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?
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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.
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.
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 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.
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.
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?
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?