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Posted: 8/13/2006 6:41:25 AM EDT
[#1]
huh, Im suprised factory ammo would do that, gotta be on the DPMS side.
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Posted: 8/13/2006 7:08:32 AM EDT
[#2]
Do i have to go thru an ffl to send it back? I hope they correct this and not blame it on the ammo.
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Posted: 8/13/2006 7:23:02 AM EDT
[#3]
Absolutely not - you aren't transferring ownership of the firearm. |
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Posted: 8/13/2006 7:30:16 AM EDT
[#4]
ok cool. Any suggestions on what might be causing this. Id like to have some sort of idea before I talk to someone. one ohter thing, do you think i should pull out the rra 2 stage trigger before it goes back? I cant see how that would cause any problems but i dont want it to come back to haunt me.
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Posted: 8/13/2006 12:02:02 PM EDT
[#5]
The blown primers is easy -- the bullet is too high in the chamber. The ogive of the bullet is sitting very near or on the lands and grooves. This causes a significant chamber pressure spike and ultimately blown primers. The reason for this is manufacturing and it is not a defect. Think about it, the rounds were intended for use in a bolt gun where the bullets are set a little longer. How many semi-auto .260 Remy's are there?
There are a couple of solutions for blown primers. Both solutions requires the depth of the chamber to measured. A Stoney Point gauge or a bullet in a empty split case will measure the depth for you. The two main solutions are 1) seat the bullets deeper into the brass, 2) have the chamber reamed a little deeper. Obviously, solution 1 is the easier. As for the ejecter pin stamp on the brass -- it's Federal brass, some of the softest brass there is. My LR308 does the same thing on Federal brass. You have chose a less than mainstream round. You will probably need to start reloading to get consistent perfornance in both accuracy and operation. abn_lcss |
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Posted: 8/13/2006 12:08:09 PM EDT
[#6]
A .260 chamber is a .260 chamber. Gas or bolt - should not be a difference. If anything - gas gun chambers are often cut looser for reliability.
You have clear signs of overpressure. As stated - you can reload lower pressure custom ammo.... However, if you want the option of shooting factory ammo - then I would send it back and find out why it is doing what it is doing. Something is not right.... I seriously doubt DPMS makes a rifle with the intent that you cannot go purchase off-the-shelf ammo and use it. |
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Posted: 8/13/2006 12:21:45 PM EDT
[#7]
Did you ever hear of FREEBORE ? Freebore - (aka ‘barrel throat’) The distance a bullet must travel between chamber and bore before its bearing surfaces contact the lands of the rifling, the purpose being to delay resistance, hence prolong pressure built-up. It cannot be too much, however, as that can cause instability in the bullet. This distance will vary from rifle to rifle, as well as according to the preference of the handloader, as some prefer the bullet to touch the lands, while others prefer it to be further away. Care must be taken not to make this distance too great, as this will adversely affect accuracy. It is not advisable to have the bullet touch the lands, either, as that can increase pressure levels significantly. Most Bolt action rifles have it - as a safety factor , and to allow different length bullets to be loaded in a cartridge. Very little Freebore , + a tight chamber might be - why he has the problem! ---- Yes ,Send it back with a detailed description of the problem - and a sample fired and Unfired round. |
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Posted: 8/13/2006 1:01:08 PM EDT
[#8]
Duh. That was exactly my point - unless you ordered a custom chamber - this amount should be within SAAMI spec - and therefore should not have overpressure signs from shooting factory ammo within SAAMI spec. What I stated was - bolt or gas is not the defining characteristic here. It is the chamber/throat dimensions. This chamber - is obviously, not. |
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Posted: 8/13/2006 2:26:09 PM EDT
[#9]
Yes, you are correct about having excessive chamber pressure. The circular mark on the base, crater around the firing pin dimple, shiny marks on the base and missing primers are all indications of excesive pressure.
Was it hot when you shot? Heat can increase chamber pressure. Don't leave ammo in the sun and don't let the cartridge heat-soak in a hot chamber while you fiddle with other stuff. I would guess the cartridges use a very slow burning powder intended for bolt-action hunting rifles, not gas-operated rifles (hint: core-loks and game kings are hunting ammo = bolt gun ammo). Does the ammo box state which powder is used? Can you disassemble a cartridge and weight the powder? That can give a clue about the powder's burning rate. A gas-operated gun should not use powder slower than IMR-4064, a medium burning rate powder. You can have someone cut a longer throat (move the lands forward to reduce pressure) but that will mess up the barrel's accuracy. |
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Posted: 8/13/2006 3:20:33 PM EDT
[#10]
It wasn't that hot at all. I would say it was around 80. I know that the rounds i used are for hunting but there isnt much of a choice in ammo for gas guns when it comes to the 260 rem. Although in a guns and ammo article about the rifle they use the core loks. I plan to reload for this caliber but until then i can't beleive they would make this rifle with the intention that it would not fire factory ammo. if thats the case i will have a problem with dpms. I don't know what kind of powder they use in those rounds so I can't tell you anything there.
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Posted: 8/13/2006 7:01:50 PM EDT
[#11]
My Fed GM Match brass looks like yours while shooting a DPMS 308b 18" carbon bull barrel. I noticed that Austrailian surplus didn't have many marks on it. Probably harder brass. No blown primers though.
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Posted: 8/14/2006 9:38:22 AM EDT
[#12]
Beat me to it. The pressure signs on your brass are the classic signs of a slower burning powder ment for bolt action rifles. I suspect there is nothing wrong with your rifle. Before sending it back I would get a box of FMJ 260 Remington (if there is such a thing), and test the rifle again.
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Posted: 8/14/2006 10:20:50 AM EDT
[#13]
I called up dpms this morning and told them everything I told you guys and the wanted me to send the upper in along with the brass to see whats going on. I definetly think it may be due to the fact most of the 260 rem ammo is meant for bolt guns but I feel i still feel they should have stated the fact that the rifle would only function normal under reloads and not factory ammp. Ive read article where others have shot this stuff and functioned fine so it beats me. Ill let you guys know what they say.
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Posted: 8/14/2006 10:32:54 AM EDT
[#14]
Explain to me how a round "designed for a bolt gun" will show pressure signs in a gas gun, but not in a bolt gun. When the bolt in a bolt gun is locked - you have a .260 round in a .260 chamber. When the bolt in a gas gun is locked - you have a 260 round in a .260 chamber. The issue of slower burning powders possibly not be correct for a gas gun isnt one of overpressure during firing - it is port pressure at that gas port... changing the pressure moving the bolt back too fast, or too slow, or not creating the appropriate timing to cycle the weapon. I fail to see how any of this would affect overpressure upon initial firing. You could block off the gas port in the barrel - essentially making this a single shot bolt gun.... and you would still have these pressure signs. |
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Posted: 8/14/2006 12:15:26 PM EDT
[#15]
Thats exactly what the guys at the range were telling me but I dont know enough to argue that.
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Posted: 8/14/2006 12:34:50 PM EDT
[#16]
I agree with FALARAK on this.
The issues with slower burning powder in gas guns are with the M1 and M1a (M14) designs, and have to do with the long op-rod. The op-rod is easily over-powered with high port pressures, even when the max chamber pressure is never exceeded. The M1 & M1a issues have nothing to do with chamber pressure, and that IS the problem here. The brass shown above is probably not reloadable because the primer pockets are now oversize - a classic sign of a serious over pressure condition. [on edit - Mustang-52, if you can, would you verify this by seating new primers into your fired cases?] There could be another cause besides the bullet resting against the lands (freebore): The barrel could be on the narrow side. I have a friend with a custom 6mm barrel that needs loads to be at (or under) the minimum load listed in manuals. The chamber is good, but the bore diameter is at the lower edge of the spec. He is happy with the gun because he must reload for it anyway (custom chamber), and because its a very accurate gun. The fact that factory ammo is producing an over pressure condition is cause for great concern, and should be handled by the rifle builder. |
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Posted: 8/14/2006 2:15:06 PM EDT
[#17]
AMEN. |
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Posted: 8/14/2006 2:36:27 PM EDT
[#18]
Quoted:
I agree with FALARAK on this. [on edit - Mustang-52, if you can, would you verify this by seating new primers into your fired cases?] I can't, they asked for the spent brass to be sent along with the rifle. sorry. |
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Posted: 8/14/2006 8:18:17 PM EDT
[#19]
Guns & Ammo had pressure signs on their test LR-260 as well.
--OverBored |
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Posted: 8/15/2006 5:45:22 AM EDT
[#20]
We should probably start a new thread on the topic of chamber pressures and factory ammo.
FALARAK: First not all chambers are cut equal. There can be and usually are significant variations in due to manufacturing tolerances. Secondly there is brass and loads that are designed specifically for a bolt gun. For example, I am still working out the proper loads for my RND Edge 2000 chambered in .338 Lapua Mag. Some of the brass out there has a "burr" on outside of the mouth of the brass, right around the cannelure line. This is used for centering into the top of the chamber on a bolt gun. They don't work well for gas guns - extensive jamming. I am also trying to determine the right load for cycling. Too little powder and voila, the bolt doesn't cycle. Such was the case for Lapua factory loads. Too much powder and we get blown primers, etc. Haven't encountered this problem, because I am working up from the bottom end with loads. Expierence with my SR-25 when I first got it a few years ago was the same. Most quality ammunition was loaded with bolt guns in mind. It would shoot the 7.62 ball without cycling problems or blown primers. Federal 168gr BTHP's shot great. Black Hills 168gr BTHP's had 100% blown primers. The difference -- bullet seat -- almost .010". EVERYONE: This is a new chamber/caliber for AR's. I don't think Cobb Mfr'ing has delivered a .260 Remington yet, so this is probably a first. Expecting factory ammo designed for four legged animal hunting in a bolt gun to be used in a semi-auto is not alway realistic. Works in some cases and not in others. Not in this case. It doesn't surprise me that G&A had issues, there is barely a handful of factory loads in .260 REM. If it is an ammo problem, everyone would expierence the same thing. abn_lcss |
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Posted: 8/15/2006 10:50:20 AM EDT
[#21]
That is exactly my point. His chamber/throat are designed in such a way that it creates too much pressure to fire factory ammo.... or at least specific brands. If this is not known and desired by the manufacturer, then the chamber is defective and should be sent back.
I call BS. That might be true of custom reloads and wilcats... but there is no ammo off-the-shelf that states "not safe for anything but a bolt gun" There is NOTHING specific about a gun locking with a manual bolt, versus a gas operated bolt. A locked chamber is a locked chamber. There may be custom, specific types of ammo - that are only safe to use in special cut chambers - but this has NOTHING to do with being a bolt gun. For instance, there may be some ammunition with projectiles that are loaded too long for mag length... but this can happen in a bolt gun or semi. There may be some ammo that is loaded to sit too far into the lands of a standard cut rifle chamber, and is designed for a specific bore - but that is not the case here. There may be specific variances in chambers due to the way weapons and ammo types are used (5.56 NATO vs .223 SAAMI) for example. However, this has nothing to do with the .260 in this example. The above 5.56 reference has to do with militray style semi/full auto weapons needing to err on the side of reliability, and use heavily standardized ammunition. This is NOT true for the custom .260 barrel - whether it is semi-auto gas operated, or in a bolt gun.
I fail to see how that example has anything to do with factory ammo in a factory gun and overpressure?
And I covered that in a previous post above. And again - this has nothing to do with the overpressure in the chamber.
Explain how a bolt gun is different - other than the examples I gave above.....
Then there was something very specific about that gun's throat that was a bit off. Any SR25 should be able to shoot Black Hills without blowing anything. Armalite specifically recommends Black Hills for their semi-auto .308 weapons. .010" in bullet seat should have ZERO impact on a SR25.... because in most cases, unless you are shooting some type of specialized long AMAX load... a factory federal round will be a LOT forther off the lands than .010". Most factory federal rounds I have measured can vary .010" round to round.
I agree with that statement - except for rounds "being designed for a bolt gun". This is a farily new caliber, and new ammo. It may be entirely possible that some new weapons will not like specific characteristics of some ammo. But it simply is the cut of the chamber involved here. If the barrel manufacturer expects that ONLY custom worked up reloads will work, then they should simply specificy that... or cut their chambers the same dimensions as the "bolt guns".... and we wouldnt be having this conversation. |
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Posted: 8/15/2006 12:22:21 PM EDT
[#22]
Timing in a gas gun is a balance between chamber pressure decay so the case no longer obturates and port pressure to accelerate the bolt carrier and unlock the bolt. Use of slow burning powder keeps the chamber pressure high while the port pressure tries to unlock the bolt. Notice the shiny spots on the base of the cartridge cases. The shiny spots are caused by the case being burnished by the bolt face as it rotates to unlock while the case is still obturated in the chamber due to high pressure. This phenomenon does not occur in bolt guns because you cannot pull the trigger and work the bolt fast enough.
If the timing is bad enough, you get the Vietnam-style M-16 jam with the rim ripped off by the extractor. That the OP's LR-260 ejected the cases shows it is probably on the ragged edge of this kind of malfunction. One other factor is soft Federal is known for certain lots with soft brass. Some guys don't like to use FC because the primer pockets loosen after a couple firings. |
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Posted: 8/15/2006 12:32:07 PM EDT
[#23]
Agreed. However - how do you explain the serious ejector flow? That is typical of high chamber pressures - while still locked.
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Posted: 8/15/2006 12:37:53 PM EDT
[#24]
Even considering Federal's softer-than-most brass, it still should never flow into the ejector recess in the bolt under pressure. That, all by itself, clearly indicates a dangerous over-pressure condition. The timing isn't in play yet when this is happening. The timing you mention involves factors that are not directly associated with excessive chamber pressure. The timing involves things like gas-port pressure, port size, port placement and gas tube length. Some of these can be related to chamber pressure, though there is no direct correlation. |
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Posted: 8/15/2006 1:47:44 PM EDT
[#25]
It's not just the soft brass that flowed into the ejector hole, look at the crater in the primer cup where the cup material flowed into the firing pin hole. That is a sign of excessive pressure too. As FALARAK stated, I would bet the ammo would show signs of overpressure in a bolt gun with a short throat. Since the chamber dimensions are pretty much set by SAAMI (within a couple thousandths), the only parameter the manufacturer has to play with is throat length, which will destroy accuracy. The shape of the pressure curve (P vs T) is one of the considerations in timing. That is why powders shouldn't be faster than IMR-4064 for a gas gun. Even John Garand had to get the Army to back off to a moderate burning rate powder for the M-2 cartridge, and this is with the gas port at the end of a 24 inch barrel. If I owned the OP's rifle, I would have never known it had a problem because I don't use store-bought ammo. I learned my lesson about hunting ammo and gas guns 25 years ago when I tried feeding my then-new M-1A some 180gr soft points (I think they were federals too). I never did find the extractor, detent and spring after shooting the 2nd round. I now reload for everything and typically use LC or Lapua brass. |
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Posted: 8/15/2006 3:01:57 PM EDT
[#26]
i'd like to add that the remington brass has the same marking on the bottom also. I would understand if they stated that anyone buying the lr260 should use reloads, but they didn't. Well all find out what they say once i hear back but if they tell me thats just how it goes, ill take a 308 instead.
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Posted: 8/15/2006 5:51:43 PM EDT
[#27]
I would like to add some findings that I found with a 260 upper made by JP Rifles. It is built on a DPMS lower and I have a 308 24 inch upper also. I wanted the 260 to shoot varmint bullets and 123 Lapua for the longer ranges. When I got the 260 upper I loaded as per several manuals. Guess what, all were very hot and I was not getting any speed!! I fought it with H414, 4350, RL19, H380 and I can not remember what else right now. Not being very happy I email John Paul at JP. John did some load work himself and send me a load to try. First seat the bullet deeper. OAL about 2.7 rather then 2.8. And he put me on to V550 for the powder. And guess what all the problems went away. Went out and shot 4 3 shot groups that averaged under half inch. With 4 different bullets. the 123 Lapua at 2800 down to the 90 TNT at just over 3200 out of the 22 inch barrel. It is now a great rifle.
I do not understand why the pressure was so great with loads that were 3 grains below where I would have started with a bolt gun. But it was. Just wanted to add to this thread as the 260 is fun in the semi auto format. Jerry |
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Posted: 9/14/2006 7:32:12 AM EDT
[#28]
I got a response back from Shane at DPMS and they found out that the chamber was indeed to tight. They are waiting on some new barrels to come in which should be around 2 weeks to replace mine. Lets hope that does it.
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Posted: 9/16/2006 9:12:21 AM EDT
[#29]
Very interesting to follow this thread, and the lack of understanding of internal ballistics is appalling! Interesting that DPMS is waiting for a new barrel instead of rechambering the "tight" chamber. Faralak, you have supreme patience with the unwashed anad unknowing...
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Posted: 9/16/2006 1:21:21 PM EDT
[#30]
Is it chrome lined? If not, it would be an interesting question why they don't just run a reamer and then ship it back to you. You are more patient than I. Good luck |
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Posted: 9/17/2006 4:58:42 AM EDT
[#31]
Its a stainless steel bull barrel. I htought they ream it out also but they just said they put a new one on and test it before they send it back. The waiting is killing me tho but what can i do. |
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Posted: 9/17/2006 5:56:32 AM EDT
[#32]
This doesnt surprise me... it most likely means they arent cutting their chambers on these special rifles... they are having that done elsewhere. Just gotta wiat for them to get stock when this happens. They most likely dont sell enough of these chamberings to keep a large stock of replacement parts. |
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Posted: 11/4/2006 5:51:31 AM EDT
[#33]
OK, finally got the upper back. They replaced the barrel and it appears the bolt assembly as well. they fired 25 rounds thru it without any problems and included the brass which looked much better. I havent had a chance to get to the range but ill report back when I do.
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Posted: 11/7/2006 2:13:20 AM EDT
[#34]
Thanks for the update. I have still not ordered my LR-260 because of the reports of pressure problems...
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Posted: 11/8/2006 1:38:38 AM EDT
[#35]
And if possible, please post some data about accuracy and velocity.
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