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6mm ARC "sane" loads ?
- Thread starter rsbhunter
- Start date
Using a .136 bolt depth, Rubber City bolt....my friend has an Aero bolt and his was doing the same thing....his did it on 2 different bbl brands.....belted cases after resize.....both of us have loaded for over 40 years, I've never seen it before, and I also shoot a 243 LBC.....it works without skipping a beat....rsbhunter
The 243 (LBC) is .125. I see what your pointing out, another reason I don't understand the design of the arc rifle/cartridge. The one thing I don't understand is why now, .243 LBC barrels aren't offered by more bbl. makers...I only know of one that is offering it. I won't post my load data, but my .243 out does the ARC by a decent amount, fps wise, and I am on my 5-6 load with cases. If I was younger and going to shoot for years to come, I would buy a reamer, have a couple bbl's made up in .243 LBC. just my experience... rsbhunter
So your setting the shoulder back .003 off of once fired brass? In effect there is pretty good chance that your pushing the shoulder back .005 or a lot more., unfired, formed brass.Base to datum was set .003 off of fired factory brass measurements
That's not really how i would approach that. I am not able to properly set my FULL Length Die up until the Brass no longer fits into the chamber. I then... and only then am able to know where Max is and have a base line number to work off of. Auto loaders make that a little harder to do but its important if you don't want ruin brass in a short order.
I just will not run an AR without a good adjustable gas block anymore. Just is not worth the hassle, particularly one in a none 223 Caliber. I have been doing this Wild Cat Cartridges in an AR Platform for well over a decade now. First with the 6mm Rat ( your cartridge with a 40Deg Shoulder) Several barrels on that one, and recently the 224 Grendel. So the previous Advice to use an adjustable gas block should not go un heeded.
Available Freebore can and does change pressure significantly in a small cartridge like this. The farther that bullet goes into the case the less combustion chamber available. So if you have a rifle with a shorter freebore than others posting load data it can pressure up quite bit faster. Its just what you have and can only be remedied by a Throating reamer. Or stick to the shorter bullets.
Contrary to popular belief some of these bullets withstand a tremendous amount of bullet jump and still shoot lights out. The 87gr Berger VLD is one of them. The 70gr Sierra Match King is another. Also the 80gr Nosler
On an AR platform, I set the shoulder back .002-.003...tried less, but in a timed match, having to "morter" a round out isn't conducive to range safety. Function is 100% importance factor, so .001-.0015 is not realistic. Besides, I'm not upset over case life, just the "one and done" factor. If I only got 3-4 firings on "book" loads, I would be fine with that...but so far, it's not a reality. Still doing load development, and hoping. I have some cases that are 2-3 fired that I am setting "set back" figures at, the others were unusable after 1st firing...rsbhunter
What I was getting at is... is that setting the shoulder back .003 off based off a case that is fired only one time is most likely setting your shoulder back .005 or more because the case has not fully fire formed all the way forward. You would think that it would be.. However in most cases this is not actually what happens as the Brass Grips the side walls of the chamber. I know I know... over 55,000 PSI it SHOULD be fully formed...On an AR platform, I set the shoulder back .002-.003...tried less, but in a timed match, having to "morter" a round out isn't conducive to range safety. Function is 100% importance factor, so .001-.0015 is not realistic. Besides, I'm not upset over case life, just the "one and done" factor. If I only got 3-4 firings on "book" loads, I would be fine with that...but so far, it's not a reality. Still doing load development, and hoping. I have some cases that are 2-3 fired that I am setting "set back" figures at, the others were unusable after 1st firing...rsbhunter
I agree that ,003 from Fully Formed brass is what you need to be for an Auto loader. I shoot timed fired strings of ten shots as well. .002 to .003 because it HAS to work every time... no if and buts about it. Over .003 is a complete waste of brass life for no advantage over .003 to .002 shoulder set back. Of course your welcome to do as you please, I am just trying to help.
Russel
I agree 100% with what your saying....it's like checking case volume on a new case, it increases with the first and second, maybe 3rd firing... I don't set shoulder bump til the 2nd or 3rd firing....I do use the Hornady tool to check bump. The biggest problem I (we) were having was pressure at Factory recommended levels....but , just dropped back and accept the results...like the name, same but add an L
I just finished building a 6.5 Grendel with 18" barrel, mid-length gas, and adjustable gas block... so all the bells and whistles.
My initial firings showed cases similar to yours with the exception of the split/crack. There was a raised rim at top of case web and the case body had a dulled hue/surface as if bead blasted. These were standard loads and should not have been an issue. There is another possibility to what you are seeing.
In my case this was not the result of excessive gas or a hot load, but rather too long of a lock-up time (case in chamber) that results in excessive pressure building inside the case. The roughed up surface on the case is it expanding hard against the chamber walls and getting dragged out under friction during its extraction. Normally the case would spring back (shrink) as it is being extracted as the pressure dropped. However, in this scenario the pressure hadn't subsided and it kept the brass pinned against the chamber wall during the entire extraction resulting in the case's roughed up surface. The pressure was looking for somewhere to escape and as the bolt began moving rearward the first chance it had to find a release was by expanding the brass at base of case, which was first part of case that was no longer supported/constrained by chamber's walls.
The diameter of the gas port in my Grendel barrel was significantly smaller than what you typically find in the .223/5.56 or 300BO barrels. The combination of the smaller gas port and longer gas system (mid-length) meant that there was marginal benefit from an adjustable gas block. Basically, I closed it down only 2-3 clicks. I changed out the buffer to a standard carbine buffer (3oz.) and a standard buffer spring. Make certain your buffer length matches your buffer tube. I also pulled the gas block and made certain that I had perfect alignment on the gas port.
My initial firings showed cases similar to yours with the exception of the split/crack. There was a raised rim at top of case web and the case body had a dulled hue/surface as if bead blasted. These were standard loads and should not have been an issue. There is another possibility to what you are seeing.
In my case this was not the result of excessive gas or a hot load, but rather too long of a lock-up time (case in chamber) that results in excessive pressure building inside the case. The roughed up surface on the case is it expanding hard against the chamber walls and getting dragged out under friction during its extraction. Normally the case would spring back (shrink) as it is being extracted as the pressure dropped. However, in this scenario the pressure hadn't subsided and it kept the brass pinned against the chamber wall during the entire extraction resulting in the case's roughed up surface. The pressure was looking for somewhere to escape and as the bolt began moving rearward the first chance it had to find a release was by expanding the brass at base of case, which was first part of case that was no longer supported/constrained by chamber's walls.
The diameter of the gas port in my Grendel barrel was significantly smaller than what you typically find in the .223/5.56 or 300BO barrels. The combination of the smaller gas port and longer gas system (mid-length) meant that there was marginal benefit from an adjustable gas block. Basically, I closed it down only 2-3 clicks. I changed out the buffer to a standard carbine buffer (3oz.) and a standard buffer spring. Make certain your buffer length matches your buffer tube. I also pulled the gas block and made certain that I had perfect alignment on the gas port.
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Mike - That is exactly what my initial reaction/thought was. Due to the gas operation and timing of moving parts, an AR15's can be counter-intuitive compared to a bolt rifle. With an AR15 the go-to answer is always too much gas... it just seems right. I got some advise from some far more knowledgeable AR15 shooters. It took a little bit to get my head around it, but they were right.Hi Oso. I'm curious how you concluded that this was caused by the gas system. Without much experience, I would look at that case and figured a hot load of way too fast powder.
The spent cases showed no cratering on the primer, no extractor claw marks or bent rim, and only minor hint of ejector swipe. The raised rim along top of case head/webbing was more pronounced on one side of case and as it circumnavigate around to the other side of the case it became less pronounced... the highest point was closet to the ejection port.
The other issue was that the rifle was not locking open. This can happen as either the result of (1) too much gas (too weak of buffer system) in which the BCG slams rearward and slingshots forward so fast that it skips right over the bolt catch or (2) not enough gas, too heavy of buffer system, or even too long of buffer (wrong length for buffer tube) which any of these can retard/restrict the BCG from being able to move sufficiently rearward to eject spent case and clear the rear of the magazine.
Something to also keep in mind is when an AR15 is excessively over-gassed the timing is can also be off (too short) which doesn't allow the case to begin its normal contraction (spring back) and the shear force/speed that the BCG is thrust backward results in the extractor claw bending the case rim as it pries the case out of the chamber.
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Those who are advocating the adjustable gas block or gas system adjustment are misunderstanding something fundamental. An Adjustable gas block has only minimal impact on rifle timing-- the *when* the bolt unlocks and such. It can have a major effect on how hard it accelerates once the unlock occurs. But the basic timing of the unlock function is not very sensitive to gas block adjustment at all.
The reason mid length gas systems were created for 16" barrels isn't because the port size was wrong, it's because the *location* was wrong and the carbine location led to early bolt unlock while chamber pressure was still too high. This can't be changed just by making a carbine orifice smaller. An adjustable gas block is nothing more than a variable orifice. Moving the gas block has a major effect on rifle timing, while orifice size is much less of a control.
If the rifle is out of time (and it sounds like it is) then the best solution is likely to go to a much faster burning powder. This will shift the peak of the pressure curve earlier and will cause a higher expansion ratio to occur, lowering port pressure. Keep in mind the original AR15 rifle length gas system was developed with IMR4198. Slower powders or shorter gas systems reduce the timing margin built into the rifle.
In theory, a larger bore caliber should have a higher expansion ratio (larger volume in the barrel) and thus should lower port pressure.
I suspect that the fast burning powders that work best for rifle timing in a 6 ARC gas gun are not preferred because of the MV penalty and obviously Hornady wants some marketing numbers. The low pressure limit also further handicaps the potential. Look at the 6ARC gas gun data from Hornady and focus on the faster burning powders and you'll see that a lot of the gain over a 5.56 disappears.
The reason mid length gas systems were created for 16" barrels isn't because the port size was wrong, it's because the *location* was wrong and the carbine location led to early bolt unlock while chamber pressure was still too high. This can't be changed just by making a carbine orifice smaller. An adjustable gas block is nothing more than a variable orifice. Moving the gas block has a major effect on rifle timing, while orifice size is much less of a control.
If the rifle is out of time (and it sounds like it is) then the best solution is likely to go to a much faster burning powder. This will shift the peak of the pressure curve earlier and will cause a higher expansion ratio to occur, lowering port pressure. Keep in mind the original AR15 rifle length gas system was developed with IMR4198. Slower powders or shorter gas systems reduce the timing margin built into the rifle.
In theory, a larger bore caliber should have a higher expansion ratio (larger volume in the barrel) and thus should lower port pressure.
I suspect that the fast burning powders that work best for rifle timing in a 6 ARC gas gun are not preferred because of the MV penalty and obviously Hornady wants some marketing numbers. The low pressure limit also further handicaps the potential. Look at the 6ARC gas gun data from Hornady and focus on the faster burning powders and you'll see that a lot of the gain over a 5.56 disappears.
Hohn How do you explain all of us that are getting good case life and no issues using Leverevolution and CFE223? I am just using Hornady cases, and unless I load and shoot until they need replace, I won't know the case life, but I am not seeing any reason that they won't last 5-7 reloadings at this point. I am using the max book loads for a gas gun, with Leverevolution, and want to try CFE223 next. I have loaded 87 Vmax, 75, Vmax and 80 Fowlers so farand all of the cases look nearly new. I have a 22" Sanders barrel with a rifle length gas system, and it runs perfect.
I suspect the rifle length system is a big part of your success. The extra length of the longer gas system provides ample time for pressure to fall and prevents ripping out a stuck case my brute force. And the difference in port pressure between a “fast” powder and a slow powder like CFE223 or leverevolution is small compared to the difference between a carbine length, mid length and rifle length system.Hohn How do you explain all of us that are getting good case life and no issues using Leverevolution and CFE223? I am just using Hornady cases, and unless I load and shoot until they need replace, I won't know the case life, but I am not seeing any reason that they won't last 5-7 reloadings at this point. I am using the max book loads for a gas gun, with Leverevolution, and want to try CFE223 next. I have loaded 87 Vmax, 75, Vmax and 80 Fowlers so farand all of the cases look nearly new. I have a 22" Sanders barrel with a rifle length gas system, and it runs perfect.
And there’s an element of time separate from port pressure. Ie a carbine system is worse than rifle even if you had identical port pressures.
I agree. Just google up some pressure trace pressure curves. Pressure drops rapidly by much more so with bullet travel(expansion area) than with burn rate. Optimize the two, and you'll have a smile machine!I suspect the rifle length system is a big part of your success. The extra length of the longer gas system provides ample time for pressure to fall and prevents ripping out a stuck case my brute force. And the difference in port pressure between a “fast” powder and a slow powder like CFE223 or leverevolution is small compared to the difference between a carbine length, mid length and rifle length system.
And there’s an element of time separate from port pressure. Ie a carbine system is worse than rifle even if you had identical port pressures.
Thanks guys. I hope I did not come across sounding like a dick.
That's Great... and I thought I was the only one guilty of doing this. Welcome to the "I Didn't Mean To Be A Dick" Club. Our membership is continuing to grow, our ranks are filled with the Technologically Challenged. If you were born before 1975 then your automatically a member!!!!Thanks guys. I hope I did not come across sounding like a dick.
Without tone or body language, the choice of words or even sentence structure can easily be misunderstood. It is not uncommon for me to read a post I made previously and realize I need to hit "Edit" in order to clean up my message and/or possibly rearrange a couple of words to eliminate the inadvertent/unintentional Jackwagon undertone.
NZ_Fclass
Silver $$ Contributor
I joined years ago...That's Great... and I thought I was the only one guilty of doing this. Welcome to the "I Didn't Mean To Be A Dick" Club. Our membership is continuing to grow, our ranks are filled with the Technologically Challenged. If you were born before 1975 then your automatically a member!!!!
Without tone or body language, the choice of words or even sentence structure can easily be misunderstood. It is not uncommon for me to read a post I made previously and realize I need to hit "Edit" in order to clean up my message and/or possibly rearrange a couple of words to eliminate the inadvertent/unintentional Jackwagon undertone.
KMart
Gold $$ Contributor
That is about the same time that the enlightened progressives started overhauling the educational systems.If you were born before 1975 then your automatically a member!!!!
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