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Posted: 8/28/2016 10:20:14 PM EDT
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Hi all, I went to the range today to sight in an 18" WOA SPR .223 Wilde rifle length gas build with some CBC 77gr OTM that I've had sitting around for awhile. I brought the chrono today (ProChrono) because last summer I popped primers, had ejector swipes, and ejector burrs that I didn't notice until I got home from the range. 81° mostly sunny, chrono set up 15' from muzzle. Here are the results from lot 013/14 PR 884 L-152/13: 2962 3003 3071 3065 2996 Had signs of high pressure on the brass, but no popped primers. I have either an H2 or 3 buffer in right now from a previously installed H1. I thought these velocities were ridiculous so I'm looking for input. I had 2 rounds left from the box so I loaded them in my 16" DD CHF, one didn't register on the chrono, but the other clocked in at 2842, at this point I'm thinking my chrono is broken. The empty brass from the 16" didn't show any signs of excess pressure. I ran a few more rounds from a different lot/powder 09/14 WC 732 L-2475/14: 2943 2949 2969 Same signs of pressure, did not put any more 77gr down range. |
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I switched to some XM193 to get some readings: 18" barrel 3463 3472 3505 No significant pressure signs, slight impression from ejector, velocities seem ridiculous for an 18" barrel. 16" barrel: 3263 3252 3276 3228 3152 3259 3252 1110 1124 1128 1122 1129 1129 1128 1176 |
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Your Chrono is about 250-300fps too fast.
XM193 should be about 3200-3250fps for 18", and the CBC 77gr OTM should be about 2600-2675fps from multiple sources I've found. I have 4 different lots of CBC 77gr OTM made in 2014 and 2015, all with WC 732 powder, never had I experienced any signs of potential over pressure issue. All of the reports of CBC have been positive, and after shooting ~1500rds of the CBC 77gr OTM, I would have to say it is GTG. You mind posting some pictures? |
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Everything I've read on it has been great as well, and it is a very accurate round despite the pressure signs. I'll post some pictures of the spent brass tomorrow after work. I think I've got 2 probable issues to sort out, one being the pressure signs I'm getting and one being the chrono readings.
I'll also try to get out in the next few days and shoot some strings with the chrono in complete shade and see if my results are different. |
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Everything I've read on it has been great as well, and it is a very accurate round despite the pressure signs. I'll post some pictures of the spent brass tomorrow after work. I think I've got 2 probable issues to sort out, one being the pressure signs I'm getting and one being the chrono readings. I'll also try to get out in the next few days and shoot some strings with the chrono in complete shade and see if my results are different. I wonder if your barrel's gas port is over sized... Of all the 77gr OTM I've tried, IMI 'Razor' is the only one that I would consider 'hot', with it being about 75-100fps faster than CBC and BH. CBC and Black Hills have almost the same velocity. |
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THIS. Quoted:
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I wish the CBC 77 OTM would come back in stock somewhere...anywhere. THIS. Maybe September. |
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I wish the CBC 77 OTM would come back in stock somewhere...anywhere. THIS. Maybe September. Like everybody else, I am ready to buy more CBC/Magtech 77gr OTM... I need a few cases of that stuff in my life 
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With the 16" gun firing the same ammo just fine with no ejector swipe, I would have to say there's something wrong with your 18" barrel...
Also, it seems to me that the cases fired by your 18" gun has a light primer strike symptom. It just doesn't look like a deep enough strike on the primer. |
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I agree on those primer strikes looking a little light. So have any of you CBC fans tested directly against the Black Hills MK262? Been curious about this, as I'm starting to run low on BH 262 ammo. I have tried CBC 77gr OTM, BH 77gr OTM, and BH 77gr TMK. I haven't tried the actual military MK262, but I figure they will be very similar to the civi version. Accuracy and velocity both goes to the CBC, at least in my KAC SR15 Mod1 16"(LW profile) and my SR15 Mod1 LPR 18"(SPR profile). Next will be the BH 77gr OTM, then TMK. Also tried the IMI 'Razor' 77gr OTM, and it has the same accuracy as the BH TMK. 
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Classic,
I shoot CBC 77 OTM at 1000 yards frequently and have left ammo in direct sunlight at 100 degrees F. It does heat up but anyhting will. I am using 37 MOA elevation to get on at 1,000 at around 85 degrees F and 3200 feet absl. I often but not always chrono while shooting at long range and CBC seldom comes in over 2870 fps from a 20 inch Wylde chamber. |
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Update, I sent the upper in for evaluation. The tech was super friendly ando they were incredibly quick to evaluate it. The barrel/bolt headspace came in at 1.466". I've also read their barrels have a larger gas port at .108. I was told they keep their chambers in this area to promote better accuracy. They said that ammo was simply just hotter than most and he noted the additional felt recoil vs their normal test fire ammo. Basically said that the ammo was too hot to shoot in this barrel, said I could do it, but I'd have to keep the brass shavings out of the FCG and scrape the extractor once in awhile along with checking the bolt over for stress/damage. I'm not thoroughly impressed by not being able to shoot a factory ammo from it, especially since I built it for the 77gr variety. I've never used an adjustable gas block, could that tone this thing down a bit or should I quit screwing around and just buy a Stealth instead? |
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I would have told em to keep that POS barrel and send me the rest of my parts back.
I shot some CBC and IMI 77gr SMK last year through a 20" LaRue, a 16&18" BCM SS410 and all three of those performed great. I also shot some through a 16" KAC Mod1 and it would be key holing at 400M. I have another BCM SS410 enroute now and will run some through it when it arrives. |
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The fact that you haven't had popped primers or severe flattening makes me think that, while it's a little hot in that chamber, the consequential issue is simply early unlocking due to a very large gas port oriented more to slower competition loading. Get an SLR clamp-on adjustable gas block and tune it one click open from where it starts failing to lock back while holding the rifle un-shouldered with the H buffer. It'll probably be somewhere around 7 clicks from closed. 1.466" headspace isn't particularly tight. |
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Update, I sent the upper in for evaluation. The tech was super friendly ando they were incredibly quick to evaluate it. The barrel/bolt headspace came in at 1.466". I've also read their barrels have a larger gas port at .108. I was told they keep their chambers in this area to promote better accuracy. They said that ammo was simply just hotter than most and he noted the additional felt recoil vs their normal test fire ammo. Basically said that the ammo was too hot to shoot in this barrel, said I could do it, but I'd have to keep the brass shavings out of the FCG and scrape the extractor once in awhile along with checking the bolt over for stress/damage. I'm not thoroughly impressed by not being able to shoot a factory ammo from it, especially since I built it for the 77gr variety. I've never used an adjustable gas block, could that tone this thing down a bit or should I quit screwing around and just buy a Stealth instead? My WOA 20" DMR ran fantastically with CBC, but I use an adjustable gas block. Are you using a WOA bolt too? |
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I have popped a couple primers, none fully came out in this round of firing. One of the ones I sent in was noticeably loose in the pocket, you could wiggle it slightly with your finger nail, the primer crimp is the only thing holding it in. It is odd primers aren't flatter, but I was told that the primer strikes appear light due to the amount of pressure pushing them back out. Other ammo does show a better primer strike. Yes the bolt was from WOA as well. |
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