190gr Sub-X cycling issue

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BrianT
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Re: 190gr Sub-X cycling issue

Post by BrianT »

I reloaded a PMAG M3 556 mag, pushing each round back as it was loaded, instead of just smacking the back of the mag as I usually do, and it did strip and load the rounds as expected. I didn't fire anything out of the 556 mag yet, but I will soon.

I haven't gotten any No Last Round Bolt lockbacks with the A1680, but initially I did intermittently with the Lil Gun and CFE BLK, and then most recently with the H110. A couple of those were decent velocities, which got me to thinking about being undergassed.

I loaded some of these Sub-Xs up same as before, 2.070", using CCI#41 and Winchester #41 primers, and still got the same FTF failure.

I don't know, maybe it's something to do with the feed ramps on this CMMG barrel.

Other than trying the 556 magazines, I guess I'll just fool with the COL, and see where it starts to fail.

I also might try a couple of slower powders, I4198 I4227, just for the data, then start all over again when my suppressor shows up...
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dellet
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Re: 190gr Sub-X cycling issue

Post by dellet »

H110 and Lilgun can be tricky without a suppressor in carbine gas. CFE BLK surprises me if would not cycle.

What is the length loading to the Cannelure? I would load longer, but not shorter.

The feed ramps worth checking. They were a big problem early on, then people figured it out. But the way parts and loading components have been, the issue could have resurfaced.

There are M4 feed ramps and standard ramps. You can mix barrel extension and upper one way but not the other, easy to track down photos on a search.

The other problem not so talked about, is using 223 feed ramps and a .308 bullet. There is a difference and it’s easy to see side by side. A lot of people opened them up more even if they were a 308 ramp.

I think your narrowing down the issue, again I think your load is probably the least of the problems.
300 Blackout, not just for sub-sonics.
BrianT
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Re: 190gr Sub-X cycling issue

Post by BrianT »

Loaded to 2.070" is very close to the middle of the cannelure.

The rounds that cycled, I loaded to 2.150". I have 10 more of them loaded up, and 10 @ 2.100", also.

I found a picture you posted in a discussion of "Feed Ramps", and my feed ramps look like the top right 'Good' picture.
Augenblick
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Re: 190gr Sub-X cycling issue

Post by Augenblick »

I'm pretty sick with covid. My brain is cooking. But I believe it is correct that you want to load longer. I'm not up for research right now but I believe that will lead to higher pressures at the gasport and may help your feed issues. Something about dwell time also.
BrianT
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Re: 190gr Sub-X cycling issue

Post by BrianT »

I loaded up more of the 208gr Hornady BTHP-M, A1680, 11.8gr, 2.250” and, as previously, they all cycled fine, same with the 220gr Berry’s SP, A1680 12.0gr, 2.220”, no problems.

The 190gr Sub-Xs cycled fine this morning with A1680 12.0gr, @ 2.150”, a little hot at 1129fps, which surprises me, but I can work with it. Standard buffer spring and 4.75oz buffer.

Load them @ 2.100”and get the same FTFeed failures.

I guess there is some kind of issue with this bullet’s profile and my rifle, when loaded shorter 2.070”-2.100”.

Hope you get feeling better Augenblick.
Augenblick
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Re: 190gr Sub-X cycling issue

Post by Augenblick »

Yeah feeling better thanks.

I think you're on the right track. While I have had success with both my rattler and psa16" pistol length seating the 190gr sub-x at 2.07" I'm currently seating to 2.185" without issue.

Seating a little longer should slightly decrease your velocity but you might need to take off a couple tenths off your charge if you're still a little hot.

I think it is exactly what you said. Something with that bullet profile and your guns.

Just a couple more tweaks and you'll have this licked.
mildot22
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Re: 190gr Sub-X cycling issue

Post by mildot22 »

I can't give you a specific to solve your cycling issue but can offer a simple thing to try.

You might change your primer to a standard small rifle primer (SRP) like CCI 400.

I have a 16" carbine gas system that has (had) some issues cycling 180 and some 190 subs without a suppressor. I was using #41 and SRMP primers to get best powder ignition but that was part of my issue. By dropping back to a standard SRP (#400) that allowed me to go up a couple of tenths on powder fill and still not go over my 1060 fps goal.

It seems that I got enough more gas from the little extra powder added than I did with less powder but a hotter primer.

After I got it cycling good with the SRP with 1680, after shooting it for about 100 rounds I even managed to get it cycling with my favorite powder (N-120). I didn't get bolt lock back with a stock carbine buffer spring but it cycled OK, it does lock back just fine with a SpringCo spring.

I did also try some CFE black and that cycled just fine but personally I don't like the CFE black if I can get N-120 to work as my SD's went up noticeably with the CFE black (but it did cycle OK and work)
BrianT
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Re: 190gr Sub-X cycling issue

Post by BrianT »

Thanks to you all for your suggestions!

I will try some Standard SRP's when I can find some, Dellet suggested the same thing, but all I have are the Magnums and #41s.

I can work with having to load them longer, 2.15"+, and maybe sometime I'll get a wild hair, and work on my feed ramps, and see how that works.
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