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6Arc AR not making sense

C

Cwballistics

Private
Minuteman
Aug 15, 2018
16
6
#1
To make a short story shorter, I’m gettting 2940+ fps from factory Hornady 108 Match ammo out of a 16” barrel. This doesn’t jive with what I have seen while not being able to sleep.

2900 numbers are from Magnetospeed. Initial doubts were caused when 1.8 mils (assuming 2425 fps) at 300 turned into .8 mils, turned into .4 mils, and still a little high.
I was always just hoping to get 2450, (jbm starting point) so I did not trust it. Shot over Magnetospeed and got 2934-2951 over 5 shot string.

Please let me know what you think.

Thanks
 
#7
I really don’t know what to think. This crap never grouped better than 1.25/1.5 in 5 shot increments, but it was always that 1 or 2 shot flier. I’m just trying to make brass. I bought a bunch of the 108 because it was available. I got some of the 105 black because it was available locally. It grouped the same, but no strings on top of another. Right now, I’m just going to continue on with load development. I am definitely considered a newbie by this forum standards, but I’m not an idiot. I’ve done a a little wildcatting, and dialed in most standard calibers and don’t consider myself lucky. But when you are hoping for 2425 and you are looking at 500fps more, you got to ask yourself, do you feel lucky? Right now, can’t say that I do.
Or don’t
 
#8
Kadams1563 said:
1.8 mils at 300 seems like a lot. I feel like data somewhere is getting messed up.
Click to expand...
Jbm was calling for 1.8mils assuming 2400fps. That is what I started with since my barrel length/gas block placement made Magnetospeed mounting a nonissue. So I just shot groups at 100 and the 300. Since, I’ve worked around that issue…. I think. With muzzle brake installed, I am within the Magnetospeed specs, easily Just like every other time I’ve use it. I.E. I have always left brake on. I wish I could say what it will do a distance, but right now I’m limited to 300 yds.
 
#9
Cwballistics said:
I really don’t know what to think. This crap never grouped better than 1.25/1.5 in 5 shot increments, but it was always that 1 or 2 shot flier. I’m just trying to make brass. I bought a bunch of the 108 because it was available. I got some of the 105 black because it was available locally. It grouped the same, but no strings on top of another. Right now, I’m just going to continue on with load development. I am definitely considered a newbie by this forum standards, but I’m not an idiot. I’ve done a a little wildcatting, and dialed in most standard calibers and don’t consider myself lucky. But when you are hoping for 2425 and you are looking at 500fps more, you got to ask yourself, do you feel lucky? Right now, can’t say that I do.
Or don’t
Click to expand...
Accuracy issues aside I'd guess the issue lies with how you had the MagnetoSpeed aligned, a weak battery and possibly incorrect settings or a combination of several of these.
Usually people will report them reading low with these circumstances (I've seen mention of anywhere from 100 to 1000 fps off), so ~390 fps high could be possible. Based on stated 2750 @24" velocity I'd think you'd be closer to 2550 than 2400 @16" barrel length.
 
#10
Cwballistics said:
Jbm was calling for 1.8mils assuming 2400fps. That is what I started with since my barrel length/gas block placement made Magnetospeed mounting a nonissue. So I just shot groups at 100 and the 300. Since, I’ve worked around that issue…. I think. With muzzle brake installed, I am within the Magnetospeed specs, easily Just like every other time I’ve use it. I.E. I have always left brake on. I wish I could say what it will do a distance, but right now I’m limited to 300 yds.
Click to expand...
Okay, that makes more sense, it started off with an incorrect assumption.

So let's say you were truly zeroed at 100 and dialed 1.8 Mils. @2550 fps the correct adjustment would be 1.3, so .5 high is within a realistic range of the .4 high you saw given we don't know exact velocity, environmentals etc.
 
#11
6.5SH said:
Accuracy issues aside I'd guess the issue lies with how you had the MagnetoSpeed aligned, a weak battery and possibly incorrect settings or a combination of several of these.
Usually people will report them reading low with these circumstances (I've seen mention of anywhere from 100 to 1000 fps off), so ~390 fps high could be possible. Based on stated 2750 @24" velocity I'd think you'd be closer to 2550 than 2400 @16" barrel length.
Click to expand...
These are the first 60 rounds down the pipe, hence the laziness, but regardless, here is how it went:

i zeroed at 100 with 108 match, shot 20 rounds of Black (3/4” low, zero held) I ran jbm, just for ease, (I have AP) and picked the lowest speed that would get me to 1000yds and it was 2400-2425. Went to 300 yard range and after dialing up 1.8 mils (jbm), dialed back down a mil, still high and inch or so, I think. Dialed down another .4 and she grouped. Still a 1.25 group, but both 108 and 105s stay consistent. I went back to jbm and reversed the numbers and at 2900 fps it was still showing 1.something mils at 300, so I figured something isn’t adding up. So today, took off hand guard, got Magnetospeed perfectly aligned, I’m fired a five shot group without a hiccup. Averaged 2940ish. That jived with the jbm that I didn’t believe to begin with, so here I am. I would love to think everything‘s copacetic, but if things seem to good to be true
 
#14
bfoosh006 said:
Pics of your fired brass ?

I can't explain the velocity readings.
Click to expand...
Thank you for asking, it actually made me remember that my whole purpose of making the original post was my fear of overpressure. But brass didn’t show anything at all, especially for soft Hornady brass. When I was going over in my head whether to make the op or not, I was thinking somebody might reverse engineer the velocity and barrel length in quickload or whatever computer programs people more experience than I can run, and tell me what the pressures I might be looking at.

As for testing against another chrono, I don’t have the option at the moment. I think I’m going to go ahead and burn through another 40 rounds or so and start load development. At least I will get to daydream about possibilities until then. I’ve got 105 vlds waiting for some CFC 223 and Leverevolution. Until then I will fantasize about varget running 2700 from a 16”. I’m sure whatever I’m missing will come to light once I actually start making rounds rather than buying them.
 
#15
Cwballistics said:
Thank you for asking, it actually made me remember that my whole purpose of making the original post was my fear of overpressure. But brass didn’t show anything at all, especially for soft Hornady brass. When I was going over in my head whether to make the op or not, I was thinking somebody might reverse engineer the velocity and barrel length in quickload or whatever computer programs people more experience than I can run, and tell me what the pressures I might be looking at.

As for testing against another chrono, I don’t have the option at the moment. I think I’m going to go ahead and burn through another 40 rounds or so and start load development. At least I will get to daydream about possibilities until then. I’ve got 105 vlds waiting for some CFC 223 and Leverevolution. Until then I will fantasize about varget running 2700 from a 16”. I’m sure whatever I’m missing will come to light once I actually start making rounds rather than buying them.
Click to expand...
Give up on that fantasy with Varget, you'd be over 90,000 psi.
 
#16
6.5SH said:
Give up on that fantasy with Varget, you'd be over 90,000 psi.
Click to expand...
yes, that wasn’t meant seriously. But, any chance you know what the Hornady factory loads would be pressuring out at given these velocities? I’ve heard multiple speculations on what Hornady uses for Powder, but never paid any attention. If I can get 2475 with Varget out of a 16 inch, I would be a happy camper. I know that’s probably not realistic either, but one can dream.
 
#17
Cwballistics said:
yes, that wasn’t meant seriously. But, any chance you know what the Hornady factory loads would be pressuring out at given these velocities? I’ve heard multiple speculations on what Hornady uses for Powder, but never paid any attention. If I can get 2475 with Varget out of a 16 inch, I would be a happy camper. I know that’s probably not realistic either, but one can dream.
Click to expand...
Hornady doesn't even give 108 load data for a gas gun, just bolt action. If you play around with any of the simulation tools you'll see there is nothing that gives the 2750 fps Hornady claims and stays within the specified 52,000 psi for gas guns. This starts to explain the issues many report with bolt failures in 6ARC, they are simply pushing it harder than it will take for what would be considered a normal lifespan.


 
#18
6.5SH said:
Hornady doesn't even give 108 load data for a gas gun, just bolt action. If you play around with any of the simulation tools you'll see there is nothing that gives the 2750 fps Hornady claims and stays within the specified 52,000 psi for gas guns. This starts to explain the issues many report with bolt failures in 6ARC, they are simply pushing it harder than it will take for what would be considered a normal lifespan.


Click to expand...
Thank you for the P max link. I will learn to use it. I will have to admit that I figured factory ammo from Hornady would be safe in gas guns, if for no other reason, then they should know that most people that are interested in this cartridge are going to be running it in the AR. I find it hard to believe the only ammo they seem to making the most readily available would be loaded to bolt gun PSI‘s, but knowing Hornady, they probably do anything to recover from the bad timing this cartridge release had. Bolt does show absolutely no signs of wear, but with less than 100 rounds, I wouldn’t expect it to, regardless of how hot loads were. Maybe I’m wrong.
Thanks again for the link, I’m starting load Developement on a 6.5SST AR10 that I am basically flying blind on. It should help quite a bit, being it’s sort of the same scenario just larger. I really appreciate all the advice.
 
#19
Just remember a simulation is exactly that.
In general when comparing the output with verified data the tool has been very accurate, but I have run into anomalies with odd-ball or newer combinations. Even QuickLoad and GRT need tweaking to reflect reality with some cartridges (ARC and Grendel among them).
 
#20
6.5SH said:
Just remember a simulation is exactly that.
In general when comparing the output with verified data the tool has been very accurate, but I have run into anomalies with odd-ball or newer combinations. Even QuickLoad and GRT need tweaking to reflect reality with some cartridges (ARC and Grendel among them).
Click to expand...
 
#21
I might bug you in the future. If multiple factory loads (black and match) by the designers of this cartridge, are running over pressure in a gas gun, then I’m gonna need a lot more help than I thought when rolling my own. I wouldn’t be surprised if I stay in their recommended limits and have 105s going sub 2500. Hell, that’s what I was hoping for from the get-go. But my experience so far has been so far out of whack that it makes me question everything. Trust me, if these things were grouping 5/8 consistently at 2900, I wouldn’t be here. I’ll be running that sucker until bolt showed wear and buying more bolts. Thanks again
 
#22
This is Seekins w/ 18” Shaw SS 1:7.5”
Magnetospeed V3
60F
Hornady Black 105 BTHP factory ammo
2512, 2554, 2559, 2558, 2532,
2556, 2536, 2545, 2547, 2550
SD 13.9

I’ve pushed Berger 108’s to 2722 with Lever but saw high pressure signs. I settled on 29.5 gr and 2650. Groups better also.
 
#23
Cwballistics said:
yes, that wasn’t meant seriously. But, any chance you know what the Hornady factory loads would be pressuring out at given these velocities? I’ve heard multiple speculations on what Hornady uses for Powder, but never paid any attention. If I can get 2475 with Varget out of a 16 inch, I would be a happy camper. I know that’s probably not realistic either, but one can dream.
Click to expand...

There's a reason almost everybody uses LVR, CFE or AA2520 in the ARC, they really make it come alive, but you have to put up with the temp sensitivity. I'm playing with Staball Match in the ARC now, it looks pretty good, but still slower than LVR.

To state it plainly, I've loaded some over pressure ARC, there's absolutely no way you're getting 2,940 fps out of the factory 108s out of a 16". I've shot it bit and I get ~2,580 fps in my 18" and ~2,670 fps in my 21". I can load up a little hotter with 108gr and LVR but you're not getting anywhere close to your numbers without losing primer pockets in one firing, and probably worse. There must be a few glitches in your measurements or drop calcs somewhere.
 
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#24
Gtscotty said:
There's a reason almost everybody uses LVR, CFE or AA2520 in the ARC, they really make it come alive, but you have to put up with the temp sensitivity. I'm playing with Staball Match in the ARC now, it looks pretty good, but still slower than LVR.

To state it plainly, I've loaded some over pressure ARC, there's absolutely no way you're getting 2,940 fps out of the factory 108s out of a 16". I've shot it bit and I get ~2,580 fps in my 18" and ~2,670 fps in my 21". I can load up a little hotter with 108gr and LVR but you're not getting anywhere close to your numbers without losing primer pockets in one firing, and probably worse. There must be a few glitches in your measurements or drop calcs somewhere.
Click to expand...
I’m sure you are right. Waiting on a new Magnetospeed display since input jack broke off the motherboard. I haven’t been able to get back out, but this next week I’m going to zero at 100 with 108s, shoot groups with the 105 blacks, record difference, and then see how many inches she drops at 300 with no elevation adjustments at all. I figured this would be the only concrete thing I could do without my chrono. I wish I could stretch out further, but I won’t waste a trip up to core shooting in baker florida until I’ve done load development, it’s about an hour or so away and I’ve still got a 150 rounds to make brass with. I would have liked one of these to group well, but didn’t expect it. I am hoping the staball works out. I’d be perfectly happy with 2500 and a vargetish temp stability, which I think the staball should be close. Interesting powder and would like to hear what you find. Thanks
 
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