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Posted: 7/31/2016 3:00:17 PM EDT
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I'm having case head swiping on my 6.8 SPC well below max pressures. I've been using AA2200. I had no problems with up to max published loads with 95gr, 100gr, and 110gr bullets. But with 120gr SST's I get swiping with even mild loads. I can go to max published loads with no other pressure signs. So its obviously a timing problem, not a pressure problem. I really want to be able to use these heavier bullets.
The barrel is an 18" mid length 1 in 11" Cardinal barrel. I am running a std carbine buffer and a F/A carrier. What are the things I can do to fix this. 1) Will opening up the gas port help?. 2) Will a heavier buffer help? 3) Will going to a slower powder help, or would I need to go to a faster powder? |
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Try in this order.
HB. HB + extra power recoil spring. 2HB. 2HB + extra power recoil spring. You need to delay the opening of the action a smidge. Using an adjustable gas block may or may not work, but considering their expense, that would be the last thing I'd try. |
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Quoted:
Try in this order. HB. HB + extra power recoil spring. 2HB. 2HB + extra power recoil spring. You need to delay the opening of the action a smidge. Using an adjustable gas block may or may not work, but considering their expense, that would be the last thing I'd try. I've got an H2 buffer and extra power buffer spring around here someplace. And I just ordered an H buffer. As soon as the H buffer shows up, I'll go through this sequence looking for a cure. |
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If you already have the 2HB, crap, try that first. The worst that will happen is that it might not lock the bolt back when empty, or at the absolute worst, short stroke.
If the 2HB solves the issue and the bolt locks back when empty, call it a day. If you still get swipes then try the XP spring. Don't spend money unless there is a reason. I don't mind spending money (just ask my wife) but not needlessly. |
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Two things come to mind.
Make sure the ejector and extractor are free from burs. Ejector usually causes case head swipes I believe, please correct if wrong. Other thing is you are overgassed, as mentioned. The action is a tad fast, I'd be curious of the OD of the gas port. Had case head swipes, mid case dents, 12-1pm ejection and went to adjustable gas, which fixed everything. Running a cheap carbine BS and Spike's ST-T2. |
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You can give this a try Tubb CWS
It takes 2 seconds to install and change out the weights and cost about the same as a new buffer |
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In a auto rifle, your dealing with two areas in regards to pressure.
The first is the actual working pressure of the round, which you do not want to go over Sammi specs, then the second is the amount of residual gas pressure once the round has passed the gas port to pressurize the B/C to get the bolt to retract. In regards to gun powders, those with slower burning rates will always have a higher gas port pressure, then those with a faster burn rate instead. The reason for this, with the slower the powder burns, the closer it's dwell max spike will be towards the gas port/away from the chamber. As for ejector marks, if the working pressure is low, then the problem is the bolt is unlocking too fast instead; with the residual pressure in the barrel too high when the bolt goes to unlock. So heaver buffer, or stronger recoil spring will slow the bolt unlock back down, using an adjustable gas block to lessen the gas pulse to the B/C, as well as just swapping to a faster burning powder as well. Simply, when the bolt is unlocking to soon, then the barrel gas port is too large for the load on hand instead. so since the match for unlock is the buffer-BC mass and the recoil spring weight, against the amount of gas against such, you can either resolve the problem by making the gas port smaller, or increase the mass/spring tension against the gas to band aid the gas port being too large isntead. |
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