How much case growth do you see from new brass after first firing?

[Winny94]

I was getting some very premature pressure signs in my 6 Grendel gasser today, so I cut the range time short and went home to check things out. I found I'm getting .0125" growth at the shoulder - I can't recall ever seeing that much growth, but I don't recall the last time I measured new vs once fired. Is this cause for concern? Would it possibly explain the premature pressure signs?

FYI, this is 6.5 grendel lapua brass necked down to 6mm. I did not bump the shoulder when necking down.
Pressure signs are extractor swipes and hard extraction - I had turned off the gas as to not lose brass in the gravel, so I was manually ejecting.

 

[ronemus]

Did you check neck thickness and/or the OD of your loads? You may be creating an interference which is causing pressure to spike.

New brass should not be 0.0125" shorter than the chamber - one or the other is incorrect. Excessive case expansion will lead to case head separations after a few reloads, which in turn can lead to damage and/or injury. It sounds like your chamber is oversized. How did you determine that you were not bumping the shoulder during the neck-down?

 

[Winny94]

Did you check neck thickness and/or the OD of your loads? You may be creating an interference which is causing pressure to spike.

New brass should not be 0.0125" shorter than the chamber - one or the other is incorrect. Excessive case expansion will lead to case head separations after a few reloads, which in turn can lead to damage and/or injury. It sounds like your chamber is oversized. How did you determine that you were not bumping the shoulder during the neck-down?

The loaded round o.d. is smaller than the fired brass o.d. by .002 if I recall (notes are upstairs).
I measured the new, out of box base to shoulder measurement on about 20 cases, then slowly worked the die down until it was working the neck, but not moving the shoulder.

 

[Texas10]

Sounds like maybe the headspace gage was not engaged into the extractor when the barrel was headspaced. An 1/8th of an inch is a loooooong way to be off on headspace. You were right to turn off the gas port for your initial tests. Saved face....in the literal term.

 

[Rocketvapor]

"An 1/8th of an inch is a loooooong way to be off on headspace."
Texas, check your decimal point

It's 1/80th of an inch.

 

[Richard Coody]

I was getting some very premature pressure signs in my 6 Grendel gasser today, so I cut the range time short and went home to check things out. I found I'm getting .0125" growth at the shoulder - I can't recall ever seeing that much growth, but I don't recall the last time I measured new vs once fired. Is this cause for concern? Would it possibly explain the premature pressure signs?

FYI, this is 6.5 grendel lapua brass necked down to 6mm. I did not bump the shoulder when necking down.
Pressure signs are extractor swipes and hard extraction - I had turned off the gas as to not lose brass in the gravel, so I was manually ejecting.

So you are shooting a 243lbc or a 6mmAR in an AR15 correct?

It is a wildcat cartridge. It probably isn't fully fireformed yet. Just make sure when it finally is fireformed you don't oversize that brass and it will easily last 10 firings.

Premature pressure signs? What powder and bullet? Sometimes too light a load can be as a bad as a heavy load. Here are some recommended powder and loads for the 105's from Robert Whitley the inventor of the round. Should get you started

Powder Charges:

27.5 to 28 gr. Hodgdon's H4895
28.5 to 29.0 gr RL 15
27.5 gr. IMR 4895
28.5 gr. N140
28.5 - 29.0 gr N540
28.0 to 28.5 gr. AA2520 (ball powder

Every barrel is different. You have to find what works for you.

AR's are notoriously hard on brass. With a case that is short like that it gets slammed around in that chamber pretty good. Could be a.contributer to those pressure signs. One thing you might try is long seat some bullets and jam them in the lands. A method of headspacing for fireforming. You are single loading anyway with your gas cut off.

 

[divingin]

Pressure signs are extractor swipes and hard extraction - I had turned off the gas as to not lose brass in the gravel, so I was manually ejecting.

Harder than other loads manually ejected?

I ask because a) not too many people try this, and b) AR's typically have pretty wimpy primary extraction, and not much leverage to apply that primary extraction. Is it possible the "hard extraction" was "normally hard extraction" (when cycling by hand)?

If the extraction was normal, that would leave the short case hammering back against the bolt upon ignition as being the likely cause of pressure signs (IMO.)

 

[Winny94]

So you are shooting a 243lbc or a 6mmAR in an AR15 correct?

It is a wildcat cartridge. It probably isn't fully fireformed yet. Just make sure when it finally is fireformed you don't oversize that brass and it will easily last 10 firings.

Premature pressure signs? What powder and bullet? Sometimes too light a load can be as a bad as a heavy load. Here are some recommended powder and loads for the 105's from Robert Whitley the inventor of the round. Should get you started

Powder Charges:

27.5 to 28 gr. Hodgdon's H4895
28.5 to 29.0 gr RL 15
27.5 gr. IMR 4895
28.5 gr. N140
28.5 - 29.0 gr N540
28.0 to 28.5 gr. AA2520 (ball powder

Every barrel is different. You have to find what works for you.

AR's are notoriously hard on brass. With a case that is short like that it gets slammed around in that chamber pretty good. Could be a.contributer to those pressure signs. One thing you might try is long seat some bullets and jam them in the lands. A method of headspacing for fireforming. You are single loading anyway with your gas cut off.

I followed Whitleys write-up and started @ 26.0gr of 4895 w/ 107gr SMK
I did:
26.0
26.3
26.6
26.8
27

pressure signs got worse as charge went up. I stopped at 27 (but did have 5 more loads up to 28.0) when I couldn't extract w/o slamming the stock on the bench.

 

[Winny94]

Harder than other loads manually ejected?

I ask because a) not too many people try this, and b) AR's typically have pretty wimpy primary extraction, and not much leverage to apply that primary extraction. Is it possible the "hard extraction" was "normally hard extraction" (when cycling by hand)?

If the extraction was normal, that would leave the short case hammering back against the bolt upon ignition as being the likely cause of pressure signs (IMO.)

yes. the 26.0, 26.3 26.6 loads were able to be extracted from the firing position. 26.8 need more leverage, and 27 need the full on buttstock slam on the bench.

 

[Richard Coody]

I followed Whitleys write-up and started @ 26.0gr of 4895 w/ 107gr SMK
I did:
26.0
26.3
26.6
26.8
27

pressure signs got worse as charge went up. I stopped at 27 (but did have 5 more loads up to 28.0) when I couldn't extract w/o slamming the stock on the bench.

I had a grendel and tried cutting the gas off. Same issue with factory ammo. Open up that gas block and buy you a brass catcher. They work great and i bet your extraction problem goes away.

One thing i have found about AR's. They need a little breaking in. Run it wet and put a couple of hundred rounds through it and it will smooth out.

Now if this is a whitley rifle follow his directions. I had a turbo 40 improved and it was the most accurate AR i ever had.

 

[Winny94]

I had a grendel and tried cutting the gas off. Same issue with factory ammo. Open up that gas block and buy you a brass catcher. They work great and i bet your extraction problem goes away.

One thing i have found about AR's. They need a little breaking in. Run it wet and put a couple of hundred rounds through it and it will smooth out.

Now if this is a whitley rifle follow his directions. I had a turbo 40 improved and it was the most accurate AR i ever had.

Just a quick follow up - when you say you had the same issue w/ gas off, was that the extraction issue or pressure sign issue?

Also here is a pic showing the extractor swipes - they get more pronounced as the charge weight increased, but, like i said, extraction didnt become an issue until 26.8.

 

[gunsandgunsmithing]

I'd go back to square one to start. Is the new brass .0125 short before necking it down as well?

 

[argrendel]

I had a grendel and tried cutting the gas off. Same issue with factory ammo. Open up that gas block and buy you a brass catcher. They work great and i bet your extraction problem goes away.

One thing i have found about AR's. They need a little breaking in. Run it wet and put a couple of hundred rounds through it and it will smooth out.

Now if this is a whitley rifle follow his directions. I had a turbo 40 improved and it was the most accurate AR i ever had.

Had? As in you sold it? I’ve gone through several Grendels looking for a shooter like that..

 

[Richard Coody]

Yes i read that the brass is short. To me no big deal. You have a wildcat cartridge in a wildcat chamber. When that brass has expanded to fit your chamber it.will quit growing. The trick is to know when it is fully fireformed where you know when it is time to set your die for a .003 shoulder bump. You can take all the measurements you want but in my opinion you should let the rifle tell you when it is fully fireformed. With a bolt gun that is easy. When the bolt closes with some resistance on a fired piece of brass then you are ready to set your die for a shoulder bump. With a gas gun it is the same principle just a little more difficult. I just drop it in the chamber and close the bolt. If the bolt closes and the fired brass ejects easily the it is not.fully fireformed. Do your neck sizing and not bumping the shoulder and load them and shoot them. Might take 3 firings to get that brass fully fireformed. If you start bumping too early it will effect the life of your brass.

Yeap in fact mortaring that stock on the bench didn't eject the brass. I had to drop the lower and then it was quite easy to tap the brass out. In fact it is much less stressful to me to drop the lower instead of beating the stock of my rifle on the bench.

I had read all that about cutting the gas off for load development and accuracy. Well maybe with a 223 but the 6AR has a lot more bolt thrust (105 gr bullet compared to a 50 or 60). Anyway i opened that gas port fully and put a 100 rounds through that rifle and the problem has not recurred. I have adjusted the gas block to fine tune ejection. Now this upper i built myself with a mur upper and jp barrel, bolt and gas block. Have the.proper jp silent capture buffer system. Rifle performs great now but i haven't had the urge to cut off the gas again.

Your brass doesn't look bad at all. Its probably just because that brass is slamming back against the bolt as it grows. With my turbo 40 i partally sized the neck leaving a donut that acted as a false shoulder for fireforming. That is why i suggested seating some 108's long and jaming them in the lands to hold that case head up against the bolt until you get the.brass fireformed. Don't be afraid.to jam them. They just have to be single loaded. Some of.my best groups were shot with 105 vld's jammed.about..020.

Now AR's are rough on brass. You will get ejector marks and bent rims and bent necks. That is the main reason i quit fooling with precision AR's. Carefully prepared lapua brass beat to $&*@.

 

[Richard Coody]

Had? As in you sold it? I’ve gone through several Grendels looking for a shooter like that..

Yep

I sold it.to someone on this forum.

If you are willing to.pay the price i am sure Robert Whitley can fix you up. His deal comes with a complete upper dies and reloading.instructions. he knows how to build an AR that shoots. And the.remarkable thing about that rifle is it wasn't picky at.all. 90 gr. Game kings to 108 bergers. Didn't matter. They all shot.great. load devopment? Followed Robert's directions. The load development was already done. I tried a.lot.of different things but always wound up with Robert's loads.

 

[Winny94]

I'd go back to square one to start. Is the new brass .0125 short before necking it down as well?

It is. The shoulder wasn't bumped when necking down

 

[Winny94]

Yes i read that the brass is short. To me no big deal. You have a wildcat cartridge in a wildcat chamber. When that brass has expanded to fit your chamber it.will quit growing. The trick is to know when it is fully fireformed where you know when it is time to set your die for a .003 shoulder bump. You can take all the measurements you want but in my opinion you should let the rifle tell you when it is fully fireformed. With a bolt gun that is easy. When the bolt closes with some resistance on a fired piece of brass then you are ready to set your die for a shoulder bump. With a gas gun it is the same principle just a little more difficult. I just drop it in the chamber and close the bolt. If the bolt closes and the fired brass ejects easily the it is not.fully fireformed. Do your neck sizing and not bumping the shoulder and load them and shoot them. Might take 3 firings to get that brass fully fireformed. If you start bumping too early it will effect the life of your brass.

Yeap in fact mortaring that stock on the bench didn't eject the brass. I had to drop the lower and then it was quite easy to tap the brass out. In fact it is much less stressful to me to drop the lower instead of beating the stock of my rifle on the bench.

I had read all that about cutting the gas off for load development and accuracy. Well maybe with a 223 but the 6AR has a lot more bolt thrust (105 gr bullet compared to a 50 or 60). Anyway i opened that gas port fully and put a 100 rounds through that rifle and the problem has not recurred. I have adjusted the gas block to fine tune ejection. Now this upper i built myself with a mur upper and jp barrel, bolt and gas block. Have the.proper jp silent capture buffer system. Rifle performs great now but i haven't had the urge to cut off the gas again.

Your brass doesn't look bad at all. Its probably just because that brass is slamming back against the bolt as it grows. With my turbo 40 i partally sized the neck leaving a donut that acted as a false shoulder for fireforming. That is why i suggested seating some 108's long and jaming them in the lands to hold that case head up against the bolt until you get the.brass fireformed. Don't be afraid.to jam them. They just have to be single loaded. Some of.my best groups were shot with 105 vld's jammed.about..020.

Now AR's are rough on brass. You will get ejector marks and bent rims and bent necks. That is the main reason i quit fooling with precision AR's. Carefully prepared lapua brass beat to $&*@.

I appreciate your time and explanation. So if you were me, what would your next step be? Load the rest to 26-26.5 grains and do the .020" jam to fireform the remaining 95 cases before any load dev.?
I need to get a modified case made from one of my once fired so I can accurately find the lands.

Thanks again

 

[Richard Coody]

I don't know what the intended purpose of this rifle is but i think i would just.work with 25 cases at.a time. That is 5 five shot groups. Plenty for load development and finding out what the rifle is capable of.

If you have a 6mm turning mandrel you could use it as an expander and get some.light neck tension and soft.seat the bullets into the lands. You will be single loading anyway. Seat the bullets way long and let the bolt finish seating them in the lands.

I would go ahead with 27.5 gr. H4895 and the gas open all the way on my gas block and see what happens. Make sure to use a primer with a hard cup like a br4 and try to ease that bolt into battery seating the bullets to avoid a hang fire. I would probably just make sure i had the rifle pointed down range and slam the bolt home on the first one and see what happens.

There is another way to roughly find the lands though. We don't need benchrest accuracy here yet. Take a cleaning rod and run it from the muzzle to the bolt face and mark it at the muzzle. Then take a short rod and firmly seat a bullet into the lands. Take that same cleaning rod and run it down the barrel till it touches the bullet and mark it. Punch the bullet back into the chamber. The difference in the 2 measurements is a rough overall cartridge length.