas system, and that's going to play more nicely with a 24" tube than the rifle length gas system.  You'll still have plenty of dwell time to operate the action, but the gun will be easier on your brass.

Regarding the actual gas system length measurement, here's what I did when I converted an AR15 with a carbine length gas system to mid length.  I'd bought the gas tube I planned to use and measured how far into the upper the gas tube was on another AR.  I then positioned the new gas tube in the upper the measured distance and marked the location on the barrel where the gas tube opening sat.  That mark was where I drilled the new gas port:

Tagged, I would like to get a 6.5 Creedmoor build going before long.  I was thinking a 24" barrel would be good.



Can someone measure the firing pin tip diameter on the DPMS and/or Armalite?  I've got what I believe is a DPMS patteren .308 BCG from AIM and I'm thinking it probably uses the DPMS diameter firing pin.  If so I'll probably use that one for a .308 and get something else for the 6.5.



Who sells extended length gas tubes?

WOA:

http://www.whiteoakarmament.com/xcart/product.php?productid=17648&cat=&page=1

Thanks  Scott_S

Yes after more research I agree a 2" longer GT would be better. Would a 4" longer tube be the way to go. I read that WOA will make custom tubes. Ruff measurements with a  +4" tube I would still have 6" of barrel

jaquforst, My Shadow Ops BCG firing pin has a 0.076 tip. Could someone explain way a smaller tip is less likely to puncture the primer?

Ok, well that didn't work.  Why won't pics hosted on photobucket show up here?

Pictures posted by new members have to be approved by three team members before they show up to everyone else.  I prevents spam and porn being posted on the board.



Post them up again and I can start the approval process.

Is that a post count thing. I have had problems posting pictures also. Don't want to post just to run up my count.

I did ask WOA about a +4" gas tube on a 24" barrel. They replied it would not be necessary and that a rifle length should work. But after the help I have read here I ordered a +2" tube. WOA sounds like good people.

Did you tell John it was a 6.5 large frame AR?

No, I told him I would chamber a 284.

I have a friend in Finland with a 6.5x284 AR10.  COL is limited by mags, so he shoots 123gr seated deep.

A 150gr 284 is list as 2.800 as is the 308 and should fit inside the mag. I ordered samples from Brownells of 140, 150,and 168 to see what OAL I can get.

Yes, I believe it is directly related to post count.  I believe purchasing a team membership solves the issue without having to post more.  People are a lot less likely to post crap after they've spent $25 on a membership.

DPMS is roughly .08" and Armalite is roughly .06".  The AIM bolt is likely to be .08, that's the 'standard' anymore.  If you need to, contact Ziarifleman to turn the .08 to .055 or so, I think that's what mine ended up at.

JP makes a good high pressure bolt too that you won't have to monkey with.

Yes $25 is not that much to pay to a site with so much information. I hope every one is aware that the gov wants to ban information sharing like this.

As inexperienced with the AR308 style of rifle as I am sure most of you have figured out. I just came to the realization that the M14 mag will not fit in a P-TAC lower. Live and learn. I have a DPMS mag on the way. By the time the sample projectiles get here I hope to have a mag that fits in the lower.

Point on the P-TAC lower. As far as I could tell the bolt catch pin hole is not threaded.

The 6.5 is on the far right.  The other two are ARs I built as well.  

Very nice!

Ordered a JP high pressure bolt and firing pin today.  Optics Planet had one left at $190 w/free shipping.  Primary Arms has them at $199.99 for now.  I'm going to cross my fingers that I don't have a headspacing issue.