+- (https://6mmarc.com)
+-- Forum: 6MMARC DISCUSSION (https://6mmarc.com/forumdisplay.php?fid=3)
+--- Forum: 6mmARC AR15s and Semi-Autos (https://6mmarc.com/forumdisplay.php?fid=4)
+--- Thread: Contradictory Issues? (/showthread.php?tid=1413)
Contradictory Issues? - tlbaker - 10-22-2024
What is my rifle telling me and how do I fix it? The build is Anderson upper/lower/parts kits, mil spec BGC, 24" barrel with a rifle length + 2" gas tube, SA adjustable gas block set at full open, JP silent capture spring with standard weights and spring. I was running 29.3 gn Lever with 105 gn Bergers at about 2770 fps using Hornady brass. The main issue I had with the Hornady brass was flat spots on the neck and an occasional cratered primer. Flat necks were the same for factory ammo and reloads. I recently got a batch of Alpha brass and tried some Varget and StaBall Match and had no pressure signs but did have flat necks. I then tried Lever charges of 27.2, 27.5, 27.8, 28.1, 28.4, 28.7, and 29.0 gn. All these were the first firing for the Alpha. The pictures show ejector marks and raised burrs on the case head and flattened necks. Ejection patterns are 4-5 o'clock. I think these are contradictory since the ejection pattern indicates an under gassed condition and the head damage indicates over pressure. I do not know what the flattened necks mean and where the brass hits to cause this. I am riding the struggle bus with this right now. Did I cause myself problems with the long gas tube resulting a need for faster burning powder?
Thanks for any advice offered.
Terry
[attachment=904]
[attachment=905]
RE: Contradictory Issues? - 300BLK - 10-22-2024
The neck dents are from the case bouncing off the shell deflector behind the ejection port. Put something on there to pad it (velcro works).
SA is full open at 8 turns out from fully closed. Additional clicks "OUT" allows gas to bleed off, so adjust OUT until it won't lock open on an empty mag, and then close it up a click or two for reliability.
Rifle +2 gas is for slower burning powders, so not your problem.
I'm not familiar with Alpha brass, so don't know if it has less capacity than Hornady or is just softer. That brass is UGLY!
What bolt are you running?
RE: Contradictory Issues? - tlbaker - 10-22-2024
The bolt is a Monster MK136. Compass Lake did the Criterion barrel and matched bolt. My velocities have always been ~200 fps higher than Hornady gas gun data. My assumption was the slower powder continues to burn and increase pressure after the bullet passes the gas port. But I would also think this show in the ejection pattern as overpressure.
Alpha brass is not cheap so I don't want to destroy it.
RE: Contradictory Issues? - grayfox - 10-23-2024
+1 on the neck flattenings, the soft side of velcro not the hooks.
The ejector marks and burrs, well, seems to indicate over-pressure but could also be unlocking late/early (not an expert here).
My own theory on the grendel and 6 Arc is you don't have a large amount of powder - smaller case volume to barrel volume so you may reach 100% burn, you'll still get some more gas expansion but it starts to be diminishing returns, sort of thing. Lever reaches 100% burn at about 16" of barrel.
On the SA gas blocks, they operate 2 ways, full closed to open as a traditional adjustable gas block gradually opening up gas flow, then beyond that they start to bleed off the gas forward of the GB, which is what 300Blk was talking about - good for suppressor ops. I have mine right now on traditional adjustment, what you want to start with is fully open which is not "all-the-way-out".
To the bullets. Which 105 berger, there are a few? tangent or secant or modified (hybrid) ogive? what seating depth are you using (how far off the lands)? what depth do they touch your lands? Using a crimp to prevent bullet sliding forward on the bcg slam?
I have some 105's, they are vld-h, and some 105 Hornady's, running them at 29.7 in my bolt gun (22), and from what I see they would extrapolate to the 24" around 2780, under 52 ksi, so wouldn't expect those burrs/ejector marks like on your brass.
O yes, one other thing, did you do a set of laddered charges, the case volume for the alpha brass is probably different than either Hornady or starline, so you need to do this to know how the powder, lot# and bullet/case react to increases in powder, to see it you're going over pressure. I don't personally think so but to know is better than to assume (my loads are with hornady brass).
From the web... accurateshooter forums, same issue but 22" criterion with +2. he tried all the buffer weights and finally enlarged the gas port to give more gas. Could be food for thought. You can follow the thread here:
https://forum.accurateshooter.com/threads/6mm-arc-new-barrel-set-up-teething-pains-not-cycling.4126202/
RE: Contradictory Issues? - tlbaker - 10-23-2024
I am 18 clicks out on the gas block and according to my directions that is full open.
Bullet is the BT target tangent ogive (24428). I never tried to find the lands but used mag length as my starting point. 1.588" CBTO was my max length and best accuracy comes between 1.570" and 1.561" CBTO measured with SAC modular headspace body/insert. Seems like I would always have a flyer or two in every group so I was question brass. Typically I do not crimp. I tried that in one reloading session but I am not convinced it did anything. I was concerned about damaging the bullet since there is not a cannelure. Maybe I need to revisit the crimp?
No ladder charge per se with the Alpha brass. I did try multiple charge weights of 27.2, 27.5, 27.8, 28.1, 28.4, 28.7, and 29.0 gn but all marked up the head and some did not lock the BCG.
I took a quick peek at the website and then reached out to Compass Lake to see if they know what they drilled. They dimpled the barrel for the SA gas block so I would think the alignment is good and hopefully the diameter is correct. Customer service email indicated Paul Ross opened his own company and they are referring customers there.
RE: Contradictory Issues? - grayfox - 10-23-2024
One observation this morning I did not think of last nite: your ejection pattern , 4-5 o'clock, appears under gassed. These steps all work towards upgrading your gas-flow volume/pressure.
1. Are you using a rifle buttstock or carbine? I have not had any luck, really, with rifle length buttstocks. Can you swap lowers or swap in a carbine stock, check for improvement.
2. With that carbine stock, swap out the JP SCS for a standard carbine spring and standard, 2.8-3 oz buffer weight.
3. There is a possibility that you are losing too much gas via leakage in/around the bolt carrier, what kind is it. See if swapping in a different carrier (keep the monster bolt) changes anything.
4. Not trying to say definitively that this is the issue, but about a year ago I had a similar problem (except for the dings on the neck, that is from hitting the deflector as described above) of gas even when full open, in a 223 wylde barrel from Ballistic advantage. Nothing worked until I opened up the gas port diameter (there are easy ways to do it and not mark or score the rifling on the bottom of the channel) like the accurate shooter thread (don't believe I had seen this thread but something similar). I got a set of "wire diameter drill bits" and gradually opened it up until it cycled properly. Now BA is not a Criterion but is a decent barrel, and other BA's I've had did not have this problem.
RE: Contradictory Issues? - 300BLK - 10-23-2024
I just looked at SA's website and their new instructions do indicate 18 clicks is full open. The LAST one I installed was full open at 32 clicks......
Not all gas ports are at the same distance from the shoulder on the barrel, and not all gas blocks are drilled a specific distance from the rear edge. I had an issue with a 223 Wylde barrel and SA gas block, so it doesn't hurt to VERIFY. By sliding the block forward you will see witness gas marks where the port and block meet, so will tell you if the port is partially obstructed.
I would NOT drill the gas port bigger until I had eliminated the other variables.
Lever is one of slower burn rate powders for the cartridge and bullet, so should be making enough gas. Your 24" barrel is the reason you're seeing more velocity than indicated in "gas gun" data. the additional 6" of barrel over 18" test barrels used for the data is the difference.
LOW pressure won't make case head brass flow back into the ejector hole; HIGH pressure will. Your primers look round, so nothing scary there. Cratered primers can be from a poor firing pin/hole fit. Soft brass?
RE: Contradictory Issues? - tlbaker - 10-23-2024
The stock is a Magpul PRS. I may be able to borrow a complete carbine lower to check that function.
BCG is a Brownell's mil-spec unit. I did find a damaged gas seal and replaced it. There was not much missing but one was not like the other two. It appears to pass the "support the bolt weight test" but I will verify that after a good clean.
I will check the gas port location and size before I shoot it again. Compass Lake processed the barrel specifically for the SA gas block so I do not expect to find anything out of the ordinary.
It will be probably be several days before I can get back to the range. I have brass in the tumble right now and I will need to work the reloading bench.
Again, thanks for all the information!
RE: Contradictory Issues? - BD1 - 10-30-2024
I have also experienced the raised burrs on the case head on my new Alpha brass. I have checked the volume of the Alpha vs my Starline and they both average 35 grains of H2O overflow, (although the starline is much less consistent). One significant difference I've found is that the rim is thicker on the Alpha which means the extractor is a closer "fit" to the groove and is holding the brass tighter to the bolt face on extraction. I am going to try a different extractor to see if it makes a difference. I also plan to polish the leading faces of the bolt lugs to take the sharp corner off to see if that helps with the gouging of the case head. I am running a 22" +2 RTR barrel with a fixed gas block, 3.8 oz buffer and stock spring in a rifle lower. I am unwilling to give up on the rifle lower as I've been competing with it for a long time. it came from my DCM Hi Power rifle, has an excellent trigger that I am very used to, and I like the lead brick in the stock for competition.
RE: Contradictory Issues? - Dino11 - 11-03-2024
Take the ejector pin out of the bolt and round the edges were it make contact with the case head.
The edges of the ejector are sharp and galling your brass. they need rounded off a little bit.
RE: Contradictory Issues? - tlbaker - 11-04-2024
I have incorporated some changes to my rifle and reloading process based on feedback here. Shout out to Gray Fox. I have not tried the carbine stock and standard buffer yet so that remains on my to do list along with some more bolt work. I started over with load development and reduced my powder charge expecting no bolt lock. Surprise, every shot locked the bolt back and the Saturday groups were some of my best ever. I filled in a few powder gaps today and results were similar with six sub MOA groups and three sub .5 MOA groups. Saturday was dead calm but today was gusty. I am pleased with the results. The rifle seems to like 26.9 to 27.1 gn Lever. I was probably pushing things way too hard before. Every ejection pattern still indicated an undergassed condition so the buffer change is on my list.
Monday update: I looked over my ejector and it appears to have the proper geometry and no sharp edges. I stoned the ejector hole as it was a little rough. The extractor also had some pretty sharp edges on the surface that contacts the head stamp. I stoned those edges just enough to break the corners. Some of my Alpha brass showed rotational smear corresponding to the edges that I worked but none of the Hornady brass had these marks. I measured the rim thickness on two pieces of brass and the Alpha is maybe .002" thicker if that is enough to matter. I will continue working with the Hornady for now and save the Alpha.
[attachment=909]