overpressure ejector marks/dwell time?

D

DarrinW

Online Training Member
Full Member
Minuteman
Dec 26, 2017
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Dayton, Ohio
#1
I have a friend that built a 6.4 creedmoor gas gun. I picked up a piece of brass and found ejector marks, a flattened primer, and what looks like scoring which i assume is what people are calling swiping. This is with factory hornady 140s in a 20" barrel with rifle length gas system.

This looks to hot for his rifle. Or is it to much dwell time and the case is just getting pushed back against the bolt for too long. Or is it something i am not mentioning? Sorry if this isn't enough info, ill do my best if anyone needs more. My concern for him is he shot this a few weeks ago when the ambient temps were in the 40-50s. I expect it to be worse as the summer arrives. It looks like a recipe for something bad.
 
#2
Pictures of brass would be most helpful.

Buffer weight?
Trigger?
Adjustable gas block?
Buffer spring - standard or extra power or ???
What brand barrel?
 
#6
FCS said:
Pictures of brass would be most helpful.

Buffer weight?
Trigger?
Adjustable gas block?
Buffer spring - standard or extra power or ???
What brand barrel?
Click to expand...
I wish i would have taken a pic of it when i was at the range. Thanks for the response.
He has a Ballistic advantage barrel. Non adjustable gas block, ( i have no idea why he didn't put one on it). A standard buffer weight, although he is looking at a JP silent capture one.
Giesselle trigger. Just curious though, why would that matter?

Thanks,
 
#8
A lot of people turn to agb and adjustable springs by default these days.
They will work but at a price.

Many threads here on S H start with "having problems with agb and springs".

For less than 30$ you can go + 1, 2 or 3 on the buffer and many an ar has been sorted
out that way. If you don't get satisfaction they can be used on other builds.

If it doesn't work you can justify the "better" more popular route.
 
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#9
DarrinW said:
I have a friend that built a 6.4 creedmoor gas gun. I picked up a piece of brass and found ejector marks, a flattened primer, and what looks like scoring which i assume is what people are calling swiping. This is with factory hornady 140s in a 20" barrel with rifle length gas system.

This looks to hot for his rifle. Or is it to much dwell time and the case is just getting pushed back against the bolt for too long. Or is it something i am not mentioning? Sorry if this isn't enough info, ill do my best if anyone needs more. My concern for him is he shot this a few weeks ago when the ambient temps were in the 40-50s. I expect it to be worse as the summer arrives. It looks like a recipe for something bad.
Click to expand...

Factory ammo or handloads? What you describe might be hot loads, but could be other causes:

Brass flow into the ejector hole can be caused by high pressure, or by excess headspace at normal pressure. Flattened primers have the same causes; a little excess headspace can look like a really hot load that actually isn't. If he's loading his own ammo without measuring shoulder bump, I'd check that first.

Brass swipes at the ejector hole can be caused by an overgassed system (unlocking too early while under pressure), or high pressure. Tuning the recoil system for correct operation (a balance of gas flow, buffer weight, and spring force) is important especially on the large frame guns; if he hasn't done this I'd look there unless you have other reasons to think the load is too hot.
 
#10
DarrinW said:
I wish i would have taken a pic of it when i was at the range. Thanks for the response.
He has a Ballistic advantage barrel. Non adjustable gas block, ( i have no idea why he didn't put one on it). A standard buffer weight, although he is looking at a JP silent capture one.
Giesselle trigger. Just curious though, why would that matter?

Thanks,
Click to expand...

Different triggers have different amounts of force required to reset hammer. Trigger information is for farther down path of the diagnosis but the trigger is part of the overall system, just easier for me to get the information on front end.

Load isn't hot, gas port is too close and too big, buffer weight is too light so gun is unlocking too soon while pressure is higher. Burn rate of powder with 140s giving you a time / pressure history curve that does match well to the port location / size and buffer weight.

BA has to port barrels to run any / all ammo so port will be big to cycle lighter bullet ammo (different time / pressure history curve).

Heavy up buffer at minimum, add an adjustable gas block would be good idea also.
 
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#11
To reiterate what many above have recommended a heavier tungsten buffer solved the same problems shooting 5.56 green tip ammo in a cheap AR15 build. For about $30.00 you can get one, two or three tungsten weights for your current buffer to tune the rifle. Cheap fix.
 
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#12
FCS said:
Different triggers have different amounts of force required to reset hammer. Trigger information is for farther down path of the diagnosis but the trigger is part of the overall system, just easier for me to get the information on front end.

Load isn't hot, gas port is too close and too big, buffer weight is too light so gun is unlocking too soon while pressure is higher. Burn rate of powder with 140s giving you a time / pressure history curve that does match well to the port location / size and buffer weight.

BA has to port barrels to run any / all ammo so port will be big to cycle lighter bullet ammo (different time / pressure history curve).

Heavy up buffer at minimum, add an adjustable gas block would be good idea also.
Click to expand...
Thanks for the explanation FCS.
 
#13
The tungston weights listed above, how do you install them? can you remove the rubber tip off the buffer and then remove the weights and use these instead?
 
#14
Yes on all of my buffers, I took a punch and knocked out the pin on the buffer.

Got a pair of pliers and wiggled the front off, takes some effort.

Slide them out in order there is a spacer between and substitute the titanium for the steal, reinsert and tap the pin back in. A block of wood with a hole in it helps if no other tooling available.

I would try 2 and see if it still locks back on empty mag, if dirty beat up brass may try 3.

If problems not solved, agb will be required.
 
#15
I am a fan of an Adj. GB , heavier buffer weights, and a "stiffer" recoil spring ( Tubbs .308 Flatwire. ) ( i also use a Lantac muzzle brake, but that shouldn't change the brass condition )

Ultimately, I'd bet.. if your friends bolt remained locked closed, just a few milli-seconds longer... those issues would go away.

It has worked for me , 9 times.

The bolt remaining locked closed slightly longer, allows for extraction to take place at far lower chamber pressures... the closed bolt also supports the primer better, those casehead ejector swipes will disappear because of the delayed bolt unlocking.

And FWIW, none of my 9 setups like that have Adj. GB / heavy buffer weight / heavy recoil spring issues.

My SLR adj. GB's all run at about 6 clicks open from a fully closed position... with 9 more clicks open more available.

In addition to all of that... it is far easier on you and the firearm. Easier on the parts, easier to shoot follow up shots, easier on you easier on the brass... it is IMHO the way to go.

If you would like any extra info.. just let me know.

With those parts I have listed.. the firearm's recoil impulse, ( and brass abuse ) will be dramatically different for the better.

And FWIW... a +1,2,3 " gas length will work basically the same ( lower gas port pressure ) , but hindsight is always .. well hindsight.
 
#17
Does your friend's AR have a HP bolt? There was a thread on here years ago about someone shooting 140's out of either a 6.5C M or a 260 Rem, exhibiting the same things you mentioned. He ended up switching to a HP bolt and that seemed to have cured a lot, if not all of his issues.
 
#18
lawdog said:
Does your friend's AR have a HP bolt? There was a thread on here years ago about someone shooting 140's out of either a 6.5C M or a 260 Rem, exhibiting the same things you mentioned. He ended up switching to a HP bolt and that seemed to have cured a lot, if not all of his issues.
Click to expand...
To tell you the truth i a not sure what BCG he bought for it.
 
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