Damage to case base - ideas needed

[DD2000]

AR15 from Rock River in 204 - 26" heavy barrel and beautiful RRA 2 stage trigger
Hornady cases, 26.5gr Reloder 10, CCI400 SR primer all pushing a 32gr VMAX hornady bullet
Shoots a dream - nice velocity and beautiful sub 1/2 MOA groups. The gun also shoots Hornady 32gr factory the same.

Both loads come out of the gun with some base rim damage after firing (not destroying the case but some rim damage)- I am not 100% sure if this is extractor or just pressure against the bolt - I suspect the former.

Next weekend I am going to fire some with gas tube removed - so single shot - that will confirm it is extractor damage.

The load shoots just too well to abandon (my fourth powder test type and 1st to really do it) - so I am looking for ideas (like a longer gas tube or adjustable gas block or something) I can do to the gun to be able to shoot this without trashing the cases.

So experts of the world please help me in my quest for that super accurate varmint destroyer

 

[dan06]

I am not familiar with RL 10 but that is a gas gun and if the pressure doesn't drop before the bolt tries to open you can play heck with the rims. My AR has worked pretty well with most powders but it refuses to cycle with IMR 4198

Try some H335 I have had good luck with that. Or some IMR4895

 

[Minesweeper3433]

If you disable the gas system and still get the scrapes on the rims then its your extractor. I had the same problem with both bolt assemblies for my uppers. Check Under the extractor for debris and for proper clearance by test fitting round to bolt face while it is out of gun. I have had this with a several rifles and all my gas guns I just adjusted the claw on the extractor with a set of needle files. But test for gas system first.

 

[ronemus]

The problem is most likely timing - the bolt is opening before the gas pressure has dropped sufficiently. The real cure is to move the gas block forward and use a correspondingly longer gas tube. This is usually done by knowledgeable AR gunsmiths, but not by the factory, with long-barreled guns. The factory port position is correct for a 20" barrel, but too far back for a 26" barrel.

There are things you can do to retard the bolt opening, primarily by adding weight to the bolt carrier group: the Tubb weight system, a heavier buffer, and a heavier M-16 carrier. These all help, but the real solution is moving the port. The port on my 26" .223 barrels is 2" forward of the "normal" rifle location.

 

[DD2000]

Thanks for the input
I have tried H335, H4895 and Clays but not managed to get anything like the group sizes I am getting with the R10
Interestingly with R10 I get better groups with CCI400 primers than Rem 71/2 or Fed Match - there seems to be something in harmony with this load and my gun at the barrel end but not the extractor end

 

[DD2000]

If you disable the gas system and still get the scrapes on the rims then its your extractor.

Thanks - I plan to do that test this weekend
It is kind of important cause I am also getting it with factory Hornady ammo

 

[DD2000]

The problem is most likely timing - the bolt is opening before the gas pressure has dropped sufficiently. The real cure is to move the gas block forward and use a correspondingly longer gas tube. This is usually done by knowledgeable AR gunsmiths, but not by the factory, with long-barreled guns. The factory port position is correct for a 20" barrel, but too far back for a 26" barrel.

There are things you can do to retard the bolt opening, primarily by adding weight to the bolt carrier group: the Tubb weight system, a heavier buffer, and a heavier M-16 carrier. These all help, but the real solution is moving the port. The port on my 26" .223 barrels is 2" forward of the "normal" rifle location.

Now this was my thinking so I was considering the 'pigtail' gas tube but the reviews are not glowing

http://www.midwayusa.com/viewproduct/?productnumber=144389

So my plan 'a' is to see if Minesweeper is right at the weekend by a 'gas off' test
Plan 'b' is to consider trying a pigtail gas tube - but how do you keep it clean ? Will it fit under my Hogue tube > etc
Kind of hoping that Minesweeper is right - the new factory gun should shoot factory ammo ! So maybe just a tolerance issue

Thanks for all input and please keep it coming

 

[DD2000]

I just had an idea based on Minesweepers comments - what happens is I use a different bolt from another gun (.223 is same base size) ?

Easier than taking gas tube off - anybody see any pitfalls (after all this is a quite hot load) ??

 

[kentuckytroutbum]

The problem is most likely timing - the bolt is opening before the gas pressure has dropped sufficiently. The real cure is to move the gas block forward and use a correspondingly longer gas tube. This is usually done by knowledgeable AR gunsmiths, but not by the factory, with long-barreled guns. The factory port position is correct for a 20" barrel, but too far back for a 26" barrel.

There are things you can do to retard the bolt opening, primarily by adding weight to the bolt carrier group: the Tubb weight system, a heavier buffer, and a heavier M-16 carrier. These all help, but the real solution is moving the port. The port on my 26" .223 barrels is 2" forward of the "normal" rifle location.

I concur with ronemus on his ideas. I used the Tubb weight system to slow the bolt opening primarily for accuracy and follow through on HP matches. But it will also slow down the unlocking sequence.

Also, when I built my 6.5 Grendel, Steve Saturn (who was making the 26" s/s barrel for me) and I discussed the gas port and its location in great detail. Steve was correct in that the gas port needs to be 2" further out to allow gas pressure in the chamber to drop before unlocking and extraction.

Also, RL10 may be too "fast" for the barrel length. I will also try some "slower" powders to take advantage of your longer barrel length to achieve higher velocities possible with the barrel.

Bill

 

[DD2000]

I concur with ronemus on his ideas. I used the Tubb weight system to slow the bolt opening primarily for accuracy and follow through on HP matches. But it will also slow down the unlocking sequence.

Also, when I built my 6.5 Grendel, Steve Saturn (who was making the 26" s/s barrel for me) and I discussed the gas port and its location in great detail. Steve was correct in that the gas port needs to be 2" further out to allow gas pressure in the chamber to drop before unlocking and extraction.

Also, RL10 may be too "fast" for the barrel length. I will also try some "slower" powders to take advantage of your longer barrel length to achieve higher velocities possible with the barrel.

Bill

Thanks Bill

I have tried several powders and from an accuracy point of view this R10 load is head and shoulders above the rest (of what I have tried), with this rifle. So I am trying to find a way to keep using this load if I can - also I am suspicious someting is wrong because the factory loaded Hornady also shows the same problem.
With regard to the speed of the powder - - wouldn't a slower burning powder keep building pressure longer down the barrel and lead to a higher pressure in the gas tube and a higher pressure in the case at extraction ? I guess it depends on where in the bullets travel down the barrel peak pressure is attained. Got any suggested loads that give good accuracy in a self loader (I am not after ultra fast just >4k ft/sec and tight group)?
I am hoping that it may just be a tolerance/machining issue with the extractor or that I could get away with slowing the system down using a 'pig tail' gas tube.

Thanks again for the suggestions/input

 

[kentuckytroutbum]

DD2000-

What I was trying to say about a slower powder is that you will have a longer "burn" of powder in the bore to gain a higher velocity. Burn characteristics and port pressure can vary, even with different lots of the same powder such as RL10.

You might talk to the Chief Ballistician at Alliant and discuss with them.

But, I still believe that the gas port is too close, and needs to be moved. I have no experience with the "pigtail" so I can't respond to whether it will work or not. I do have a concern about the gas tube wrapping around the barrel though. As you noted, there may be a clearance issue with the barrel, handguard, etc.

Another option, may be to move the gas port. Plug the old one, and install a new gas block further out. I used a low profile gas block from Larue Tactical. (And it will require a longer gas tube which are available). Google his website, and give him a call. He does answer his calls.

Bill

 

[DD2000]

OK a few more tests
Some pics attached - not sure how they will look at your end - hope they are clear

First one (upper) is a new Hornady case fired once with 26.5gr R10 - Monday
Second one (lower) is a factory hornady cartride fired and then reloaded 26.5 R10 - both today

Upper case is damaged (once fired) and lower (twice fired) is not !!

After reading Minesweepers comment I dissasembled the extractor to the max number of parts, gave serious clean and found some small bits of brass attached - took some removing. Note case now looks OK.

I would like to know how the brass got there ! If only to make sure it doesn't happen again.
Possible over load during ammo testing last week ?

On another note - the primer in first pic shows some back flow - do people think this is OK or too much and if 'too much', do you know a thicker primer than a CCI400 ?

Thanks for the support !

 

[CatShooter]

The CCI #400 is a very thin primer (0.020")... go to the #450, it is 0.025".

 

[jonbearman]

Why not a jp enterprises adjustable gas block. It worked on mine.

 

[Minesweeper3433]

I almost forgot. Do this somewhere safe. Cycle brand new loaded rounds threw the gun to see if the marks show up without firing. if there are marks on the outer edge of the case then the extractor is a wee bit tite. I had this problem with a 6.5 build as the bolt came with a 223 extractor and had to be opened up to the larger diameter rim. works fine now.