These were fired in an AR10. Gouges occured on several cases loaded with 43.5gr of Varget but not on brass loaded with 45gr. Any ideas?
Thanks
Thanks
- Thread starter locolife
- Start date
Sure look like ejector marks to me. Can't say why the lighter charge is exhibiting a classic pressure sign, while the heavier charge isn't.
Did you load them long, jamming the lands, casuing a pressure spike? Did you drop more powder than you intended? Was that ammo sitting in the hot sun?
Chris
Did you load them long, jamming the lands, casuing a pressure spike? Did you drop more powder than you intended? Was that ammo sitting in the hot sun?
Chris
Last edited:
Damn, you are right...they are jammed and I just looked at the chrono data and this was the string for 43.5
2840
2839
2868
2840
2850
2866
and this was for string for 43.6
2547
2540
2543
2523
2539
2569
Is a spike that high even possible? I am beginning to wonder if I somehow screwed up the setting on the dispenser.
2840
2839
2868
2840
2850
2866
and this was for string for 43.6
2547
2540
2543
2523
2539
2569
Is a spike that high even possible? I am beginning to wonder if I somehow screwed up the setting on the dispenser.
What bullet?
How are you dropping powder?
Do you own a set of check weights, or a check weight?
~2850 fps for a 150/155gr bullet isn't unheard of, even out of a 20" barrel, it's getting up there for a 16" tube. ~2850 fps for a 168/175gr is way high for even a 20" barrel, especially a gas gun.
When you post this stuff, you need to provide us with all of the pertinent information and not leave us guessing.
Chris
ETA:
The average difference between those two strings is ~300 fps. That's not just a grain over, it's a good bit more than that over. Figure out what you're doing wrong and correct it ASAP, otherwise that AR10 will be ready for the beer can maker.
Last edited:
Thoes cases would be in my scrap bin! Nasty ejector marks. Will be following this one to see what you find out.
Congrats. You just proof fired your AR at least 5 times and you both survived.
I was just writing back and forth with someone highly knowledgeable about AR-10s and this is what I was just informed of: "the carriers never wear out, and since the SR's are not proof fired those bolts lasted for ever - usually past 15-20k rounds, whereas on the M110 as Big Army wanted it proof fired they start throwing lugs around the 7k-10k mark (hence the 5k refurb recommendation) depending if fire more of less with the can."
Based off what he said, I would say you just drastically shortened your weapons life and may possibly want to get a new bolt for it.
I was just writing back and forth with someone highly knowledgeable about AR-10s and this is what I was just informed of: "the carriers never wear out, and since the SR's are not proof fired those bolts lasted for ever - usually past 15-20k rounds, whereas on the M110 as Big Army wanted it proof fired they start throwing lugs around the 7k-10k mark (hence the 5k refurb recommendation) depending if fire more of less with the can."
Based off what he said, I would say you just drastically shortened your weapons life and may possibly want to get a new bolt for it.
Had a friends AR10 do this. Ended up being a head spacing issue with soft brass. Chamber is an odd one.
Was using 175SMK's over Varget, no crimp. Powder dispensed by RCBS Chargemaster. I didn't turn off the machine, so I just went out and verified the calibration on another scale and it was accurate. No other over pressure signs other than the velocity. No popped or flattened primers.
That leaves the spike issue as the most likely. I am amazed that it can manifest itself from a one grain differential. Am going to back away from the lands today and see how that affects it.
That leaves the spike issue as the most likely. I am amazed that it can manifest itself from a one grain differential. Am going to back away from the lands today and see how that affects it.
Port pressure is too high. Get a adjustable gas block. It is a secondary pressure spike. If you had a trace, you would see this. AR10's are notorious for scrapping brass.
Sorry, you're not getting an increase in velocity of ~300 fps by adding one grain of powder.
Ain't happening.
Chris
I doubt you can get enough Varget in a 308 case to get a 175SMK to go 2850 in an AR10. Something else is going on. Not buying those velocities.
I have an adjustable gas block on it as well as a heavy buffer. Bullet is a 175 SMK.
Ladder showed a nice node at 45gr so I loaded up a handful to try tomorrow. Also cleaned a little burr off the edge of the ejector hole.
Thanks for all the advice.
Ladder showed a nice node at 45gr so I loaded up a handful to try tomorrow. Also cleaned a little burr off the edge of the ejector hole.
Thanks for all the advice.
Agreed, but what? Im using a magnetospeed chrono which has been extremely accurate so I have no idea where the additional 300fps came from. I tested 10 different loads and all of them except the 43.5 had normal readings.
This is very common on AR's in 308 or like calibers. I wish it wasn't but unfortunately it is. If you are going to run that load there is no way around that happening. There is enough pressure to cause slight brass flow into the ejector area and the bolt is rotating to boot at the same time. Most of them do it with factory ammo so I don't or wouldn't find it overly alarming. You do need to know that you are closing the gap to blowing primers though so don't get to carried away and keep pushing things. If you continue further there will be enough pressure to push the primer out when the bolt rotates.
I could understand it being an over-gassing issue with the brass deformation, but it doesn't explain the extra 300 fps over the chrono.
Any chance you mixed up powders when you loaded those rounds and used something with a different burn rate? This is assuming your scale was good as you said you checked.
Any chance you mixed up powders when you loaded those rounds and used something with a different burn rate? This is assuming your scale was good as you said you checked.
I have never owned an AR10, but... I did have a semi in 223 which showed extractor marks like yours. Two things contributed to this.
One, a slightly larger than normal chamber, and two, man made head space. I didn't know about shoulder bump gauges back then. Needless to say I was over sizing my brass, and this was also compounded with a sloppy chamber.
Have you measured your fired cases and compared them to your unfired cases? What ever the measurements are on the fired case, bump the shoulders back by 0.002-0.003
A one grain difference in powder weight won't give you a 300 fps change, something else is at play here.
These are only my thoughts.
One, a slightly larger than normal chamber, and two, man made head space. I didn't know about shoulder bump gauges back then. Needless to say I was over sizing my brass, and this was also compounded with a sloppy chamber.
Have you measured your fired cases and compared them to your unfired cases? What ever the measurements are on the fired case, bump the shoulders back by 0.002-0.003
A one grain difference in powder weight won't give you a 300 fps change, something else is at play here.
These are only my thoughts.
No chance...
Thanks Jared, that explaines the marks. What about the pressure spike which caused a +300fps increase, have you ever seen that?
How long is barrel?
Varget in a 308 with 175s you see about 80 fps per gr of powder. 300 fps velocity increase, you either have bad chronograph data or you had a different powder then Varget in that ammo. There is NO way you are going to see a 300 fps increase going from 43.5 to 45. Jammed is only worth about 0.5 gr so at most a 2 gr shift, roughly a 160 fps difference.
45 gr not showing burr on case high might be because you are sticking case to chamber wall harder with increase in pressure but that is a reach.
How heavy is buffer?
Varget in a 308 with 175s you see about 80 fps per gr of powder. 300 fps velocity increase, you either have bad chronograph data or you had a different powder then Varget in that ammo. There is NO way you are going to see a 300 fps increase going from 43.5 to 45. Jammed is only worth about 0.5 gr so at most a 2 gr shift, roughly a 160 fps difference.
45 gr not showing burr on case high might be because you are sticking case to chamber wall harder with increase in pressure but that is a reach.
How heavy is buffer?
I dont know how to explain those velocities. My magnetospeed has been accurate as well, but I dont see how if things are as you say they are that a 175smk was really doing 2850. I have tested my dpms 18in sass with as much as 45 gr (which is too much) and it didnt break 2600. I will be interested to find out the explanation, if you do that is. Maybe someone with quickload can shed some light. I would say shoot some more with a different chrony but that doesnt seem like a good idea given the uncertainty. If you are POSITIVE you couldnt have loaded a faster powder then are you equally positive those are 175s?
Jered, what damage do you normally see on over pressured semi-auto bolts? What would you look for?
Jered, what damage do you normally see on over pressured semi-auto bolts? What would you look for?
These were the 45gr velocity readings:
2689
2676
2675
2698
2682
Powder was definetely Varget and bullets were definetely 175 SMK's.
I have Quickload and it says 43.5 is near max while 45 is over and should not be used.
